Woodin .1974.Polychaete Abundance Patterns in a Marine Soft-Sediment Environment - The Importance of Biological Interactions

8/3/2019 Woodin .1974.Polychaete Abundance Patterns in a Marine Soft-Sediment Environment - The Importance of Biologic

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Polychaete Abundance Patterns in a Marine Soft-Sediment Environment: The Importance of

Biological InteractionsAuthor(s): Sarah Ann WoodinReviewed work(s):Source: Ecological Monographs, Vol. 44, No. 2 (Spring, 1974), pp. 171-187Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/1942310 .

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Ecologiac Al'Monog0.raphIs (1 974 ) 44: 17 1-I 87

POLYCHAETE ABUNDANCE PATTERNS IN A MARINESOFT-SEDIMENT ENVIRONMENT: THE IMPORTANCEOF BIOLOGICAL INTERACTIONS'SARAH ANN WOODIN2Departmient f Zoology, Uniiversity f Washinglton, eattle, Washhigtoni 8105

Abstract. Samples of infauna and measurementsof temperature, xygen, salinity, nd algalcover were takenfromJanuary 1969 to December 1970 at -1.2-ft tidal elevation in a mud flatdominated by polychaetes in Mitchell Bay, San Juan Island, Washington. Mortality of adultsafter pawningand variable larval settlement uccess probablyexplained muchof the variation npopulation numbers of the four large and numerically mportantpolychaete species, Lumbri-neris lflata, Ariot/lc/la utbrocinicta,latynereisbicanialiculata, nd A i-mandia rel' is. No corre-lationswerefoundbetween theabundances of numerically mportant pecies and physicalfactors.Exclosures constructed of 3-mm mesh plastic screeningplaced on the flat became coveredwith diatoms. Settling uveniles of tube-building pecies, such as P. bicanaliculata, Axiothellarubrocincta, nd L. inflata, built tubes in this layer of diatoms and thus did not reach the en-closed sediment,while settling uveniles of a burrowing species, Armandia brevis, burrowedthrough hediatom layer and reached the sediment. Thus, cleaning the cage surfaces or remov-ing thecage after ettlement educed abundances of tube-building pecies withoutdisturbing hesediment ince adults of all threenumerically mportant ube builders experiencemortality fterspawning. The manipulation of tube-builder abundances showed that the burrowing speciesresponded to space vacated by tube builders by increased settlement uccess. Results from ex-perimentalvariation of A. br-ei'is umbersper unit volume of sediment in the laboratory andabundance data from unmanipulated natural areas also demonstrated the presence of inter-specific nd intraspecific ompetitionforspace.Changes in physicalfactorsdue to algal cover had some impact on population levels but thecompetitive nteractions nd behavior patterns, evealed only by observations on thebehavior oflivingorganisms and manipulation of the infauLna, emonstrated the importance of biologicalinteractions o the determination f species abundance patterns n a soft-sediment nvironment.Key wortls: Competitioi; interlactions, biological; polychacte; soft-sedimnent;pace.

INTRODUCTIONIn general marinebenthicecologistshave not sep-arated two importantquestions: which species ofthoseavailable for ettlementan tolerate hephysicalextremesof an environment nd what determineswhich of these species exist n an environment.Thefirstquestion is concerned with physical tolerancelimits and life styles. For example, a soft-sedimentorganisms an unlikelynhabitant fa rockface. The

work reportedhere evaluatesa portionof thesecondquestion, .e., how important hysicalfactors nd bio-logical interactions re as determinants f speciesabundance patternswithin one habitat, an intertidalsoft-sedimentnvironment.Physicalvariables fluctuatewithgreater mplitudein the intertidal han in the bordering ubtidal,thussubjecting rganisms nhabiting he ntertidal nviron-ments to greaterphysical stress. Research in bothtypes of marine soft-sedimentnvironments as pro-duceddescriptions f the faunaand flora and charac-1ReceivedMarch 5, 1973; acceptedAugust16, 1973.2 Currentddress: Department f Zoology,UniversityofMaryland, ollegePark,Maryland 0742.

terization f thephysicalvariables (Thorson 1957).With littleor no biological informationvailable onlife-stylesr species interactions, variety f mathe-matical techniques have been used in an attempt oexplain patterns of species distribution nd abun-dance. The resultant orrelationsprovide onlyweakinferences nd do not necessarilyexpose the causalrelationships. However, much efforthas been di-rected toward explaining species distribution andabundance patternsusing correlationswith easilymeasured physical factors,such as sediment grainsize distribution.Depending upon its life style, an organismmayrequire a givensize range of sedimentfor tubebuild-ing, burrowing,or feeding (Wieser 1959). In ananalogous manner an organism inhabiting rockyintertidal egionmay require a horizontal or verticalsurface, revices,or tidepool. As Levins (1968) ex-plains,a determinationuch as therequired sedimentrange represents nly a portionof the fitness urvefor thatorganism nd thatparameter. It may be theoptimalportionphysically utsucha characterizationalone is less interestinghanthequestions: (1) whatdetermines he boundariesof an organism'srealized

171


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172 SARAH ANN WOODIN FcologicailMonograplh%Vol. 44, No. 2

_....*_-'O..Wh DFIG. 1. Aerialphotographf study rea on MitchellBay.San Juan sland,Washington,howing he 1.2-ft(A) and -1.7-ftB) stations.

substrateistributionelativeo itspotentialubstratedistributionnd (2) whatdetermineshe organism'sabundancewithinhese oundaries.The same rgumentanbe madeformost hysicalpropertiesf an environment. here he tolerancelimits or particularhysical ariable, uch as tem-perature,ave beendeterminedor n organism,heorganism'sealized istributionas beenfound obemuchmorerestrictedhan ts potential istributionrelativeo thisfactor Newell 1970). Thus, the m-portancef another hysical actor nd/or iologicalprocesss indicated.The distributionndabundance atternsforgan-isms nhabitinghe rockyntertidalone are notde-terminedolelybytheir oleranceo physical tress.Connell 1961), Paine 1966), Dayton 1971), andothers aveshown xperimentallyhat he biologicalprocessesf competitionnd predationlay signifi-cant role. However, efore heuse of experimentalmanipulativeechniquesherockyntertidalonewasas poorlyunderstoods the soft-sedimentnviron-ment for example, ewis 1964).Biological rocessesmaybe equally mportante-terminantsf such patternsn the marine oft-sedi-

ment nvironmentut he vidence s lessconvincing.Sharp real boundariesccur betweenpecies Vas-sallo 1969, anders tal. 1962), species how ggres-

sive territorialehavior Reish and Alosi 1968,Ockelmannnd Vahl 1970), organismsan toleratefluctuationsreater han he xtremesf thephysicalenvironmentNewell 1970),andpredatorsre knownto affectpopulationdynamics Trevallionet al.1970). As Dayton 1971) has demonstratedn therockyntertidal,he hypothesishatbiological nter-actions re mportantoes nothave s a corollaryherelative nimportancef physical actors.Both thebiological nd physical ropertiesf the ubstrate,orexample,re mportanturingarval ettlementWil-son 1968, Gray 1966).Conventionalampling echniques ield esults npopulation luctuationsut noton the relativem-portance f biological nd physical eterminantsfspecies abundancepatterns.Manipulations bothmore romisingnd more ifficulttechniqueousein soft-sedimentnvironments. he substrate sthree-dimensionalnd the sediment ayeringmustnot be disturbedr, f t is, thecontrolmust e dis-turbedna replicatemanner. nfaunal rganismsrerarely bserved nd often mpossibleo map. Unlessa very argearea is used,an experimentannotbesubsampled.Despite the anticipated echnical problems tseemed ppropriateo undertaken examinationf asoft-sedimentommunitysing xperimental anipu-


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Spring1974 POLYCHAETE ABUNDANCE PATTERNS 173lation. Crustaceans,polychaetes, nd bivalves usuallydominate nfaunal assemblages and are therefore hemostuseful rganisms or xperimental tudy. Amongthese,numerous nfaunalcrustaceans nd polychaetesare relatively short-livedwhile bivalves are oftenlong-lived Thorson 1957). In addition polychaetesare intermediate n size and represent an aureamediocritasfor the investigator etween the crusta-ceans, whose small size makes taxonomy difficult,and the argerbivalves, where sample size becomes aproblem. Since polychaetes re short-lived, ecoloni-zation of a disturbed rea can be expected within areasonable time. Also, the polychaete portion of theinfauna often encompasses a number of species be-longing to a varietyof life styles and trophic evels(Jones 1961).This work reports ata on the physicalvariables ofa mud flat and on the polychaete inhabitants. Thedescriptivefindings are interpretedn the light ofmanipulation f polychaete abundances, observationson living individuals, ife cycles of the componentspecies, and abundances of infauna.

STUDY AREAThe flat s located inMitchellBay on the northwestcornerofSan Juan sland, Washington.The extremetidal range s approximately 3 ft from+10 to -3 ft(0.0 ft = MLLW) (U. S. Coast and Geodetic SurveyTide Tables 1971).3 The main studyareas were at-1.2-ft and -1.7-ft tidal elevation (Fig. 1). They

were chosen because of the number of polychaetespecies and their dominance. The numerically m-portantpolychaete families were Syllidae, Nereidae,Lumbrineridae,Maldanidae, and Opheliidae.MATERIALS AND METHODS

Tidal heightwas establishedfromhightide markto -2.0 ft with a transit.A transect ine dividedinto10-m ntervalswas laid down fromhightidemark to-1.8 ft,a distance of approximately 0 m. At the-1.2-ft study area a 70-m line perpendicularto thetransectwas mapped and each meterwas assignedanumberfromone to seventy. Numbers from one toseventy were then drawn from a random numbertable and used to choose sites where samples ofinfauna were taken every other monthfromJanuary1969 to December 1970. Samples were not takenseasonallyat the-1.7-ft station. Sedimentgrainsizewas determined or 14-cmdeep cores in June 1969,October 1969, and November 1969 using the stan-dard drysieve and pipette echnique Krumbein andPettijohn 1938). Mean grain size and sorting (ameasure of deviationfrom the mean in phi units)were calculated accordingto Inman (1952).

3Tidal heightsre given n feet o conformwithU.S.Coast and Geodetic urvey ide Tables.

Temperature and P02 were determined n situ inareas with and without lgal cover in 2-cm deep in-crements to 16 cm. Measurementswere taken atfour stages n the tidalcycle: as thearea was uncov-ered, at the time of extreme ow water, ust beforethe area was covered, and with SCUBA at high tide.Depending upon availability f equipmentthermistoror mercurythermometers raduated to 10 C wereused to measure temperature.The thicknessof thealgae, i.e. thenumberof layers,was notedifpresent,and measurementswere taken from areas with andwithout algae. The pO. was determinedwith Ra-diometeroxygenelectrodes attached to a Beckmanor Radiometer gas analyzer and calibrated withwater-saturated nown gas mixtures. Surface watersamples were obtained using a glass syringe, stop-cock filled withglass wool, and a probe. The glasswool did not cause aerationonce it was wet; it pre-ventedcontaminationby sedimentwhich fouled theelectrode membrane. In-sediment water sampleswere originally btained with a syringe, stopcock,and a 12-cm graduated probe, but in contrasttoBrafield (1964) surfacewater contaminationwas amajor problem, compounded by small pore size ofthe surface sediment, apillary action, and contami-nation by sediment. This method, therefore,wasabandoned and the electrode was placed directlyinto the sediment Jansson 1967).Samples of waterfordetermination f salinityweretakenin plastic syringes t different epthsfromthesediment surface in a variety of pool sizes duringtidal exposure. The samples were analyzed in thelaboratoryusing a milliosmometer.Algal cover was monitored y photographing hreerandom areas at each station veryothermonthfromJune1969 to June 1971. Samples of algae for taxo-nomicpurposes were taken at these sites.For samplingthebenthicfauna, a galvanized steelframe 0.05 m2 by 14 cm deep was pressed into themud and the enclosed volume was collected either sa whole or in 2-cmdeep layers. In eithercase it wasfixed in formaldehyde olution bufferedwith hexa-methylenetetraminend dilutedwith ea water to4%.After fixation the sample was sieved through a1.0-mm and a 0.5-mm sieve and the retained frac-tionswere stained with rose bengal and sorted undera dissectingmicroscope. If algae were present, heywere removed prior to the sieving process so thatsamples with algae would not retain small organismsto a greater xtent than samples without algae. Be-fore sorting, ome samples were passed throughamodifiedflotationprocess using a saturated sucrosesolution n a 20-cm column. In all cases the entirefraction etained by the sieve was sorted. The fixedorganisms retainedby the sieve were counted and


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174 SARAH ANN WOODIN Ecological MonographsVol. 44, No. 2TABLE 1. Abundance f polychaete nfaunarelative osieve ize withinne 0.05 m2samplefrom 1.2-ft idalelevation n February 3,1969

Sieve izeSpecies 1.0mm 0.5 mm

Exogone ourei 125 426Lumbrinerisnflata 39 55Axiothella ubrocincia 11 12Platynereisicanaliculata 11 12Armnandiare is 5 9Nereisvexillosa 1 1Scolelepis uliginosa 7 8Naineris endritica 18 19Eupolyniril eterobranchia 8 9Ophiodrontusugettensis 8 22Harmothoembricata 3 8Cirratulusirratus 0 6Capitella apitata 3 3Syllisheteroclaeta 3 12

identified. Only heads and whole organismswerecounted.Identification f polychaete pecies was based pri-marilyon thekeys of Berkeley nd Berkeley 1948,1952) and Banse and Hobson (unpublished). Addi-tionalreferences sed were Pettibone 1953), Usha-kov (1955), Day (1967), Hartman 1968 and 1969)and Kozloff and Lambert 1969).First a 0.5-mmsieve thena 1.0-mm ieve was usedsince the majorityof the sediment s retained by a0.5-mm sieve butnot by a 1.0-mm ieve,while 12 ofthe 13 species and 41% of the totalpolychaetenum-bers retainedby a 0.5-mm sieve are retainedby a

TABLE 2. Replicate0.05 m2 amplesfrom 1.2-ft idalelevationApril , August 1,1969a 1969'

Sample Sample SampleSampleSpecies 1 2 1 2Exogone ourei 661 622 208 263Lumbrinerisnflata 46 52 14 33Axiothella ubrocincta 50 59 37 49Platynereis icanalicu-lata 28 33 1 3Armandia revis 38 43 15 12Nereisvexillosa 3 2 2 1Scolelepis uliginosa 0 0 2 2Naineris endritica 4 6 2 2Eupolymniah'tero-branchia 4 5 1 0Op/hiodromusugettensis 5 3 5Hartnothloinmbricata 4 5 12 15Sphaerosyllisirifera 23 0 1 6Capitella apitata 4 0 0 2Dorvillearudolphi 1 1 1 1Phyllodocemaculata 1 3 2 1Glycera apitata 0 0 1 0Syllisheterochaeta 1 9 1 0Cirratulus irratus 0 1 0 0

ab = 0.795; P > 0.999.bTab = 0.741; P > 0.999.

FIG.2. Photographfthe hree age types: opless,sideless,nd omplete.1.0-mmieve Table 1). OnlythedataforApril7,1969, are from 0.5-mm ieve.Replicabilityetween.05-m2 y 14-cm eepsam-ples a minimumf5 mapart) wasgood nough hatone sampleadequately epresentedt least the nu-mericallyominantmembersf thepolychaete or-tionofthe nfaunaTable 2). Themeasure f simi-larity sed was Kendall's oefficientf association,Taib. The use andsignificancef this tatistics dis-cussed n Looman andCampbell 1960) and Nich-ols 1 70).Cages were used to excludepredators, etainknownnumber f predators, revent ettlementfcertainpolychaetes,nd increase lgal abundance.Theywere onstructedf 3-mmmeshplastic creen-ing attached ybraidednylon ord to framesmadefrom ishpans y cuttingut thebottom nd sidesandtopflangeFig. 2). Eachcorner f the agewasattached y a stainlessteel lipora length fnyloncord o a 0.5-m oweldrivennto hemud.The finalin situ agedimensions ere28 cmby34 cmby 12cmhighwith heedgeof thecagepenetrating cminto hemud. Unless lgalor diatom overwasde-sired, age surfaceswerecleaned pproximatelywoto four imesach month.After,6,or 9 monthshesediment ithinhe cage was sampled s describedabove.

PHYSICAL FACTORS AND ALGAL COVERThe slopeof theflatwas 70 fromhigh idemarkto +1.0-ft, ? from 1.0-ftto -1.2-ft,nd 1? from-1.2-ft o -1.7-ft idal levation.The sedimentizedistributiont the 1.2-ft tationis presentednTable 3. Presumablyue to thefinegrain izeand thusarge apillary orce fthe urfacesediments,mallpoolsof water angingn diameterfrom cmto I m and indepth rom .1 cm to 8 cmremained n the urfacef theflat rom 1.0 to-1.7ftduringow tides.Due to this urfacewater nd thewater etained y the ediments,nfauna id notex-


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Spring 1974 POLYCHAETE ABUNDANCE PATTERNS 175surface A. B. C.water 04_s ^

0 0~~~~~~~~~~~~~0 A

8-

xsurfaceDE.Fwater A9 t'2 o t {2 B fsr D . . .6) 8 10 01

waterk ~ ~ ~ ~ ~ ~~emeatr -?

Qr) A.0

16 A A

6810 14 18 ~~~ ~~~~~~~~~~~226 ~430 2 16 2'0 24 8Temperature0C)FIG. 3. Sediment emperaturesith lgal cover open symbols) nd without lgal cover closed symbols) tthe -1.2 and -1.7-ft stations (A and D-Feb. 2, 1970; B and E-June 20, 1970; C and F-July 19, 1970).Measurements were made as the tide exposed the area (circles), at the time of extreme low water (squares),

and just beforethe area was covered with water (triangles).perience desiccation. Sedimenttemperatures ariedwithtidal height,weather, imeof exposure,season,sediment epth, nd algal cover.Surfacewater and surface sediment, s expected,exhibitedmaximalvariationswhilethe sedimentayersprotecteddeeper levels (Fig. 3). This is in accordwiththe findings f Johnson 1965) and Pamatmat(1968). The duration of tidal exposure and theweather strongly nfluencedthe sedimenttempera-tures;for example, duringthe wintermonthswhenair temperatureswere below those of the overlyingsea water at hightide, surfacesedimentreadingsap-proximatedair temperaturesduringtidal exposure.In addition, he presenceof algal coverprovidedsomeinsulation n the summer s is documented n Fig. 3.Sea watertemperaturesn 2 m or moreof waterathigh tide and extreme urface-sedimentemperaturesduringtidal exposure for monthswhen samples ofinfaunawere takenwere maximal in Aprilto Augustand minimal nOctoberto February Table 4).Oxygen determinations f surface water samplesand directelectrode mmersion evealed oxygenten-sionsfromgreater han saturationduringthe day toless than 20 mm Hg duringthe night. During day-

light,bubbleswith tensionsgreater han 400 mmHgwere observedboth at high tideand duringperiodsoftidal exposure on diatom and large algal surfaces.Thus, oxygen tensionsof surfacewaterseemed to bestrongly ffectedby algal respiratory nd photosyn-theticdemands. Sedimentsbelow 2-5 mm had nooxygen. This usually coincided with the top of theblack layer.The salinity f surface-sediment ater duringtidalexposureranged from28%, to W6Yohe salinityoftheoverlyingwater rangedfrom 30% to 34%. San-ders et al. (1965) and Johnson 1967) demonstratedelsewhere that although surface water varied insalinity he sediment salinityremained stable unlessthechangeswere of long duration.Algal cover is both a physicaland a biologicalfac-tor in the sense that it is a biological entitybut itspresencechangesthe physicalproperties f the sedi-ment. Its presence reduces thedepthof the oxygen-ated sediment ayer o that he anoxic layer begins m-mediately eneath he algae. Algal cover also insulatesthe sediment,but in addition it increases the avail-ability f food both to herbivores nd depositfeeders(Khailov and Burlakova 1969).


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176 SARAH ANN WOODIN Ecological MonographsVol. 44, No. 2TABLE 3. Summary of sediment grain size analysis:mean grain size, sorting, nd percentage by weightpergrain size category, sediment from-1.2-ft tidal eleva-tion

June 27, October November1969 17, 1969 14, 1969Pebble (4.0 to 64.0 mm) 34.8 49.9 41.4Gravel (2.0 to 4.0 mm) 8.5 5.4 16.0Verycoarse sand (1.0to 2.0 mm) 8.9 4.9 5.2Coarse sand (0.5 to1.0 mm) 13.0 5.6 6.4Medium sand (0.25to 0.5 mm) 11.0 7.9 7.4Fine sand (0.125 to0.25 mm) 10.9 9.9 9.3Very fine sand (0.0625to 0.125 mm) 8.2 8.7 8.8Silt (0.0039 to 0.0625mm) 4.0 6.9 4.4Clay (< 0.0039 mm) 0.7 0.8 1.0Mean grain size 1.21mm 1.21mm 1.24mmSorting phi units) 3.03 3.29 3.13

As Fig. 4 illustrates, lgal cover varied seasonallyat both the -1.2-ft and -1.7-ft stations, nd the sea-sonal variation n algal cover was similarfor the twostations. Three genera were involved: Ulva, Mono-stroma, and Enteromorpha. All were temporary nthatfewattachment iteswere available and most ofthealgae werenotfastened o a solid objectbut float-ing in the water. The amount of attachedalgae wasincreased by the behavior of two nereid polychaetespecies, Platynereis bicanaliculata and Nereis vexil-losa. Individualsof all sizes actively caught" floatingalgae withtheir aws, pulled thepiece down, and at-tached it to their ubeopenings. Depending upon thegrowthand size of the algal piece relative to theappetite fthenereid, healgae might how noticeablegrowth Roe 1971). Over 80% of theattachedalgaewas fastened to such tubes. During winter stormsalgal clumpsand their ssociatedfauna werewashedTABLE 4. Extreme sea water (during high tide) and sur-face-sediment during low tide) temperatures t -1.2-fttidal elevation

Temperatures ? C)Date Sea water Sediment

January 1969 6.2 3.0February 1969 8.5 4.5April 1969 9.5 9.5June 1969 11.2 26.0August 1969 15.0 25.5October 1969 7.5 7.5December 1969 6.0 6.0February 1970 7.2 6.8April 1970 8.5 23.0June 1970 13.5 28.0August 1970 13.8 28.4

-12 ft statin . I

J F N D JF M A M J A S 0N D JF M A M J6020 1

J J A S N D J F M A M J J A S 0 N D J F M A M J J---969 - -7 0 - -D----17

FIG. 4. Percent lgal cover t -1.2-ft nd -1.7-ft idalelevation.Mean and range represented.out of the intertidal one; thus, some animals werelost in this manner.Massive mortality f polychaetes was never ob-served either during or after very cold (minimalsurface-sedimentemperature f +30 C) or veryhot(maximal surface-sediment emperatureof +28.4?C) or very rainy periods that coincided with tidalexposure, xcept for one species thatwill be discussedlater. Whether or not polychaetesare as successfulin withstanding hysicalstress s the lack of obviousmortalitywould indicatecannot be answered incenomeasurements f reproductive utput weremade.

SAMPLING DATALif history nf rmationIn terms of species number, samples of benthicmacrofauna (organisms retainedby a 1.0 mmsieve)fromthe -1.2-ft station re dominatedby polychaeteannelids,while themost numerous pecies is a tanaid,Leptochelia savignyi Table 5). As Table 5 illustrates,

three polychaetes (Exogone lourei, Lumbrineris n-flata, and Axiothella rubrocincta) are numericallyimportant hroughouthe yearwhile two polychaetes(Platynereisbicanaliculata and Armandia brevis) arenumericallymportant xceptduringperiodsof adultmortality fter spawning,when prior to settlementtheirpopulation evels are low (e.g. June and August1969 and June 1970). Three are herbivores nd eatprimarilygreen algae, and two are deposit feeders(Table 6). Exogone lourei adults are less than 1 cmlong and 1 mm wide, while adults of the otherfourspecies are much larger. The majority f the nfaunalcrustaceansand bivalves are not discussedbut theirabundances in seasonal samples and experimentalareas are available from the author.


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Spring 1974 POLYCHAETE ABUNDANCE PATTERNS 177TABLE 5. Abundances of infauna at -1.2-ft tidal elevation: January 1969 to December 1970, 0.05 m2 samples

1969 1970Species J F A J A 0 D / F A J A N D Mean no./m;

PolychaetesExogone lourei 129 125 661 603 263 663 211 303 639 433 575 484 877 8842Lumbrinerisnflata 57 39 46 91 33 135 37 104 45 33 45 92 168 1465Axiothellarubrocincta 32 11 50 127 49 158 49 79 104 79 52 158 123 1702Platynereisbicanaliculata 30 11 28 9 3 72 143 69 165 8 133 313 358 2190Armandia brevis 3 5 38 1 12 55 99 77 29 8 44 52 47 720Nereis vexillosa 2 1 3 3 1 0 1 1 1 1 5 1 6 38Scolelepis fulginosa 2 7 0 20 2 37 12 18 12 7 7 11 17 253Naineris dendritica 29 18 4 0 2 5 6 25 1 0 1 1 18 177Eupolyninia lieterobranchli 3 8 4 2 0 4 1 1 3 2 0 2 19 75Ophiodrornuspugettensis 1 8 7 22 5 15 16 16 19 24 18 18 15 295Hlartnothoembricata 1 3 4 14 15 5 7 4 4 1 6 14 4 130Glycera capitata 1 0 2 0 2 0 1 1 0 0 0 0 12Phyllodoce maculata 3 0 1 2 1 4 2 8 0 1 0 2 1 40Capitella capitata 19 3 4 4 2 5 8 19 9 2 3 9 22 175Syllis hieterochaeta 21 3 1 41 0 22 0 9 1 1 0 17 6 202Doritillearudolphi 1 0 1 14 1 5 2 8 5 10 5 11 6 113Cirratulus cirratus 0 0 0 8 0 5 1 9 2 1 0 0 10 60Sphaerosyllispirifera 0 0 23 0 6 0 7 0 6 13 3 0 5 67TanaidLeptochelia satiggnyi 520 519 365 1321 942 1421 3381 2203 26680Bivalves" 18 45 63 49 42 33 84 94 1070Miscellany') 87 56 165 193 172 242 369 436 4300

Data for April 1969 were excluded from this calculation since a ().5 1mm ieve was used on that datc.Abundances of species are available from the author.

Leptochelia savignyi,Exogone lourei, Lumbrinerisinflata,Axiothella rubrocincta,and Platynereisbi-canaliculata buildtubes thatopen onto the surface orintothe oxygenated ediment ayers. The burrower,Arinandiabrevis, oes not builda tube but ndividualsare normally oundwithin cm of the sediment ur-

face (Table 6). Depth distributions f the six nu-merically mportant pecies are shown in Table 7.Most of the individualsfoundbelow 6 cm may havebeen pushed down by the samplerframe. The vastmajority f thepolychaetesoccur within cm of thesediment urface.

TABLE 6. Life stylesof thedominantpolychaetespeciesSpecies Trophic level" Reproductivetype Comments

Exogone lourei Berkeleyand Berkeley Herbivore Broods itsyoungon ventral Builds a tube; often n thetop(Syllidae) surfaceNov.-Feb. 2 cm of sedimentor in theoxy-genated layersurroundinglargerorganisms'tubesLumbrineris nflataMoore Herbivore Lays egg masses attachedto Builds temporary ubewith(Lumbrineridae) undersidesof rocksand shells surfaceopeningthroughout heyearAxiothella rubrocincta Johnson) Deposit feeder Reproductivemethod not Builds permanent ubewith(Maldanidae) known;settlement ccurs surfaceopeningthroughout heyearPlatynereisbicanaliculata (Baird) Herbivore Discrete synchronous pawns Builds permanent ubewith(Nereidae.) with peaks June-Sept.;plank- surfaceopening; shows terri-tonicdevelopmentwith et- toriality nd "gardening"be-tlement fter3 to 4 weeks; havior(Roe 1971)Armandia brev s (Moore) Deposit feeder Free spawnApr.-Nov.; Does not build a tube; burrows(Opheliidae) planktonicdevelopmentwith in top 3 cm of sedimentsettlement fter 3 to 4 weeks;

mayhave 2 or 3 generationsper summer Hermans 1964and 1966)a Determined ygut contents nd observations n livingorganisms.


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178 SARAH ANN WOODIN Ecological MonographsVol. 44, No. 2TABLE 7. Abundances of numerically important species on April 7, 1969 (A), August 11, 1969 (B), December 7,1969 (C), and April 22, 1970 (D) in 0.05 m2samples analyzed in 2-cmdeep increments t -1.2-ft tidal elevation

Surface 2 cm 4 cm 6 cm 8 cm 1Ocm 12 cmto to to to to to to2 cm 4 cm 6 cm 8 cm 10 cm 12 cm 14 cm TotalExogoeI lourei A 551 67 23 6 2 8 4 661B 201 62 0 0 0 0 0 263C 209 2 0 0 0 0 0 211D 600 16 17 1 1 0 4 639Luinbrinerisnflatk A 16 13 6 5 3 0 3 46B 16 17 0 0 0 0 0 33C 37 0 0 0 0 0 0 37D 25 7 0 4 6 2 1 45Axiotllal rubrocincta A 22 9 7 2 5 2 3 50B 18 31 0 0 0 0 0 49C 49 0 0 0 0 0 0 49D 82 16 2 1 1 1 1 104Platynereishicanaliculata A 21 6 0 1 0 0 0 28B 3 0 0 0 0 0 0 3C 143 0 0 0 0 0 0 143D 150 14 1 0 0 0 0 165Arnmandia revis A 21 9 7 0 1 0 0 38B 12 0 0 0 0 0 0 12C 99 0 0 0 0 0 0 99D 27 2 0 0 0 0 0 29Leptochelia savignyi B 462 52 3 1 1 0 0 519C 358 7 0 0 0 0 0 365D 1170 121 13 5 5 4 3 1321

No data available for April 1969.

Natural historynformation or the six numericallydominant pecies was used to design the manipulationexperiments nd to interpret he abundance data.Due to the small size of Exogone lourei (adults areless than 1 cm long and 1 mm wide) many were notretainedby a 1.0-mm ieve. Thus the abundance pat-terns of E. lourei in natural and manipulated areaswill not be discussed.

Abundance levelsThe abundances of thefour numerically mportantand large polychaete species (Lumbrineris intlata,Axiothella ubrocincta, latynereis icanaliculata, ndArmnandia revis) are quite variable throughout he

year (Fig. 5).One hypothesis o explain the consistency f repli-cate values in space is that the causes of the popula-tion fluctuationsfor each species occur uniformlyor in patch sizes of less than 0.05 m2 at -1.2-ft tidalelevation. Given adequate settlement y the impor-tant species this result is not surprising ince mostphysical fluctuationsoccur relative to tidal heightchanges,and randomphysical events,such as dam-age by logs on rockyshores (Dayton 1971), are notcommon on mud flats.A moreattractive lternative ypothesiss thatran-dom eventsdo occur but are unimportant elativetonon-random ffects. Since Platynereisbicanaliculataextends its tube up into the algae and herbivorous

divingducks are present, t least some P. bicanalicu-lata individuals are eaten with the algae by suchducks and presumably this is a random event at agiven tidal height. A major non-random vent thatmay account for a large percentage of the seasonalvariation is reproduction,patch size for this eventbeing ess than 0.05 M2.All four species experience mortality fter pawn-ing. Spawning in P. bicanaliculata and Armandiabrevis populations occurs within discrete periods.Armandia brevis begins spawning n April and con-tinues to Novemberwithsettlement -4 weeks aftereach spawning. The juveniles attain reproductivematurityn one and one-halfmonths nd thenspawnand die (Hermans 1966). The events are relativelysynchronouswithina population. Marked fluctua-tions in A. brevis abundance, then, should be ex-pected from Aprilto November or December due tomortality f overwintered dults afterspawning,fol-lowedby settlement, aturation, pawning, nd deathof two or three new generations f A. brevis. Suchfluctuationswere found (Fig. 5). This does not ex-plain whyA. brevis abundance was so much greaterin December 1969-February 1970 than in January1969-February 1969 and December 1970. Nor doesit explain the reduction n A. brevisnumbersfromDecember 1969 toApril 1970 (Fig. 5).Platynereisbicanaliculata die after spawning,andthe expected population fluctuations due to adult


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Spring 1974 POLYCIIAFTE ABUND)AN(C IA'I-PATERNS 179200 -|VA100 _

000 B 114iI . 0 I{0.10 tot-all valueILs f r.

to conditions nl hemud flatthatwere notmeasuredor to conditions in the plankton.Reproductiveevents,then, can be used to explainn-uch of the seasonalvariation, uchas thepopulationfluctuations ofA. brevis nd P. hicanaliculata duringthe summer, nd theyprobablymask the effects frandom events. Fluctuations that cannot be ex-plained by reproductive vents are the decline in A.berevisnumbersfromDecember 1969 to April 1970,the decline in P. iicanaliculata numbers from De-cemiber 1 69 to February 1 70 and their ubsequentincrease n April 1970 (Fig. 5). Samplingvariability,of course, is anotherpossibility.Correlation (of pecies abundance with otherspecies and with physit l factors

Abundances of the five dominant polychaetespe-cies were examinedforcorrelationwith physicalfac-tors found to fluctuate easonally, i.e., seasonal ex-tremesof surface-sedimentemperatures,urfaceseawater emperaturesTable 4), and percent lgal cover(Fig. 4). No significantorrelationswere found Ta-ble 8).Summer ow tidesduringhot sunnyweather,how-ever,do contribute o mortality speciallyof surfaceorganisms, such as Ophiodlrola us pugettensis-a poly-chaete that spends a large proportionof its timecrawlingon the surfaceand often s not in the sedi-TABILL 9. Individualsn surface howing hermal tresson JUIly19, 1970 whenSUrface-sedimentemperaturesreached 28.40 C

Species CommentOphlio(droto)illlsllgvettteu.sis$ Manydead; numberwith ur-face epidermiseeling ffArmtandia1)er is Two dead; individualsn sur-faceshowing scapebehavioraPiityttercis ,,caltalicullt Individualson surfaceshowingLAmrintle//lsbtiflat ta escape behavioraAxitheOtla rulbroc lc tal

SLttC behavior was not seen at any other time dUring low tidesexccpt wshena prctlator was present.


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180 SARAH ANN WOODIN Vol. 44, No. 2Ecological MonographsTABLE 10. Pearson product-moment orrelationcoefficients f infaunalalbundances from January1969 to December1970 and the significance f the correlationcoefficients

L. vsaviiiivi E. lourei L. jiuflaita A. ruhroti-bwta P. hicoooiliclo1ta A. brevisLeptoclelia SO i'igy +0.618 +0.648 +0.916 +0.821 +0.175Exogoile lourei n.s. +0.578 +0.668 +0.493 +0.099Lumhri/ieristIflata +0.710 +0.550 +0.282A-xioti/llarbtbrocnctti +0.524 +0.206Plbtaviiereisicaiiablicidlbtia: +0.466Arnaclidiahre'i~s n.s. n.s. n.s. n.s. n.s.

* 0.1 > P > 0.o5.* 11.05 P >1.0(11.n.s. P> 0.1.;I Abundances of L. savigtiyi were only available for January 1969, AuLguIst 1969. Deceoiber 1969, and April 1970 to December 1970;N =8.

mentbut only under the algae during ow tides. Forexample, on July 19, 1970, mortality ttributed oheat stressdid occur (Table 9). Such mortality asbeen observed n the fieldonly when surface-sedimenttemperatures xceeded 270 C. Laboratory experi-mentswith 0. pugettensis bserved under a dissectingmicroscope n sea water n heated watch glasses con-firmed that temperatures n excess of 27? C arelethal. Thus, the insulationprovided by algal cover(Fig. 3) could affect 0. pugettensis urvival duringhot periods of tidal exposure.Since mostwormsother han 0. pugettensis re noton thesurfaceduring ow tide, nd since the sedimentprovides an excellent nsulating ayer (Fig. 3), mor-talitydue to thermal tress was rarely observed pre-sumably because the temperature f the deeper sedi-ment evels rarely xceeded infaunal tolerance imits.0. pugettensis s the only species for which mortalitywas observed more than once. Other species, how-ever, do experience thermal stress, f not mortality(Table 9) even thoughtheir easonal abundances donot correlatewith seasonal temperatures Table 8).Abundances of the same five species were exam-ined for correlation with one another by Pearsonproduct-momentorrelation oefficients Table 10).Positivesignificant r nearly significant orrelationswere found between all species except Armandiabrevis Table 10). Since the reproductive ycles ofExogone lourei, Lumbrineris nflata,Axiothella ru-brocincta, nd Platynereis icanaliculatadifferTable6), the correlations annot be explainedon that basis.All fourspecies are tube dwellerswhile A. brevis-the one uncorrelated pecies-is a burrower Table6). Fhe numericallydominant tanaid, Leptocheliasavignyi, s a tube dweller and its abundances arepositivelycorrelated with three of the four tube-dwelling polychaetes but not with the burrower(Table 10).Summaryof sampling nformation

The majority f the seasonal changes in the abun-dance levels of the four large dominantpolychaetescan be interpretedn termsof reproductive vents.

The year to year changesmay possiblybe due to poorsettlement uccess or poor survival of larvae in theplankton for some unknown reason. The seasonalchanges in population levels of the five dominantpolychaetes do not correlate significantlywith sea-sonal changes in temperature nd algal cover (Table8). All four tube-dwelling olychaetespecies showpositive significant r nearly significant orrelationsof their easonal abundances, and the abundances ofthree of these species correlatepositivelywiththoseof the tube-dwelling anaid (Table 10). The burrow-ing species' abundances do not correlatesignificantlywith the tube builders' abundances. Conditions thatfavor one tube dweller,then, seem to favor all fivesuch species, but do not affect the burrower eitherpositivelyor negatively. The data do not indicatewhat theseconditions re.EXPERIMENTAL MANIPULATION

Introduction o tile hypothesis hat pace is limitingThe tube-building activities of polychaetes andtanaids give structure o the upper 4 cm of sediment.The tubes intertwine nd often the majorityof thesurface ediment eems to be composed of tube struc-tures and fecal pellets. Since the animals requireac-cess to thesurfacefor food (e.g. greenalgae) and/oroxygen, he apparently ullutilization fsurface paceby tubes suggests hehypothesis hat surface space isa limiting esourcebothfortube-building pecies andburrowing pecies. Tube-building species in generalneed at least one tube extension o the surface; thus,theirneed forsurfacespace is continuousover timeif thetube is permanent.Burrowingpecies need ac-cess to the surface and/or the shallow oxygenatedlayerbeneath it only intermittently.he hypothesisfurther redicts hat f the abundance of tube buildersdecreases, the sedimentvolume available forburrow-ing will increase thus ncreasing ase of access to thesurface ayers. Withthisincrease in space availabil-ity, more burrowing ndividuals should occur. Onemeans of testing hishypothesis s to remove all orsome of the tube builderswithout isturbinghe sedi-ment. This effectivelyemovestheirtubes since un-


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Spring 1974 POLYCHAETE ABUNDANCE PATTERNS 181occupied tubes were found to degradewithin everaldays. However, no methodof removing ube build-erswithout isturbing hesedimenthas been reportedpreviously.

Manipulation of abundances of infaunaIn the field,the surfaces of enclosures (Fig. 2)became covered with diatoms almost immediately.The larvae of Armandia brevis, burrowing pecies,respondedto this ayer as a sediment ayer and bur-rowedthrough t;thus they eachedtheenclosed sedi-ment. Larvae of tube-building pecies,such as Platy-nereis bicanaliculata, also reacted to the diatomgrowth s a sediment ayer and built tubes in it; thustheydid not reach theenclosedsedimentbelow. Theabundances of tube-building pecies, then, can bereduced byplacingcages in the fieldprior to periodsof settlement y the larvae of tube-buildingpecies.

Given variable diatomgrowthon the cage surfaces,thereduction n the abundance of tube-building pe-cies should be directlyrelated to the amount ofdiatomcover. If the hypothesis s correctthat bur-rowing pecies are competingwithtube-building pe-cies for access to the surface sediment,one shouldfind an increase in the volume or numbersof theburrowingspecies with reduction of tube-buildingspeciesabundance or volume. Since thesepolychaetesare relativelyedentary, ne would not expectto ob-serve immigration ut rather response in the formof either increased settlement uccess or increasedgrowth or both. Obviously the response is not re-strictedo theburrowingpecies,sincealreadysettledtube-building pecies are present nd can respondbygrowth o the increase n available space.Among the tube builders,Platynereisbicanalicu-lata is the easiest one to manipulate since its majorspawning period is in August and is quite discrete,and settlement s usually completedby the end ofOctober. The other large, numerically importantpolychaete tube builders, Lumbrineris inflata andAxiothella rubrocincta,reproduce throughouttheyear. Armandia brevis, the numerically mportantburrower, inishes ettlement n November and thushas larvae available to respond to increased spacevacated by P. bicanaliculata adults,which die afterspawning. The space is not filledby P. bicanalicu-lata juvenilesas they are kept out bythepresenceofcages.The firstexperimental ages were placed at the-1.2-ft station n August 1970, and the enclosed sedi-mentwas sampled nNovemberand December 1970.The four month interval encompassed the majorspawning nd settlement eriods of P. bicanaliculataand A. brevisbutonlya fraction f the reproductiveperiodsofL. inflata nd A. rubrocincta. As Table 1shows, P. bicanaliculata abundances were reduced

TABLE 11. Abundances f three tube-buildingpeciesand one burrowingpecies within .05 m2caged anduncaged reas at -1.2-ft idal elevation fter monthsBurrowingTube-building pecies species

L. A. rubro- P. bicana-inflata cincta liculata Total A. brevisWithout cage *168 123 358 649 4792 158 313 563 52With cage 168 136 47 351 143132 153 25 310 160113 141 19 273 12964 104 54 222 139

-0.098a -0.089a -0.974a -0.920tn.s. n.s. * **n.s. P > 0.05.**0.01 >P>0.00l.*** 0.001 > P.a CorrelationwithA. brevis abundance; Pearson product-momentcorrelation oefficient.

due to the presence of cages (settled ndividualswerefound on the cage surfaces), while as predicted bythe hypothesis hat space is limiting, he abundanceof the burrowing pecies was increased. A possiblecomplication, he filling f the vacated space by in-creased growthof organisms enclosed initially, uchas L. inflata nd A. rubrocincta dults, pparently idnot occur. The correlation f the total abundance ofL. inflata,A. rubrocincta, nd P. bicanaliculata withthe abundance of A. brevis is highly significant yPearson product-momentcorrelation (0.01 > P >0.001), and 85% of the variance in A. brevis abun-dance is explained by the relationshipbetweentubebuilderand A. brevisdensity.To test the hypothesisfurther, ages were main-tained at the -1.7-ftstationforsix monthsfrom Au-gust 1 70 toJanuary1971 so that thesettlement uc-cess ofLumbrinerisnflata nd Axiothellarubrocinctawould be reduced as well as that of Platynereisbi-canaliculata. Since all threespecies experiencemor-tality fter pawning,with decreased settlement uc-cess thepopulationabundances are reduced. Due tothe longer time,however,the possibilityof growtheffectsby already settled individuals is increased.Unfortunately,ince both L. inflata and A. rubro-cincta fragment asily during fixation,no volumemeasurementswere made. A gradient n settlementsuccess of L. inflataand A. rubrocinctawas foundforthesefive xperimental reas,whiletheabundanceof P. bicanaliculata, ettled ver a much shorter ime,was reduced in all areas. The diatom cover variedoverthe 6-moperiod,thus the arvaeofL. inflata ndA. rubrocincta hat settledover theentire6 mo wereexposed to a wide variety f diatomcover,while thelarvae of P. bicanaliculata that settled over a muchshorter eriodwere not (e.g. Fig. 6). If thehypothe-sis of space limitation s correct, heabundance of A.brevis should followthegradientof increasing pace


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182 SARAH ANN WOODIN Ecological MonographsVol. 44, No. 2A B

C D

* 3

E F10.~~~~~~~~~~~~~~~~~~'

FIG 6 Photographs of c'igesurfa.es ifter three months (A, C, and E) and four months (B, D, and F).

FIG. 6. Photographsfcagesurfaces fter hreemonthsA, C, and E) and fourmonthsB. D, and F).availability, .e., the reverseof the gradient f abun-dance of tube-buildingpecies. As Table 12 shows,this is the case. This correlation of numberof A.breviswithtotal numbers of the threetube-huildingspecies is significant0.05 > P > 0.025), explaining70% ofthevariation nA. breviisbundancc.Another ong-termxperiment n which the cagesweremaintainedforfourmonthsbut the areaswere

not sampled for 5 mo afterremoval of the cages(August 1971 to April 1972) at the -1.2-ft stationshowed the same result Table 13). The correlationcoefficient is significant 0.025 > P> 0.01), and79'/ of the variation n A. brevis abundance is ex-plained by the relationshipbetween A. brevis andtube-builder ensity.The abundance of theburrowingpecies,Armandia


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Spring 1974 POLYCHAETE ABUNDANCE PATTERNS 183TABLE 12. Abundancesof threetube-buildingpeciesand one burrowingpecies within .05 m2caged anduncaged reas at -1.7-ft idal levation fter months

BurrowingTube-building pecies speciesL. A. rubro- P. bicana-inflata cincta liculata Total A. brevis

Without age 309 65 101 475 45With cage 263 37 12 312 49163 54 23 240 41171 20 24 215 39148 3 8 159 8660 8 20 88 101

-0.712 t -0.766t -0.403a -0.8341n. . n.s. n.s. *n.s. P> t).05.*0.05 > P >0.025.Correlationof tube-buildingpecies abundances in caged areaswith A. brevis abundances; Pearson product-moment orrelationcoefficients.

brevis, espondsto the abundances of all three argetube-building olychaete species. The form of theresponse varies temporally.Armandia brevis abun-dances fluctuate rom ate April to November due toreproductivevents Fig. 5 and Table 6). In Novem-ber the final settlement ccurs, and if larvae andspace are available, settlement uccess should begood. Thus, inthe falltheresponseto available spaceis in the form of settlement uccess or failure. Ar-mandia breviscan respond onlyto vacated space be-tweenNovember and April by growth. If competi-tionfor space increasesbetweenNovember and Aprildue to increasedgrowthof tube-building pecies ordue to increasing . inflata nd A. rubrocincta bun-dances (P. bicanaliculata settlement oes not occurduringthistime), A. brevis numbers should declinedue to mortality r emigration f its settled ndivid-uals. A population decline of this type apparentlyoccurredfromDecember 1969 to April 1970 (Fig. 5andTable 14). As was mentioned arlier, hisdeclinecan notbe interpretednterms freproductivevents.TABLE 13. Abundancesof threetube-buildingpecies

and one burrowingpecieswithin .05 rrn agedareasat -1.2-ft idal levationfter monthscage waspres-entonlyduringhefirst months)Tube-building pecies Burrowing pecies

L. A. rubro- P. bicana-inflata cincta licualata Total A. brevis137 174 46 357 90141 133 36 310 7391 109 34 234 152129 71 9 209 12568 40 21 129 175

-0.947 -0.7931 -0.503a -0.890;* on.s. n.s.*).01 > P >0.005.* 0.025 > P > 0.t 1.n.s. P >0.05.tCorrelationwithA. brevisabundance; Pearson prodtuct-moroentcorrelation oefficients.

TABLE 14. Seasonal variations n the abundances ofthreetube-buildingpecies and a burrowingpecies,0.05 m' samples t -1.2-ft idalelevationBurrowingTube-building species species

L. A. rubro- P. bicana-Date inflata Lincta liculata Total A. brevisOctober 1969 135 158 72 365 55December 1969 37 49 143 229 99February 1970 104 79 69 252 77April 1970a 38 93 160 291 49

a Mean of two replicates.

Laboratory experimentsAn obvious corollary f the hypothesis hat pace islimiting s that therewill be intraspecific ompetitionfor space. To test this in the laboratory,measuredindividualsof Armandia brevis were introduced nto

containersof sediment. The sedimenthad been col-lected a maximum of 2 days previously from the-1.2-ft station; the grain size was less than 1.0 mm.A. brevis numbers declined due to the inability ofsome individualsto maintain themselves n the sedi-ment. These appeared on thesediment urfacewithinthe initial3 days. In all six experiments tabilizationof the population levels occurred within three days,afterwhich there was no further eduction n num-bers even after 8 days. The data indicate a constantrelationship etween the volume of the sediment ndthefinal total volume of A. brevis specimens Table15). Individual A. brevis, herefore, ffectotherA.brevisnegatively, nd given a volume of sediment,certainabundance ofA. brevisof a given size can bemaintainedwithin t. In nature,the volume of sedi-ment available to A. brevis is determinedby theabundancesof tube builders Tables 11, 12, and 13)whose tubes remove sediment romthe volume avail-able for burrowing.Several attemptswere made tocontrol thepossibility f food limitation f A. brevisabundance using the same experimentaldesign butwith sterile sediment. A. brevis, however,will notburrow into such sediment; thus, food cannot becompletely liminated as a limiting actor, althoughTABL-E 15. Relationshipf Arinandia breviisvolumetosediment olume n containersnthe aboratory

TotalFinal mean A. brevisSediment individual volumevolume A. brei's no. A. brevis volume(cm) initialfinal volume cm3) sediment228.9 10 5 0.17 0.004121.5 10 4 0.16 0.00542.9 4 1 0.16 0.00442.9 4 1 0.16 0.00442.9 4 1 0.16 0.00442.9 4 1 0.16 0.004


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184 SARAH ANN WOODIN Ecological MonographsVol. 44, No. 2TABLE 16. Axiothella ubrocinctabundance elative opercent lgal cover in manipulatedreas maintainedfromAugust 1970 to January1971 at -1.7-ft tidalelevation

A. rubrocinctao./Manipulationype 0.05m'sampleAlgae removed 157138Control: natural lgal cover 65Algae added" 1510

Topless and sideless cages that retained algae.

given the rapidity f the response and the size of thesedimentvolume, it seems unlikelyto be one.Algal cover

Animals that cannot survive anaerobically mustpossesseither tubeopening on thesurface or accessto the oxygenated-surfaceayers. When algal coveris present,the situation s more complex since theanimals must then extend their ubes or access routesup through everal ayers of algae. Most polychaetesdo, but Axiothella rubrocinctadoes not and is in-capable of emigration; hus, ndividuals of A. rubro-cincta must eitherbe able to withstand noxia or die.Comparisonof data fromtopless and sideless cagesthat retained algae and from areas that were keptclear of algae shows that A. rubrocincta bundancesare negatively ffectedby the continuedpresenceofalgae, presumablydue to an inabilityto withstandanoxia and the failure to move and/or to extenditstubeup through healgae (Table 16).No obvious relationship exists between seasonalabundancesof A. rubrocincta nd percent lgal cover(Table 8). The majorityof the algae is loose; so,therelationshipmay be masked by thevariability fthealgal cover (Fig. 4).

Summaryof manipulation ataTables I I to 15 show theimportance f space and

the effect f tubebuilding n theaccessibility f sedi-mentvolume to burrowing pecies. Access to the sur-face is important orat least one tube dwelleras wellTABLE 17. Diet ofOligocottus lnculosuIs as determinedbyfecalpellet nalysis

Frequency of prey type in dietNo. ofNo. food Other P.of organisms Cope- crusta- hicana- L.fish counted pods ceans liculata inflata21 665 (0.98 0.02 0.002 0.02(0 1574 0.91 0.09 0.001 0.00128 3455 0.94 0.06 ().001 0.023 1963 0.96 0).03 0.001 0.022 1711 0.99 ().(I 0.0 0.0

TABLE 18. Estimatesf the rea defended yPlatynereisbicanaliculata t -1.2-ft idal levationApril April June December1969 1970 1970 1970

Mean P. bicanaliculatalength 2.2 cm 2.3 cm 3.5 cm 1.7 cmP. bicanaliculata abundance 28 165 8 358Estimates of area defended, 67 cm2 445 cm2 49 cm2 520 cm2Area available 500 cm2 500 cm2 500 cm2 500 cm2(P. bicanaliculata abundance)(0.4 mean P. bicanaliculata length)2,r.

(Table 16), and the presence of algae can preventaccess. Nereid "gardening" behavior influences hepermanence f the algal cover; thusthephysical ffectis biologicallymediated.PREDATION AND CANNIBALISM

No estimate of fishpredation was made becauseflatfishwere observed only three times in over 150hours of diving, no evidence of siphon predationeither n theform f evasive feedingbehavior Hughes1969) or siphon regenerationTrevallionet al. 1970)was found n the bivalvepopulation, nd the only fishconsistently een on the flatwas a cottid,Oligocottusmaculosus,which ate copepods as it followedthe in-comingtide edge (Table 17).Bird predationwas observedbut only on one spe-cies, Clinocardium nuttalli, surfacebivalve. Bothravens and gulls ate this pecies by picking ndividualsoffthe surfaceat low tide and crackingtheir shellsby dropping hemonto rocks. Gulls also occasionallyate individuals fNereis vexillosa,which s active ustbefore he mud flat s covered bywater. Herbivorousducks were frequently een diving in the bay, andalong withpieces of green algae theyprobablycon-sumed various infaunal species, in particularthosethat build permanent ubes in the algae, e.g. Platy-nereisbicanaliculata. The activity f suchbirdscouldin partaccount for thepatchinessof the algal cover.Fewer than ten shorebirdswere seen on the flat.Roe ( 1971 showedthatParanemertes eregrina,predatorynemertean,ate Platynereisbicanaliculataand accounted npartforthereductionn P. bicanali-culata abundances after settlement. However, P.peregrinadoes not accountentirely or thisreductionsince individuals fP. bicanaliculataare lostto divingducks and stormswhen their tubes are in the algae.In addition,P. bicanaliculata s aggressive Reish andAlosi 1968). Observationsmade both in the labora-toryand fieldshowed thatP. bicanaliculatadefendsthe area in frontof its tube. It feeds in this area,which has a radius one-thirdto one-half its bodylength. It will attackand eat other P. bicanaliculatathat trespasswithinthis territory.

Calculationsof P. bicanaliculata ize and abundanceshow thatafter ettlement December 1970) and inthe spring before spawning mortality nd after a


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Spring 1974 POLYCHAETE ABUNDANCE PATTERNS 185TABLE 1 . Mortality f Nereis i'exillosa juveniles in con-tainers with and without Platynereisbicanaliculata andOphiodromnus ugettensis dults

Density of Nereis vexillosa uvenilesWith5 P. With5 0.Controls" hicanaliculata pugettensi.s

A B C DDay 1 500 500 500 500 500Day 19 151 223 0 0 0

(2 P. bicanaliculata added to A)Day 24 1 179'13 Setiger ndividuals; ust settling.b Dishes without P. bicanaliculata or 0. pugettensis initially.

period of growth April 1970), theareas theoreticallydefended by the populations are close to those en-closed by the sample (Table 18). With spawningmortality nd severewinter storm damage and per-haps poor settlementnitially tc., the abundances arereduced so that the areas defended are much lessthan the area available (April 1969), as is trueduringthe spawning season before settlement as occurredbut afterreproductivemortalityJune 1970) (Table18).The area defended by P. bicanaliculatawas calcu-lated by multiplying . bicanaliculata abundance bythe area of a circlewhose radius was taken to be 0.4times the mean body length, ince the radius of thecircle P. bicanaliculata defendsranges from 0.33 to0.5 times hebody length f the individual. This is acrude estimatebecause the defended zone reallyhasthreedimensions ottwo.Polychaetes re important redators n otherpoly-chaetes as well. If uvenileNereis vexillosaare placedin a dish withapproximately .5 cm of sediment ndallowed to build tubes, and then adult Platynereisbicanaliculata and/ r Ophiodromus pugettensis reintroduced, ll the N. vexillosa are eaten. Theirsetaeappear in thefecesofP. bicanaliculata and 0. puget-tensis. Numerous N. vexillosa in identical disheswithoutP. bicanaliculata and/or 0. pugettensis ur-vive although theyexperience mortality Table 19).The cause of thismortalitymaybe cannibalismof thetypedescribed above forP. bicanaliculatasince Roe(1971) has observed tforN. vexillosa.Althoughthe crab Cancer magister s rarelyseenon theflat, t s an important redator n specieswithtubesthatopen onto the surface. A small C. magisterintroduced tself nto an experimentalage inSeptem-berafter hecage had been in positionone month atthe -1.2-ft station. When the cage was sampled inNovember,the abundances of tube-dwelling pecieshad been greatly educedwhilethe abundance of theburrowing pecies,Armandiabrevis,was no differentthan in the controlcaged areas withoutC. magister(Table 20). The presenceof C. magister f it were

TABLE 20. Infaunal abundances in 0.05 m2 caged areaswith and withoutCancer magister t -1.2-ft tidal eleva-tionafter4 monthsWith WithoutTube-buildingspecies:

Exogone lourei 140 597 531 737 949Lumbrineris nflata 43 113 64 132 168Axiothella rubrocincta 27 141 104 153 136Platynerei~sicanaliculata 2 19 54 25 47Leptochelia savignyi lII 2619 1736 2010 3163Burrowing pecies:Armandia brevis 112 129 139 160 143

common obviouslywould favor burrowing rganismsat the expenseof tube-building rganisms.This leadsto the prediction hat in areas where C. magister sabundant, the infaunalcommunity hould be domi-nated by burrowingorganismsrather than by thecompetitivelyuperior tube builders.

GENERAL DISCUSSIONThe abundancesofthenumericallymportant argepolychaete species (Lumbrineris inflata,Axiothellarubrocincta, latynereis icanaliculata, nd Armandiabrevis) were foundto be quitevariable (Fig. 5). Themajorityof the fluctuations ould be interpretedntermsof reproductive vents, uch as mortality fterspawning and differences in settlementsuccess.Abundance levels of these pecies and Exogone loureidid not correlate ignificantly ithpercent lgal cover

(Fig. 4) or withtemperaturesf the overlyingwateror thesediment urface Tables 4 and 8).Abundances of the four numerically importanttube-building olychaetes (Exogone lourei, Lumbri-neris nflata,Axiothella rubrocincta, nd Platynereisbicanaliculata) did correlate positively with oneanother (P < 0.01; Table 10). The abundance ofthenumericallymportantanaidLeptochelia savignyi,a tubebuilder,was positively orrelatedwiththreeofthese tube-building olychaetes. The abundances ofthenumerically mportant urrowing olychaete,Ar-mandia brevis,did not correlatewith those of thetube-buildingpecies (Table 10). If no other datawere available, a possible hypothesismightbe thatconditionsthecharacteristicsfwhich are unknown)that favor one tube-building pecies favor all suchspecies but do not affect burrowingspecies eitherpositively r negatively.A corollary s that the abun-dances of tube builders affect ne anotheronly posi-tively nd do not affect he abundances of burrowingspecies (Table 10). The way tube-building peciesaffectone another could be a synergistic hysicaleffect n thesediment or example. This impliesthatcompetition or imiting esources, uch as space andfood, is not important etweentube buildersor tubebuildersand burrowers.Observations on one tube-building pecies, Platy-


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186 SARAH ANN WOODIN Ecological MonographsVol. 44, No. 2nereisbicanaliculata,revealed the presence of strongintraspecificompetition or surfacespace in whichP. bicanaliculatafeeds (Table 18). In addition, ex-perimentalmanipulation f the abundances of threetube-building olychaetes Lunihibrinerisnflata,Axio-thella ubrocincta,nd P. bicanaliculata)demonstratedthat their total abundance negatively affected theabundanceof theburrowing pecies, Armandiabrevis(Tables 11 to 13). The expected result f correlationanalysis in light of these data would have been anegative orrelation f theseasonal abundancesof thetube-building pecieswith thatof theburrowing pe-cies. This result was not found (Table 10). Onepossibility s thattemporal vents probably primarilyreproductive)mask therelationship. An obvious im-plication s thatone should interpretuchcorrelationsvery cautiouslywhen no otherdata are available.The experimental esults Tables 1I to 1 ), labora-tory tudies Table 15) and abundance data (Tables14 and 18) demonstrate he presence of biologicalinteractions, ncluding nterspecific nd intraspecificcompetitionfor space. These data show that suchbiological interactionsre important eterminants fpolychaete infaunal species abundance patternsinsoft-sedimentnvironments.Additional cattered vi-dence ofbiological nteractions xists nthe iterature,such as sharp areal boundaries betweenspecies (forexample,Sanders et al. 1962), betweendeposit-feed-ing and suspension-feedingrganisms (Rhoads andYoung 1970) and the spacing of Tellina tenuis, abivalve, to avoid siphon overlap (Holme 1950).The significance f theexperimentalmanipulationsof naturalpopulations s that such evidence is muchstronger hancorrelative videnceand maycontradictthe facile conclusions of correlations. Further, heydemonstrate he population effectswhich observa-tions on small numbersof animals do not. Physicalfactors re also importantTables 9 and 16) butper-haps not as important s biological interactions.None of the species discussed are confinedto theintertidalHartman 1968 and Lie 1968). Thus, onewould expectto find uch interactions n deeper soft-sediment nvironmentss well. My observationsonP. bicanaliculata in subtidal areas, for example, re-vealed thesame intraspecificggressive ehaviorseenin the intertidal.Since the amplitudeof the fluctua-tions of physical factors s greater n the intertidalthan ntheborderingubtidal,one would predict hatbiological nteractions ould be even more importantin the subtidal.In 1968 and 1969 Sanders proposed the stability-timehypothesis o explain patterns f diversityn thesoft-sedimentmarine benthos. He demonstratedpattern f diversity or infaunalpolychaetesand bi-valves fromsoft-sediment nvironmentswith borealshallowwater and estuarineassemblageshaving low

diversity alues, and tropical hallowwater and deep-sea assemblageshavinghigh diversity alues (Sanders1968). The formerassemblages were called physi-cally controlled nd the atter biologically ccommo-dated." If the polychaete and bivalvedata presentedin Table 5 are analyzedaccordingto Sanders' rarefac-tion technique Sanders 1968, Fager 1972 contains acritical evaluation of this method), the values fallwithin hose given by Sanders (1968 and 1969) forboreal shallow water assemblages i.e., physicallycontrolled communities. Physical factorswere dem-onstrated o have some impact on population levels(Table 16), but the experimental ata (Tables I I to13 and 15) and abundance data (Tables 14 and 18)presentednthiswork demonstrate heimportance fbiological interactionso the determinationf speciesabundance patternswithin one habitat,an intertidalsoft-sediment arine environment.

ACKNOWLEDGMENTSI wishto thankmy fellowgraduate tudents nd thefaculty f the Department f Zoology, UniversityfWashington,ormany timulatingiscussions hich on-tributed o the development f my ideas. Particularthanks re due to Alan J. Kohn whose encouragement,constructiveriticismnd moral supportwere ever pres-ent, o Robert . Painewhospentmanyhoursdiscussingtheproblemwith me and to Karl Banse who read mydissertation. he facilitiesfthe Universityf Washing-tonFridayHarbor Laboratorieswere kindlymadeavail-able by the Director,Robert L. Fernald. Carl F. Ny-bladeand PamelaRoe helped n collection f fielddata.

Karl Banse and FredericH. Nichols aided in polychaeteidentification.inally, hanks reduetomyparentswhoprovided rout ishing nd refuge orthe weary,withoutwhich this work wouldnot have been completed.Thisworkwas supportedn partby an NDEA fellowship,nNSF predoctoral ellowship,nd the UniversityfWash-ingtonNSF GrantNo. GB-20978.LITERATURE CITED

Berkeley,E., and C. Berkeley. 1948. Annelida, Poly-chaeta Errantia. 100 p. In Can. Pac. Fauna 9b(1).Fish. Res. Board Can., Toronto.Berkeley, E., and C. Berkeley. 1952. Annelida, Poly-chaeta Sedentaria. 139 p. In Can. Pac. Fauna 9b(2).Fish. Res. Board Can., Toronto.Brafield, A. E. 1964. The oxygen content of intersti-tial water in sandy shores. J. Anim. Ecol. 33:97-115.Coast and Geodetic Survey. 1968, 1969, 1970, 1971, and1972. Tide tables, west coast, North and South Amer-ica (including Hawaiian Islands), U. S. Gov. PrintingOffice, Wash., D.C.Connell, J. H. 1961. Effects of competition,predationby Thais lapillus and other factorson natural popula-tionsofthe barnacle Balanus balanoides. Ecol. Monogr.31:61-104.Day, J. H. 1967. A monograph on the polychaeta ofSouthern Africa. Part 1. Errantia. Trustees of theBritishMuseum (Natural History), London. 458 p.

Dayton, P. K. D. 1971. Competition, disturbance, andcommunityorganization: the provision and subsequentutilization of space in a rocky intertidalcommunity.Ecol. Monogr. 41:351-389.


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Spring 1974 POLYCHAETE ABUNDANCE PATTERNS 187Fager, E. W. 1972. Diversity: a sampling study. Am.Nat. 106:293-3 10.Gray, J. S. 1966. The attractivefactor of intertidalsands to Protodrilussymbioticus. J. Mar. Biol. Assoc.U. K. 46:627-645.Hartman, 0. 1968. Atlas of the errantiate polychae-tous annelids from California. Univ. Southern Cali-forniaPress, Los Angeles. 828 p.1969. Atlas of the sedentariata polychaetousannelids from California. Univ. Southern CaliforniaPress, Los Angeles. 812 p.Hermans, C. 0. 1964. The reproductiveand develop-mental biology of the opheliid polychaete, Armandiabrevis (Moore). Masters Thesis. Univ. Washington,Seattle. 131 p.1966. The natural history nd larval anatomyof Armandia brevis (Polychaeta: Opheliidae). Ph.D.Thesis. Univ. Washington,Seattle. 175 p.Holme, N. A. 1950. Population dispersion in Tellinatenuis DaCosta. J. Mar. Biol. Assoc. U. K. 29:267-280.Hughes, R. N. 1969. A studyof feeding in Scrobicu-

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Nichols, F. H. 1970. Benthic polychaete assemblagesand their relationship o the sediment n Port Madison,Washington. Mar. Biol. 6:48-57.Ockelmann, K. W., and 0. Vahl. 1970. On the biologyof the polychaeteGlycera alba, especially its burrowingand feeding. Ophelia 8:275-294.Paine, R. T. 1966. Food web complexityand speciesdiversity.Am. Nat. 100:65-75.Pamatmat, M. M. 1968. Ecology and metabolismof abenthic community on an intertidal sandflat. Int.Revue Ges. Hydrobiol. 53:211-298.Pettibone,M. H. 1953. Some scale-bearingpolychaetesof Puget Sound and adjacent waters. Univ. Washing-ton Press, Seattle. 89 p.Reish, D. J., and M. C. Alosi. 1968. Aggressive be-havior in the polychaetous annelid family Nereidae.Bull. South. Calif. Acad. Sci. 67:21-28.Rhoads, D. C., and D. K. Young. 1970. The influenceof deposit-feeding enthoson bottom sedimentstabilityand community trophic structure. J. Mar. Res. 28:150-178.Roe, P. 1971. Life historyand predator-prey nterac-tions of the nemertean Paranemertes peregrina Coe.Ph.D. Thesis. Univ. Washington,Seattle,Wash. 129 p.Sanders, H. L. 1968. Marine benthic diversity: acomparative study. Am. Nat. 102:243-282.1969. Benthic marine diversityand the sta-bility-time ypothesis,p. 71-81. In Diversity and sta-bility in ecological systems. Brookhaven Symposia inBiology no. 22.Sanders, H. L., E. M. Goudsmit, E. L. Mills, and G. R.Hampson. 1962. A study of the intertidal fauna ofBarnstable Harbor, Massachusetts. Limnol. Oceanogr.7:63-79.Sanders,H. L., P. C. Mangelsdorf,Jr., nd G. R. Hamp-son. 1965. Salinity and faunal distributionin thePocasset River, Massachusetts. Limnol. Oceanoogr.10 (Supplement):R216-R229.Thorson, G. 1957. Bottom communities (sublittoralor shallow shelf), p. 461-534. In J.W. Hedgpeth [ed.],Treatise on marine ecology and paleoecology. Vol. 1.Geol. Soc. Am. Mem. 67.Trevallion, A., R. R. C. Edwards, and J.H. Steele. 1970.Dynamics of a benthic bivalve, p. 285-295. In J. H.Steele [ed.], Marine Food Chains. Univ. CaliforniaPress,Berkeleyand Los Angeles.Ushakov, P. V. 1955. Polychaeta of the far easternseas of the U. S. S. R. Keys to the Fauna of theU. S. S. R. no. 56. Acad. Sci. U. S. S. R. [transl.fromRussian by Israel Prog. for Sci. Translations, 1965].

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Woodin .1974.Polychaete Abundance Patterns in a Marine Soft-Sediment Environment - The Importance of Biological Interactions