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11 S. Fish and Wildlife Service Biological Report 82 (11.53) National Wetlands Research Center TR EL-82-4 June 1986 700 Cajun Dome Boulevard
Lafcryette, Louisiana 70506
Species Profiles: Life Histories and Environmental Requirements of Coastal Fishes and Invertebrates (North Atlantic)
SOFTSHELL CLAM
Coastal Ecology Group Fish and Wildlife Service Waterways ~xperiment Station U.S. Department of the Interior U.S. Army Corps of Engineers
B i o l o g i c a l R e p o r t 82(11.53) TR EL-82-4 June 1986
Spec ies P r o f i l e s : L i f e H i s t o r i e s and E n v i r o n m e n t a l Requi rements of C o a s t a l F i s h and I n v e r t e b r a t e s ( N o r t h A t l a n t i c )
SOFTSHELL CLAM
C a r t e r R . Newel1 Maine She1 l f i s h Research and Development
Damar i sco t ta , ME 04543
and H e r b e r t H i d u
Depar tment o f A n i ma1 and V e t e r i n a r y Sc iences U n i v e r s i t y o f Maine
Orono, ME 04469
P r o j e c t O f f i c e r John Parsons
N a t i o n a l C o a s t a l Ecosystems Team U.S. F i s h and W i l d l i f e S e r v i c e
1010 Gause B o u l e v a r d S l i d e l l , LA 70458
Per formed f o r C o a s t a l Eco logy Group
Waterways Exper imen t S t a t i o n U.S. Army Corps o f Eng inee rs
V icksburg , MS 39180
and
N a t i o n a l .Coastal Ecosystems Team Research and Development F i s h and W i l d l i f e S e r v i c e
U.S. Department o f t h e I n t e r i o r Washington, DC 20240
Th is s e r i e s should be referenced as fo l lows:
U.S. F i s h and W i l d l i f e Serv ice. 1983-19 . Species p r o f i l e s : l i f e h i s t o r i e s and environmental requirements o f c o a x a l f i s h e s and i nve r teb ra tes . U. S. F i s h W i l d l . Serv. B i o l . Rep. 82(11). U. S. Army Corps o f Engineers, TR EL-82-4.
Th is p r o f i l e should be c i t e d as fo l lows:
Newell, C.R., and H. Hidu. 1986. Species p r o f i l e s : l i f e h i s t o r i e s and env i ronmental requ i rements of coas ta l f i shes and i nve r teb ra tes (Nor th A t l a n t i c ) -- s o f t s h e l l clam. U.S. F i s h W i l d l . Serv. B i o l . Rep. 82(11.53). U.S. Army Corps of Engineers, TR EL-82-4. 17 pp.
PREFACE
This species p r o f i l e i s one o f a se r i es on coasta l aquat ic organisms, p r i n c i p a l l y f i s h , o f spor t , commercial, o r eco log ica l importance. The p r o f i l e s are designed t o prov ide coasta l managers, engineers, and b i o l o g i s t s w i t h a b r i e f comprehensive sketch o f t h e b i o l o g i c a l c h a r a c t e r i s t i c s and environmental requirements o f t h e species and t o descr ibe how popu la t ions o f t h e species may be expected t o r e a c t t o environmental changes caused by coasta l development. Each p r o f i l e has sec t ions on taxonomy, 1 i f e h i s t o r y , eco log ica l r o l e , environmental requirements, and economic importance, i f appl i cab le . A t h r e e - r i ng b inde r i s used f o r t h i s se r i es so t h a t new p r o f i l e s can be added as they are prepared. Th is p r o j e c t i s j o i n t l y planned and f inanced by the U.S. Army Corps o f Engineers and t h e U.S. F i sh and W i l d l i f e Service.
Suggestions o r quest ions regard ing t h i s r e p o r t should be d i r e c t e d t o one of t he f o l l owing addresses.
I n fo rma t ion Trans fer Special i s t Nat ional Coastal Ecosystems Team U.S. F i sh and W i l d l i f e Serv ice NASA-Slidell Computer Complex 1010 Gause Boulevard S l i d e l l , LA 70458
U. S. Army Engineer Waterways Experiment S t a t i o n A t ten t i on : WESER-C Post O f f i c e Box 631 Vicksburg, MS 39180
CONVERSION TABLE
M e t r i c t o U.S. Custanary
m i l l i m e t e r s (mn) c e n t i m e t e r s (n) mete rs (m) k i 1 ometers ( km)
2 square m e t e r s (m ) 10.76 square k i 1 ometers ( km2) 0.3861 h e c t a r e s (ha ) 2.471
l i t e r s ( 1 ) c u b i c me te rs (m3) c u b i c lneters
To O b t a i n
i nches inches f e e t m i l es
square f e e t square ' n i l es acres
ga l 1 ons c u b i c f e e t a c r e - f e e t
m i l l i g r a m s (mg) 0.00003527 ounces grams ( g ) 0.03527 ounces k i l o g r a m s ( k g ) 2.205 pounds m e t r i c tons ( t ) 2205.0 pounds m e t r i c tons 1.102 s h o r t t o n s k i 1 oca l o r i e s ( kca l ) 3.968 B r i t i s h thermal u n i t s
Ce ls ius degrees 1.8("C) + 32 Fahrenhe i t degrees
U.S. Customary t o M e t r i c
i nches 25.40 inches 2.54 f e e t ( f t ) 0.3048 fathoms 1.829 m i l e s ( m i ) 1.609 n a u t i c a l m i l e s ( m i ) 1.852
square f e e t ( f t 2 ) acres 2 square m i l e s (mi )
g a l l o n s ( g a l ) 3.785 c u b i c f e e t ( f t 3 ) 0.02831 acre- f e e t 1233.0
ounces ( o z ) 28.35 pounds ( l b ) 0.4536 s h o r t t o n s ( t o n ) 0.9072 B r i t i s h thermal u n i t s ( B t u ) 0.2520
m i l 1 i m e t e r s c e n t i m e t e r s me te rs me te rs k i l o m e t e r s k i 1 o n e t e r s
square meters hec ta res square k i l o m e t e r s
1 i t e r s c u b i c me te rs c u b i c meters
grams k i 1 ograms m e t r i c tons k i 1 oca l o r i es
Fahrenhe i t degrees 0.5556("F - 32) C e l s i u s degrees
Page
. . . . . . . . . . . . . . . . . . . . . . . . PREFACE i i i CONVERSION TABLE . . . . . . . . . . . . . . . . . . . i v . . . . . . . . . . . . . . . . . . . . ACKNOWLEDGMENTS v i
NCIMENCLATLIRE /TAXONOMY /RANGE . . . . . . . . . . . . . . 1 MORPHOLOGY/ IDENTIFICATION AIDS . . . 1 REASON FOR INCLUSION I N SERIES . . . . . . . . . . . . 3 LIFE HISTORY . . . . . . . . . . . . . . . . . . . . . 3
Spawning . . . . . . . . . . . . . . . . . . . . . . 3 . . . . . . . . . . . . . . . . Fecund i ty and Gametes 4 Larvae . . . . . . . . . . . . . . . . . . . . . . . 4 . . . . . . . . . . . . . . . . . Juveni 1 e Seed Clams 5 A d u l t Clams . . . . . . . . . . . . . . . . . . . . . 5 . . . . . . . . . . . . . . COMMERCIAL/SPORT FISHERIES 6
POPULATION DYNAMICS . . . . . . . . . . . . . . . . . . 6 GROWTH CHARACTERISTICS . . . . . . . . . . . . . . . . 7 ECCILOGICAL ROLE . . . . . . . . . . . . . . . . . . . . 8
Food and Feeding Hab i t s . . . . . . . . . . . . * a . 8 'P reda to rs . . . . . . . . . . . . . . . . . . . . . . 8
ENVIRONMENTAL REQUIREMENTS . . . . . . . . . . . . . . 9 S a l i n i t y . . . . . . . . . . . . . . . . . . . . . . 9 Temperature . . . . . . . . . . . . . . . . . . . . . 9 Oxygen . . . . . . . . . . . . . . . . . . . . . . . 10 Subs t ra te and Cur ren t . . . . . . . . . . . . . . . . 10 P o l l u t i o n . . . . . . . . . . . . . . . . . . . . . . 11
. . . . . . . . . . . . . . . . . . . LITERATURE CITErl 13
ACKNOWLEDGMENTS
Dana W a l l a c e and W a l t e r F o s t e r , Ma ine Depa r tmen t o f M a r i n e Resou rces , Augusta, Maine, and John Mor i ng, Maine Coope ra t i ve F i s h e r y Research U n i t , U n i v e r s i t y o f Maine, k i n d l y rev iewed t h e manuscr ip t .
F i g u r e 1. The s o f t s h e l l c l a m ( f i g u r e c o u r t e s y of t h e Maine Sea G r a n t Program).
SOFTSHELL CLAM
NOMENCLATURE/TAXONOMY/RANGE MORPHOLOGY/IDENTIFICATION AIDS
S c i e n t i f i c name ..... Mya a r e n a r i a L. ... P r e f e r r e d common names s o f t s h e l l
c lam, s teamer c lam, nannynose ( F i g u r e 1 ) .
O t h e r common names .... sand gaper, long-necked c lam
C l a s s ........................ R i v a l v i a (Pe lecypoda)
Orde r ............... E u l a m e l l i b r a n c h i a Suborder.................. H e t e r o d o n t a F a m i l y ......................... My idae
Geograph ica l Range: I n t e r t i d a l and s u b t i d a l t o depths o f 100-199 m a l o n g t h e A t l a n t i c c o a s t f rom L a b r a d o r t o S o u t h C a r o l i n a a n d e x t e n d i n g , i n l e s s e r abundance, s o u t h t o F l o r i d a ; t h r o u g h o u t w e s t e r n Europe; s u c c e s s f u l l y i n t r o d u c e d i n P a c i f i c c o a s t a l w a t e r s o f A l a s k a and C a l i f o r n i a (Theroux and Wig ley 1983) . The s o f t s h e l l c l a m i s most abundant i n t e r t i d a l l y a l o n g t h e New E n g l a n d c o a s t ( ~ i gu re ' 2 ) and s u b t i d a l l y i n Chesapeake Bay.
The s o f t s h e l l c l a m sometimes exceeds 10 cm i n l e n g t h ( a l l l e n g t h s i n t h i s r e p o r t a r e s h e l 1 l e n g t h s ) . The s h a p e o f t h e s h e l 1 i s e l o n g a t e a n d e l l i p t i c a l . The c l a m h a s a l a r g e s i p h o n a l gape on t h e s l i g h t l y p o i n t e d and e l o n g a t e p o s t e r i o r end, and t h e r e i s a s m a l l p e d a l gape on t h e a n t e r i o r end ( S t a n l e y 1970) . The s h e l l e x t e r - i o r has numerous g rowth l i n e s ; each u s u a l l y r e p r e s e n t s 1 y e a r ' s g rowth o r an e n v i ronmenta l d i s t u r b a n c e ( S c h u s t e r 1951 ) . On l i v e specimens, t h e e x t e r i o r of t h e s h e l l i s rugose and i s cove red w i t h a p r o t e i n l a y e r c a l l e d t h e p e r i o s t r a c u m . Empty s h e l 1s t u r n c h a l k w h i t e a f t e r t h e p e r i o s t r a c u m erodes away. On l a r g e 1 i v e specimens, t h e l o n g c o n t r a c t i l e s i p h o n may e x t e n d as f a r as 20 cm t o reach t h e sed iment su r face . The f o o t i s s m a l l and muscu la r and t h e m a n t l e l o b e s a r e fused e x c e p t a t t h e p e d a l gape and a t t h e ends of t h e s i p h o n tubes . The end o f t h e i n c u r r e n t s i p h o n has a r i n g of t e n t a c l e s . The a d d u c t o r muscles a r e unequal i n s i z e ( a n i s o m y a r i a n ) .
T h e c o m p a c t h i n g e 1 i gament i s a t t a c h e d t o t h e r i g h t v a l v e a n d i s
A TL A N TIC OCEAN
Figure 2. D i s t r i b u t i o n o f the s o f t s h e l l clam i n the North A t l a n t i c Bight.
C
enclosed by t h e chondrophore o f t h e l e f t v a l v e ( F i g u r e 1 ) . The va lves a r e t h i n , and mean r a t i o s o f l e n g t h ( L ) , w i d t h (W), and h e i g h t (H) are: L/H- 1.65, H/W-1 (S tan l ey 1970). Newel 1 and Hidu (1982) observed mean L/W r a t i o s o f 2.6 i n c lams f r o m g r a v e l beds, and 3.2 i n clams f r o m sand beds.
REASON FOR INCLUSION I N SERIES
The s o f t s h e l l c lam i s a dominant member of many e s t u a r i n e so f t - bo t t om communities and i s commercial l y impo r t an t a long much of t h e No r t h A t l a n t i c coast . I n New England, i t i s t h e second most impo r t an t commercial c lam a f t e r t h e ha rd c lam (Mercenar ia mercenar ia) . I n 1978, U.S. land ings y i e l d e d a b o u t 4.6 m i l l i o n k g (10 .1 m i l l i o n 1b) o f meats ( P i l e g g i and Thompson 1979). I t s i n sho re d i s t r i b u t i o n , however, makes i t vu lne rab le t o con tamina t ion by mun i c i - p a l sewage, i n d u s t r i a l e f f l u e n t , and coas ta l c o n s t r u c t i o n p r o j e c t s . A lso , t h e a d u l t s u s u a l l y l i v e i n permanent burrows, s o excess ive s i l t a t i o n can s u f f o c a t e them. The c o n s t r u c t i o n of p i e r s and j e t t i e s t h a t a l t e r c lam h a b i t a t i s l i k e l y t o have l ong - t e rm adverse e f f e c t s on c lam popu la t i ons .
LIFE HISTORY
The s o f t s h e l l c lam has seven l i f e h i s t o r y stages (Table 1 ) .
Spawning
Sexual m a t u r a t i o n of t h e s o f t - s h e l l c lam depends upon t h e s i z e of t h ~ c lam r a t h e r t han i t s age. Clams l o n g e r t h a n 20 mm i n s h e l l l e n g t h a r e u s u a l l y capable of spawning (Coe and Turner 1938). Clams f rom c o l d Maine w a t e r s may spawn a t a s m a l l e r s i z e t han those f rom Massachusetts o r Long I s l a n d Sound. Sexes a r e separate, t h e sex r a t i o i s 50:50, and males a r e i n - d i s t i n g u i s h a b l e from females un less a gonad smear i s examined under a m ic ro - scope. A low i n c i d e n c e of hermapro- d i t e s i s common i n most popu la t i ons . The seasonal development of t h e gonads was summarized f o r t h e n o r t h A t l a n t i c coas t by Ropes and S t i c kney (1965). Gametogenesis begins i n l a t e w i n t e r and e a r l y sp r i ng , and spawning peaks f r o m June t o September, depending upon l o c a t i o n .
Clams u s u a l l y spawn once a y e a r n o r t h o f Cape Cod, and t w i c e a y e a r sou th of Cape Cod. The d i f f e rences i n
Tab le 1. The l i f e s tages and c h a r a c t e r i s t i c s o f t h e s o f t s h e l l
clam.
--- Ase o r
-- Stage c h a r a c t e r i s t i c
F e r t i 1 i zed egg Trochophore Vel i ger
Prodissoconch I Prodissoconch I 1
Spat (Dissoconch)
0-12 hours o l d . 13-24 hours 01 d, top-shaped. 10-20 days o l d , has a she l 1 and swims by u s i n g a velum. Has s t r a i g h t - h i n g e d she l 1. Has umbo on she l 1. Bottom-dwel 1 i ng, a f t e r se t t l emen t and velum c a s t o f f .
Juven i 1 e Up t o 20 rrm s h e l l l eng th . A d u l t - Sexual l y mature.
t h e spawning h a b i t s demons t ra te t h e i m p o r t a n c e o f w a t e r t e m p e r a t u r e on gonad m a t u r a t i o n and s p a w n i n g . I n M a r y l a n d , t h e c l a m s spawn i n A p r i l when w a t e r t empera tu res reach 10 "C and t h e abundance of l a r v a e peaks when t h e s p r i n g w a t e r t empera tu res a r e abou t 20 "C. Gametogenesis f o r t h e second spawning began when w a t e r t e m p e r a t u r e s dropped be low 25 O C ,
a c c o r d i n g t o P f i tzenmeyer (1965) . Brousseau (1978a) obse rved an e a r l y spawn (March-Apr i 1 ) i n clams f r o m I p s w i c h Ray, n e a r G l o u c e s t e r , Massa- c h u s e t t s , c h a r a c t e r i z e d by r a p i d m a t u r a t i o n and gamete r e 1 ease. The c lams spawned a g a i n i n June and J u l y , and t h e number o f eggs and sperm r e l e a s e d i n t h e summer was g r e a t e r t h a n i n t h e s p r i n g .
Food a v a i l a b i l i t y a l s o i s an i m p o r t a n t f a c t o r i n d e t e r m i n i n g t h e t i m i n g and i n t e n s i t y o f spawning. I n s h e l l f i s h h a t c h e r i e s , c lams a r e ex- posed t o s l i g h t l y e l e v a t e d tempera- t u r e s and o p t i m a l a l g a l d i e t s t o s t i m u l a t e r a p i d gametogenesis and egg " r i p e n e s s . " Exposure o f b roods tock t o p e r i o d s o f a l t e r n a t i n g warm and c o l d tempera tu res s t i m u l a t e s spawning ( L o o s a n o f f a n d D a v i s 1 9 6 3 ) . On a Ma ine mud f l a t , spawning was observed when i n s h o r e seawa te r t empera tu res reached a seasonal maximum o f 2 1 "C (Newel1 1982) . D u r i n g t h e l a s t week o f June, w a t e r t empera tu res f l u c t u a t e d between 13 "C ( a t h i g h t i d e ) and 2 1 "C ( o n a sunny day i n s h a l l o w w a t e r a t l o w t i d e ) . Males u s u a l l y spawn f i r s t , r e l e a s i n g pheromone and sperm i n t o t h e w a t e r , wh ich cause t h e females t o spawn.
Fecund i t y and Gametes
I n Massachuset ts , a f e m a l e c l a m 63 mm l o n g i s capab le o f r e l e a s i n g as many as 3 m i l 1 i o n eggs a y e a r ( B e l d i n g 1930) , b u t a c c o r d i n g t o Brousseau (1978a) , f e m a l e c lams f r o m t h e m i d d l e i n t e r t i d a l zone a t Cape Ann, Massachuse t t s , p r o d u c e o n l y 120,000 eggs a y e a r . I n an exper imen t i n a
s h e l l f i s h h a t c h e r y i n Maine, females 60-70 mm l o n g r e l e a s e d about 1 m i l l i o n eggs ( S . Chapman, I r a D a r l i n g C e n t e r , U n i v e r s i t y o f Maine, Walpole, p e r s . comm.) . F e r t i 1 i z a t i o n i s e x t e r n a l . The egg, when r e l e a s e d i n t o seawater , i s s p h e r i c a l , 66 m ic rons i n d iamete r , w h i t e , and g e l a t i n o u s ( B e l d i n g 1930) . The sperm has a head wh ich measures 4 m ic rons i n d i a m e t e r and a l o n g w h i p l i k e t a i l . The f e r t i l i z e d eggs may b e c a r r i e d b y t h e c u r r e n t many m i l e s f r o m t h e s p a w n i n g s i t e . The p e r c e n t a g e o f ene rgy t h a t goes i n t o t h e r e p r o d u c t i ve p rocess i n c r e a s e s w i t h an - i n c r e a s e i n t h e s i z e o f t h e clam. The o l d e s t and l a r g e s t i n d i v i d u a l s i n t h e p o p u l a t i o n c o n t r i b u t e most o f t h e eggs.
L a r v a e
The l a r v a l s t a g e l a s t s f r o m 12 t o 14 days i n s o u t h e r n Massachuset ts , about 14 days i n Chesapeake Bay, and up t o 21 days i n Maine ( B e l d i n g 1930; Pf i t z e n m e y e r 1972) . A f t e r h a t c h i n g ( i n 9 h a t 27 OC) t h e embryo q u i c k l y changes i n t o a top-shaped, s p i n n i n g t rochophore (Be1 d i ng 1930; Hanks 1 9 6 9 ) . Soon, t h e l a r v a f o r m s i t s f i r s t s h e l 1 and a swimming o rgan ( t h e velum) and becomes a p r o d i s s o c o n c h I ( o r e a r l y v e l i g e r s t a g e ) . It i s about 80 m ic rons l ong . When g rowth beg ins a t t h e s h e l l p e r i p h e r y , t h e p r o d i s s o c o n c h I I ( o r 1 a t e v e l i g e r ) l a r v a l s t a g e beg ins . Loosano f f and D a v i s (1963) and L o o s a n o f f e t a l . (1966) g i v e measurements and p i c t u r e s of development o f t h e s o f t s h e l l c l am up u n t i 1 s e t t l e m e n t . The l a r v a e may be i d e n t i f i e d by h i n g e s t r u c t u r e ( L u t z e t a1 . 1982) . A l o n g t h e New Hampshi r e c o a s t f r o m June t o Oc tober , t h e number o f v e l i g e r s i n t h e l a n k t o n was as h i g h as 1,00O/m (Normandeau A s s o c i a t e s I n c . 1978) . D e n s i t y peaked i n l a t e summer. L a r v a l abundance was h i g h e r i n i n s h o r e wa te rs t h a n o f f s h o r e wa te rs and h i g h e r a t dep ths f r o m 5 t o 9 m t h a n n e a r t h e s u r f a c e o r a t a dep th o f 13 m.
A t t h e end of t h e l a r v a l s tage , t h e c lam a t t a c h e s t o t h e bot tom. Then t h e v e l u m i s c a s t o f f a n d a f o o t deve lops as t h e c lam metamorphoses i n t o a b o t t o m - d w e l l e r c a l l e d a s p a t . Metamorphosis may be d e l a y e d l a t e r t h a n u s u a l , u n t i l t h e c lam l o c a t e s a s u i t a b l e s u b s t r a t e f o r a t tachmen t .
J u v e n i l e Seed Clams
A f t e r metamorphosis, t h e young s p a t (0.25 - 1 mm l o n g ) may undergo a f l o a t i n - c r a w l i n g s t a g e f o r 2 t o 5 weeks q s e l d i n g 1 9 3 0 ) . D u r i n g t h i s s t a g e t h e c lam s p a t sometimes h o l d s on t o t h e s u b s t r a t e u s i n g a b y s s a l t h r e a d . The s p a t may f i r s t a t t a c h t o e e l g rass , f i l a m e n t o u s a lgae, and o t h e r o b j e c t s i n t h e s u b t i d a l zone ( K e l l o g g 1900) , b u t as i t c o n t i n u e s t o grow t h e s p a t d rops t o t h e b o t t o m and bur rows i n t o t h e s e d i m e n t . A f t e r t h e c l a m becomes l a r g e enough t o be n o t i c e d by c l a m d i g g e r s ( 5 mm), i t i s r e f e r r e d t o as c l a m "seed" u n t i l i t reaches market s i z e . J u v e n i l e seed clams may mi g r a t e up t o s e v e r a l hundred y a r d s towards s h o r e (Dow a n d W a l l a c e 1 9 6 1 ) . T h e byssus i s used f o r a t tachmen t w h i l e j u v e n i l e clams up t o 13 mm' l o n g move o r bur row.
The movement of seed clams 2-15 mm l o n g on a Massachuset ts beach was s t u d i e d by M a t t h i e s s e n (1960b) . Clams 2-3 mm l o n g t h a t s e t i n l a t e summer remained i n t h e seaward p o r t i o n o f t h e s a m p l i n g a rea f o r 8-10 months ( u n t i l A p r i l , May, o r June) and grew t o 5 mm o r more, and t h e n moved s h o r e w a r d . T h i s movement was a t t r i b u t e d t o hydrodynamic f o r c e s such as sed iment s o r t i n g by s h o a l i n g waves . D u r i n g s p r i n g s torms, c lams 5-15 mm l o n g showed a n e t h o r i z o n t a l d i sp lacemen t shoreward a l o n g w i t h coa rse sed iment p a r t i c l e s ( M a t t h i e s s e n 1960b). The movement o f f i r s t - y e a r j u v e n i l e s peaked i n September and Oc tober d u r i n g e a r l y growth, and a g a i n i n May a f t e r growth resumed i n t h e s p r i n g (Dow and Wa l lace 1961).
Because a d u l t s o f t s h e l l clams a r e seden ta ry , main tenance of each p o p ~ ~ l a t i o n depends upon t h e s e t t l e m e n t o f s p a t o r t h e movement o f j u v e n i l e seed c lams. Because of t h e i n f l u e n c e o f oceanograph ic c o n d i t i o n s on r e - c r u i t m e n t , t h e abundance of s e t t l i n g clams ( t h e s e t ) may be i r r e g u l a r i n some l o c a t i o n s ; f o r example, enormous q u a n t i t i e s o f seed clams may be c o n c e n t r a t e d i n s m a l l a rea. A c l a m 4 s e t o f 538,00O/m was r e p o r t e d i n an eddy a d j a c e n t t o a sand b a r i n Plum I s l a n d Sound, Massachuse t t s ( B e l d i ng 1 9 3 0 ) . The d e n s i t y o f s e e d c l a m s u s u a l l y i s g r e a t e s t i n edd ies , a l o n g t h e s i d e s of sand b a r s o r i s l a n d s , a t t h e mouths o f r i v e r s o r Streams e m p t y i n g i n t o s h a l l o w w a t e r , o r i n s l a c k w a t e r a d j a c e n t t o a s w i f t c u r r e n t . Because o f t h e i r s m a l l s i z e and s h a l l o w b u r r o w i n g dep th , j u v e n i l e c lams a r e s u b j e c t t o i n t e n s e p r e - d a t i o n . The d e n s i t of j u v e n i l e c lams approached 6.000/$ i n t h e s u b t i d a l zone i n V i r g i n i a , b u t dropped t o z e r o 1 month l a t e r , p resumably due t o p r e d a t i o n (Lucy 1976); however, c lams i n Chesapeake Bay were a b l e t o a v o i d most p r e d a t o r s by b u r r o w i n g as deep as 10 cm (B lundon and Kennedy 1982) . V e g e t a t i o n reduced p r e d a t i o n of s o f t s h e l l clams i n t h e s u b t i d a l zone. The movement of j u v e n i l e s f rom s u b t i d a l a reas t o t h e i n t e r t i d a l zone i n New Eng land a l s o reduces p r e d a t i o n . F o r example, e x p e r i m e n t a l p l a n t i ngs o f seed clams a t d i f f e r e n t i n t e r t i d a l e l e v a t i o n s a l o n g t h e Ma ine c o a s t r e v e a l e d t h a t growth i s s l o w e s t b u t s u r v i v a l i s t h e g r e a t e s t i n t h e u p p e r i n t e r t i d a l zone (Newel 1 1982) .
A d u l t Clams
I n s u i t a b l e h a b i t a t , t h e s o f t s h e l l c l am makes a s u b s t a n t i a1 c o n t r i b u t i o n t o t h e b i o m a s s o f t h e b e n t h i c community. F o r example, 1 ha of m u d f l a t c o n t a i n i n g c lams ab2ut 62 mm l o n g a t a d e n s i t y o f 269/m con- t a i n s 1,442 bushe ls o f c lams i n t h e
s h e l l , and 21,635 l b o f meats (D. Wal lace, Maine Dep. Mar ine Resources, Augusta, pers. corn.).
COMMERC IAL/SPORT FISHERIES
The s o f t s h e l l clam i s a va luab le s p o r t and commercial species. The s p o r t f i s h e r y i s l o c a l l y impor tan t t o coas ta l r e s o r t towns, where clam ordinances s t r i c t l y r e g u l a t e t h e c a t c h . I n New Eng land , where t h e resource i s p r i m a r i l y i n t e r t i d a l , t h e commercial ins t ruments a r e gene ra l l y cons t ra ined by law t o hand implements. Clam f o r k s and hoes a re common inst ruments, a1 though dredges a re sometimes used i n s u b t i d a l areas i n Massachusetts and exper imenta l l y i n Ma ine . I n Chesapeake and De laware Bays, where t h e resource i s s u b t i d a l , clams a re harves ted w i t h a h y d r a u l i c e s c a l a t o r dredge. Damage t o clams i s less f rom dredges than f rom f o r k s and hoes (Ky te and Chew 1975). B u r i a l and breakage o f c lams d u r i n g hand h a r - ves t i ng sometimes reduces p roduc t i on i n New England (Glude 1954). Hydrau- l i c clam rakes have been used i n Cana- da (Bourne 1967) and Maine t o c o l l e c t seed clams f o r t r a n s p l a n t i n g . Annual U.S. commercial land ings o f s o f t s h e l l clams f r om 1977 t o 1981 averaged 4.2 m i l l i o n k g (9.3 m i l l i o n l b ) , wor th $15 m i l l i o n . A t one t ime, sewage p o l l u t e d many of t h e clam beds a long t h e New England coast, so they were c losed t o d iggers and dredgers. I n t h e 19701s, t h e c o n s t r u c t i o n o f mun ic ipa l sewage t rea tment p l a n t s sha rp l y increased and p o l l u t i o n decreased, so some o f t h e beds have been reopened and p r o d u c t i on has p r o p o r t i o n a t e l y increased.
POPULATION DYNAMICS
Because t h e m o r t a l i t y o f eggs, la rvae , and seed clams i s ext remely h igh , clam popu la t ions a re mainta ined on ly because o f a tremendous fecun-
d i t y . Less t h a n 0.1% o f t h e eggs produced i n a spawning season r e s u l t i n a successfu l se t t lement , and about 1% o f t h e s e t t l e d spa t must mature and reproduce i n o rde r t o s u s t a i n t h e popu la t ions . M o r t a l i t y i s heavy i n t h e p l a n k t o n i c s tage and immediate ly a f t e r se t t lement , and decreases as t h e clam grows o lder . As t h e s h e l l becomes t h i c k e r and t h e clam d igs deeper, i t s s u r v i va l r a t e increases. S u r v i v a l r a t e f o l l o w s an exponent ia l decay, l e v e l i n g o f f a f t e r 3 years o f age. M o r t a l i t y r a t e s a r e h i ghes t i n summer when p reda to rs a r e most abundant (Brousseau 1978b). Fecund i ty increases w i t h clam s i z e , so t h e i n t r i n s i c r a t e o f n a t u r a l inc rease i s h igh. The h i g h m o r t a l i t y o f l a r v a e i s o f f s e t by t h e h i g h i n t r i n s i c r a t e o f n a t u r a l inc rease (Brousseau 1978b).
D e n s i t i e s o f l a r v a e ranged from 0 . 1 t o l,000/m3 i n New Hampshire waters o f t h e G u l f o f Maine (Normandeau Associates, I nc . 1978) and t h e l a t e v e l i g e r l a r v a l d e n s i t i e s ranged f rom 0 t o 1,400/m3 i n Chesapeake Bay ( P f i tzenmeyer 1962) d u r i n g t h e summer months. I n one s tudy , se t t l emen t d e n s i t i e s as h i g h as 107, 600/m2 (10,00O/f t2), decreased t o 21,500/m2 (2,000/ f t2) 2 months l a t e r , and then t o 0 a f t e r 1 y e a r (Turner 1953).
I n Chesapeake Bay, spa t ( l e s s than 10 mm long) d e n s i t i e s decreased from 500/m2 i n December t o l e s s than 10/m2 by June (Blundon and Kennedy 1982). I n New Hampshire, d e n s i t i e s o f young-of- the-year spa t ranged f rom 21/m2 t o 8,200/m2 f rom 1971 t o 1980. High l a r v a l d e n s i t i e s (530/m3 i n t h e summer o f 1975) were f o l l o w e d by h i g h spa t d e n s i t i e s (8,200/m2) i n 1976; a d u l t s o f t h e 1975 yea r c l a s s produced a s t rong f i s h e r y f rom 1978 t o 1980 (Savage 1981).
I n Casco Bay, Maine, s tand ing crops i n 1979 averaged 90 t o 120 bushels of clams p e r acre. The t o t a l i nven to ry o f t h e bay was 107,500 bushels (Card 1980).
A d u l t s have an aggregated d i s t r i - bu t i on , l i m i t e d p r i m a r i l y t o i n t e r - t i d a l and sha l l ow s u b t i d a l areas ( S a i l a and Gaucher 1966; Commito 1982). The degree of aggrega t ion may depend upon t h e s l o p e of t h e i n t e r - t i d a l area and c u r r e n t (Newcombe 1936). Juveni l e s may concen t ra te near a s teep shore p r o f i l e (Mat th iessen 1960b). Aggrega t ion a l s o may be caused by p reda t i on , by hand h a r v e s t i n g o r d r e d g i n g , and by t h e c o n c e n t r a t i o n o f spa t by hydrograph ic c o n d i t i o n s . GROWTH CHARACTERISTICS
The s o f t s h e l l c lam grows r a p i d l y i n a f a v o r a b l e e n v i r o n m e n t . Clams u s u a l l y reach market s i z e ( 2 inches l ong ) i n 1.5 yea rs i n Chesapeake Bay, (P f i t zenmeyer 1972), i n 2 t o 3 years i n Rhode I s l a n d and Massachusetts (Turner 1948; Brousseau 1979), i n 3 t o 6 yea rs i n Maine, and i n 5 years i n New Brunswick , Canada (Turner 1948; Spear and Glude 1957; Cormi to 1982). Growth may be modeled u s i n g t h e expo- n e n t i a l von B e r t a l a n f f y growth equa t ion , expressed by t h e f o l l o w i formula: S h e l l l e n g t h = a ( l - be-kf? where a, b and k a r e cons tan ts d e r i v e d f r o m growth data, and t = t ime. Growth r a t e cons tan ts of clams f r om d i f f e r e n t geographi c areas were c a l c u l a t e d by Brousseau (1979). Data f i t t o t h e growth equa t i on u s i n g bes t f i t computer-generated curves demon- s t r a t e widespread d i f fe rences i n growth r a t e s among 1 o c a t i ons . Seasonal v a r i a t i o n s i n growth r a t e s can be i n c o r p o r a t e d i n t o t h e von B e r t a l a n f f y equa t i on by add ing temperature, i n day-degrees (Munch-Peterson 1973).
Seasonal v a r i a t i o n s i n growth r a t e s have been a t t r i b u t e d i n p a r t t o f o o d ava i 1 ab i 1 i t y by Newcombe (1935), Mat th iessen (1960a), and S t i c kney (1964) ; t o tempera tu re by B e l d i n g (1930), Dow and Wal lace (1961), S t i ckney (1964), and Munch-Peterson (1973): and t o spawning by Brousseau
(1979). I n New England, s o f t s h e l l clams g e n e r a l l y grow f a s t e s t i n l a t e s p r i n g and e a r l y summer, and s lowes t i n t h e c o l d w i n t e r months ( B e l d i n g 1930; Brousseau 1979). The months o f r a p i d c lam growth a r e c o i n c i d e n t w i t h r i s i n g abundance of phy top l ank ton and seawater tempera tu re i n t h e Gulf of Maine ( B i gelow 1917; P e t r i e 1975). A lso, r a p i d c lam growth i n a s a l t pond i n Ma r t ha ' s Vineyard, Massachusetts, was c o i n c i d e n t w i t h a h i g h abundance of f l a g e l l a t e s (Mat th iessen 1960a). Seasonal v a r i a t i o n s i n growth r a t e s a r e p o s i t i v e l y c o r r e l a t e d w i t h seasonal changes i n b iochemica l (g l ycogen) 1 eve l s and c o n d i t i o n i n d i c e s (measurements o f s h e l l f i s h " f a t ness " ) . Glycogen l e v e l s and meat y i e l d s a r e h i g h i n t h e s p r i n g ; t h e glycogen i s conver ted t o gametes w i t h a subsequent drop i n meat y i e l d s d u r i n g t h e spawning season, and t h e meat y i e l d recovers a f t e r spawning (Newel l 1982). I n Maine, g lycogen i n s h e l l f i s h o f good market q u a l i t y peaks i n l a t e s p r i n g and i s lowes t i n w in - t e r .
Growth r a t e s a r e a l s o c l o s e l y r e l a t e d t o cu r ren t , sediment t ype , and i n t e r t i d a l h e i g h t . C lams grow t h e f a s t e s t i n s o f t sediments on t h e lower shore (where food i s r e l a t i v e l y abundant) under good c u r r e n t c o n d i t i o n s i n New England ( B e l d i n g 1930; Newel1 1982). I n a l a b o r a t o r y experiment, clams grew f a s t e r i n s o f t mud o r sand t han i n g rave l (Newel1 and H idu 1982). S i m i l a r l y , s h e l l form and pe rcen t she l 1 we igh t v a r i e d w i t h growth r a t e , sediment t ype , and i n t e r t i d a l h e i g h t . Slow-growi ng clams f r o m c o a r s e s e d i m e n t s and f r o m t h e upper shore (where f ood i s r e l a t i v e l y scarce) have h i g h e r pe r cen t she l 1 we igh ts and l a r g e r she l 1 1 ength-depth r eg ress i on s lopes ( g r e a t e r s h e l l g l obos i t y ) t h a n fas t -g row ing clams f r o m t h e l o w e r s h o r e and f r o m s o f t sediments.
Excess ive d e n s i t y can a l s o l i m i t growth r a t e s because of c o m p e t i t i o n f o r food and space. Matu re c lam
d e n s i t i e s o f 161-269/m2 a r e genera l l y cons idered f a v o r a b l e f o r good growth (Be1 d i ng 1930).
ECOLOGICAL ROLE
Food and Feeding H a b i t s
Clam l a r vae , j u v e n i l e s , and ad1~1 tS feed by f i l t e r i n g seawater. Postmetamorphic clams draw i n th rough t h e i n c u r r e n t s i phon by b e a t i n g t h e g i 11 c i l i a . The wate r passes th rough t h e g i l l s , where f ood p a r t i c l e s a r e removed, t r apped i n mucus, and swept t o t h e mouth. P a r t i c l e s t o o l a r e f o r 4 i n g e s t i o n , i n o r g a n i c p a r t i c l e s ow i n n u t r i t i o n , and p a r t i c l e s o f any t y p e i n dense concen t ra t i ons i n seawater a r e u s u a l l y r e j e c t e d by c i l i a t e d s t r u c t u r e s c a l l e d t h e l a b i a l pa lps . These p a r t i c l e s a r e expel l e d th rough t h e i n c u r r e n t s iphon as pseudofeces by a r a p i d c o n t r a c t i o n of t h e adduc to r muscles. Phy top lank ton c e l l concen t ra t i ons g r e a t e r t h a n 30,000/ml of seawater cause a r e d u c t i o n o f f i l t r a t i o n r a t e s and t h e f o rma t i on o f pseudof eces as und i gested a1 gae and mucus (S t i c kney 1964).
F l a g e l l a t e s and d ia toms a r e t h e p r e f e r r e d d i e t , a l though clams can o b t a i n n u t r i t i o n from b a c t e r i a and o rgan i c d e t r i t u s i n res1.1spended mudf la t sediments (Eaton 1981) and d i s s o l v e d o rgan i c molecules (S tewar t and Bamford 1976). I n a s a l t pond i n Massachusetts, clams grew f a s t e r on a d i e t o f f l a g e l l a t e s t h a n on diatoms (Mat th iessen 1960b).
Food f i l t r a t i o n r a t e s a r e i n - f luenced by wa te r temperature, seawater p a r t i c l e d e n s i t i e s , and p a r t i c l e t y p e (Eaton 1981). I n Maine, Eaton found t h a t clams f i l t e r and a s s i m i l a t e h i g h e r q u a n t i t i e s of a l gae i n t h e summer (14-20 O C ) t han i n t h e s p r i n g (3-8 OC). Clams a r e d i s c r i m i - n a t o r y feeders and i nc rease f i l t r a t i o n r a t e s when a l gae a r e added t o d i e t s of suspended s i l t p a r t i c l e s . Clams a r e we1 1-adapted t o hand1 i ng resuspended
s i l t s i n h i g h concen t ra t i ons , and may s o r t a l g a l c e l l s f r o m i n o r g a n i c p a r t i c l e s p r i o r t o i n g e s t i o n . Clams of 0.3 g d r y meat we igh ts con t i nue t o f i l t e r even i f seawater s i l t p a r t i c l e d e n s i t i e s exceed 300 mg/l (Eaton 1981) . High l e v e l s o f o i l p o l l u t i o n i n sediments (hydrocarbon l e v e l s over 1,500 ppm) caused a r e d u c t i o n i n f ood f i l t r a t i o n r a t e s and a lower carbon f l u x i n clams f r o m Maine ( G i l f i l l a n e t a1 . 1976). Accord ing t o Mat th iessen (1960b), f i l t r a t i o n r a t e s o f clams dec l i n e d when exposed t o s a l i n i t i e s between 8 and 15 p p t and s topped when s a l i n i t i e s were below 4 pp t . Clams con t i nue t o f i l t e r when seawater temperatures a r e below 3 O C , bu t food a s s i m i l a t i o n i s low.
Preda to rs
P reda t i on i s one o f t h e most impo r t an t f a c t o r s i n t h e c o n t r o l of n a t u r a l popu la t i ons o f s o f t s h e l l clams. P l a n k t o n i c l a r v a e a re s u b j e c t t o p r e d a t i o n by o t h e r p l a n k t o r s , f i s h , and f i 1 t e r i ng i n v e r t e b r a t e s ; young spa t may be devoured by b i r d s , f i s h , shr imp, worms, crabs, s n a i l s and f la tworms. As t h e j u v e n i l e c lam grows, i t burrows deeper i n t o t h e subs t r a t e , where i t f i n d s fh',otection f r o m most p reda to r s . r a p i d j u ven i 1 e growth and postponement of gametogenesis a r e cons idered t o be adap ta t i ons f o r s u r v i va1 (Commito 1982). I n one i ns tance , i t was es t ima ted t h a t i n Massachusetts t h e mummi chog (Fundulus h e t e r o c l i t u s ) , a sma l l f i s h , consumed as many as 546,000 s o f t s h e l l clams ( < I 2 mm l o n g ) p e r km o f s h o r e l i n e p e r day ( K e l s o 1979). I n a l a b o r a t o r y exper iment , sandworms ( N e r e i s v i r ens ) consumed s o f t s h e l l clams <15 mm l o n g (D. Dean, Department o f Zoology, U n i v e r s i t y of Mai ne, Orono; pers . comm.) . I n Massachusetts, R e l d i ng (1930) r e p o r t e d t h a t ma jo r p reda to r s a r e t h e b l ue c rab ( C a l l i n e c t e s sa i d u s ) , l a d y c rab (Ova l i es ocel* horseshoe c rab (fi& pol(yphemus), and espec ia l l y moon s n a i l s L u n a t i a spp.).
The green c r a b (Carc inus maenas) i s one of t h e most d e s t r u c t i v e clam p reda to r s . F o r example, p e r i o d i c de- c l i n e s i n c l a m p r o d u c t i o n i n Ma ine c o i n c i d e d w i t h inc reases i n t h e abundance o f green crabs when seawater temperatures were unusual l y h i g h (Welch 1969). A green c rab may consume as many as 15 c lams p e r day. When abundant, green c rab d e n s i t i e s can exceed 24,70O/ha (Spear 1953). Moon sna i 1s ( L u n a t i a spp.) p r e f e r e n t i a l l y p r e y on c lams 20 t o 50 mm l o n g and p r e f e r s o f t s h e l l clams over o t h e r foods (Edwards and Heubner 1977). The moon s n a i l feeds by d r i l l i n g a h o l e i n t h e c lam s h e l l w i t h a r a s p i n g organ and e a t i n g t h e c o n t e n t s . A s i n g l e s n a i l may consume up t o 100 clams a y e a r (Edwards and Heubner 1977). Clams over 60 mm l o n g a r e no t u s u a l l y a t t acked by crabs o r moon s n a i l s . The b l u e mussel ( M y t i l u s e d u l i s ) and ano ther c lam (Gemma gemma)compete w i t h t h e s o f t s h e l l c lam f o r space and food i n t h e lower i n t e r t i d a l zone i n some coves i n Maine (Dow and Wal lace 1961).
ENVIRONMENTAL REQUIREMENTS
S a l i n i t y
The s a l i n i t i e s of s o f t s h e l l c lam h a b i t a t decrease f r o m n o r t h t o south. P r e f e r r e d s a l i n i t i e s were 20-32 p p t i n Maine (Dow and Wal lace 1961; G i l f i l l a n e t a l . 1976), 10-33 p p t i n Massachu- s e t t s ( B e l d i n g 1930; Brousseau 1978b), and 4-15 p p t i n upper Chesapeake Bay ( P f i tzenmeyer 1972). When clams were p l a n t e d i n s a l i n i t i e s near t h e i r lowest t o l e r a t e d l e v e l , t hey showed s i gns o f d i s t r e s s and reduced t h e i r pumping ra tes . The r a t e o f pumping was s h a r p l y reduced when t h e s a l i n i t i e s were r ed l~ced t o t h e s t r e s s p o i n t . The s t r e s s p o i n t i s about 15 p p t f o r Massachusetts clams (Mat th iessen 1960a), 22-24 p p t f o r Medomac R i v e r , Maine, clams (Welch and Lewis 1965). and 5 p p t f o r Chesapeake Bay c lams (Schubel 1973). Larvae a r e more s e n s i t i v e t o low s a l i n i t i e s t han
a d u l t s ( B e l d i n g 1 9 3 0 ) . The l o w e s t t o l e r a b l e s a l i n i t y f o r a d u l t s o f t s h e l l clams i s about 5 p p t .
Growth r a t e s o f clams i n Massachusetts a r e p o s i t i v e l y c o r r e l a t e d w i t h s a l i n i t i e s f r om 3 t o 14 p p t i n Massach~rset ts (Mat th iessen 1960a). M o r t a l i t i e s were as h i g h as 90% i n Chesapeake Bay when s a l i n i t i e s dropped t o 2 p p t because o f t h e f reshwate r r uno f f a f t e r Hu r r i cane Agnes (Shaw and Hammons 1974). S o f t s h e l l c lam s u r v i v a l a t ex t reme ly low s a l i n i t i e s i s i n v e r s e l y r e l a t e d t o wa te r t e m p e r a t ~ l r e ( A l l e n and G a r r e t t 1971). For example, s o f t s h e l l clams i n t h e Bay o f S t . Lawrence s u r v i v e d s a l i n i t i e s below 1 p p t f o r as l o n g as 1.5 days a t low temperatures. However, a t h i g h e r wa te r temperatures, clams a c c l i m a t e f a s t e r t o moderate ly decreas ing s a l i n i t i e s (Creaser and C l i f f o r d 1977). F o r example, i n an exper iment i n Maine, a c c l i m a t i o n t i m e f o r a change f r om 30 p p t t o 22 p p t was reduced f r o m 60 hours a t 4 "C t o 10 h o u r s a t 10 " C . The t o l e r a n c e o f clams 2-25 mm l o n g t o low s a l i n i t i e s i s somewhat p o s i t i v e l y c o r r e l a t e d w i t h t h e i r s i z e (Mat th iessen 1960a). A m i no ac i ds r e g u l a t e t h e osmot i c charac- t e r i s t i c s o f t h e c lam hemolymph (b l ood ) i n d i f f e r e n t s a l i n i t i e s .
Temperature
Water tempera tu re r egu la tes t h e t i m e o f spawning and i n f l u e n c e s t h e d i s t r i b u t i o n of clams a l ong t h e n o r t h A t l a n t i c coast . The range o f s o f t - she1 1 clams i s l i m i t e d i n t h e n o r t h by temperatures t o o low (12 "-15 "c) f o r spawning; t h e range i n t h e south i s l i m i t e d by excess i ve l y h i g h tempera- t u r e s (Lawson 1 9 6 6 ) . Clams do n o t u s u a l l y s u r v i v e wa te r temperatures above 28 "C i n Chesapeake Bay; h i g h temperatures r e s t r i c t t h e i r d i s t r i b u t i o n and abundance t h e r e (P f i tzenmeyer 1972). I n 1-day b i oassays, t h e LT50 e t h a l tempera tu re f o r 50%) was 32.5 (k f o r a d u l t clams f r o m Chesapeake Bay and
34.4 "C f o r j u v e n i l e s (Kennedy and Mihursky 1972). Clams f r o m t h e sub- t i d a l zone and t h e h i g h i n t e r t i d a l zone b e t t e r w i t h s t o o d h i g h tempera- t u r e s ( > 25 "C) t han clams from low and m idd le i n t e r t i d a l l e v e l s . Water temperatures r e g u l a t e t h e l e n g t h of l a r v a l l i f e ( l ow tempera tu re - long l i f e ) ; o p t i ma1 tempera tu re f o r maximum g r o w t h i s a b o u t 20 "C (S. Chapman, I r a D a r l i n g Center , Univ. of Maine, Walpole; pers . comm.). I n a s tudy o f s o f t s h e l l clams from Mary land t o Nova Sco t i a , wa te r tempera tu re was t h e dominant env i ronmenta l f a c t o r a f f e c t i n g 68% o f t h e observed rowth r a t e f l u c t u a t i o n s (Appeldoorn ?982). Food ava i l a b i 1 i t y , wa te r c u r r e n t v e l o c i t y ( B e l d i ng 1930), and sediment t y p e (Newel1 and Hidu 1982) c o n t r i b u t e t o l o c a l f l u c t u a t i o n s i n growth r a t es , which sometimes mask t h e genera l l a t i t u d i n a l t r e n d (Dow and Wal l a c e 1961 ).
The pump ing r a t e s of s o f t s h e l l clams i nc rease w i t h i n c r e a s i n g t e m p e r a t u r e u p t o 1 6 "C ( H a r r i g a n 1956 ) . A t l o w e r t e m p e r a t u r e s ( 1 - 2 "C), clams con t i nue t o pump bu t r a t e s o f f ood a s s i m i l a t i o n a r e low (Gi l f i l l a n e t a1 1976). Water temper- a t u res o f -1.7 "C a r e t o l e r a t e d i n New England. Du r i ng i c i n g , clams s u r v i v e by u t i 1 i z i n g glycogen energy reserves (Newel 1 1982).
R a f t i n g o f t h e s u r f a c e sediment and t h e d e s t r u c t i o n o f sha l l ow bu r row ing j u v e n i 1 es by i c e have been observed i n mudf la ts i n Maine.
Changes i n tempera tu re r e g u l a t e t h e r a t e o f burrowing. I n Chesapeake Bay, bu r row ing was g r e a t e s t a t 18 "C, moderate f r o m 9 t o 21 "C, and g r e a t l y reduced a t h i g h e r o r lower temperatures (Pf i tzenmeyer and Drobeck 1967).
Oxygen
Oxygen u t i l i z a t i o n by s o f t s h e l l clams i s governed l a r g e l y by body
s i z e , wa te r temperature, r a t e of rnetabol ism, and oxygen concen t ra t i ons i n t h e wa te r . A p o s i t i v e l i n e a r r e l a t i on between oxygen consumption and wet we igh t was r e p o r t e d by Fong (1976). Oxygen i n t a k e i n s o f t s h e l l clams i s independent of oxygen c o n c e n t r a t i o n down t o about 2.8 m g l l i t e r (van Dam 1935). Oxygen i n t a k e i s g r e a t e s t a t a b o u t 20 "C (Kennedy and Mihursky 1972). I n t e r - t i d a l s o f t s h e l l clams can f u n c t i o n as f a c u l t a t i ve anaerobes ( C o l l i p 1920), s w i t c h i n g t o anaerobic m ~ t a b o l i c pathways a t low t i d e . Calc ium carbon- a t e f r o m t h e s h e l l i s used t o b u f f e r t h e a c i d i c p roduc ts o f anaerobic r e s - p i r a t i o n . Upon reimmersion, clams undergo r a p i d v e n t i l a t i o n t o o x i d i z e t h e end p roduc ts o f anaerobic r e s p i r a - t i o n . Excess i ve l y low oxygen concent r a t i o n s cause i ncreased s t r e s s and hea r t bea t r a t e s (Lowe and Trueman 1972). S o f t s h e l l clams s u r v i ve pe r i ods o f anaerob ios is l onge r a t low temperatures, f o r example, d u r i n g i c i n g . High r e s p i r a t i o n r a t e s i n t h e s p r i n g have been a t t r i b u t e d t o warm wa te r a c c l i m a t i o n ( G i l f i l l a n e t a l . 1 9 7 6 ) o r m e t a b o l i c c o s t s o f a c t i v e gametogenes i s .
Subs t r a t e and C u r r e n t
Clams l i v e i n s o f t muds, sands, compact c lays , and coarse g rave l , and between s tones. Sediment t y p e i s impo r t an t i n c o n t r o l 1 i ng growth r a t e and she1 1 a1 lomet ry . Under i d e n t i c a l c u r r e n t cond i t i ons , clams grow f a s t e r i n f i n e sediments t h a n i n coarse sediments (Newel1 and Hidu 1982), and grow f a s t e s t i n sand o r sandy mud ( B e l d i n g 1930; Dow and Wal lace 19611, though few l i v e i n s h i f t i n g sand. Slow-growing clams f r om grave l have more g lobose s h e l l s t han t hose f r om mud o r sand , and c lams i n sand a r e l onge r and narrower t han t hose i n mud. D i f f e r e n c e s i n growth r a t es and s h e l l form a r e a t t r i b u t e d t o t h e p h y s i c a l p r o p e r t i e s o f t h e s e d i m e n t and i t s r e s i s t a n c e t o burrowing. Sediment p a r t i c l e s i z e a l s o a f f e c t s t h e
rebur row ing of clams. C 1 ams had d i f f i c u l t y bur row ing i n sediments l a r g e r t han 0.5 mm (P f i t zenmeyer and Drobeck 1967).
The c h a r a c t e r i s t i c s o f t h e sediments r e f l e c t t h e r a t e s o f t h e bot tom c u r r e n t s and e s t a b l i s h , f r o m a p h y s i c a l s t andpo in t , t h e s u i t a b i l i t y of t h e b o t t o m f o r c l ams . C o a r s e r sediments usua l l y r e f l e c t f a s t e r c u r r e n t s , which suppo r t g r e a t e r p o p u l a t i o n d e n s i t i e s and cause f a s t e r growth (Appeldoorn 1982). I f t h e c u r - r en t s a re t o o slow, t h e sediments u s u a l l y have a h i g h s i l t - c l a y con ten t , which, i n excess, can c l o g t h e g i 11s of t h e clams, reduce growth ra tes , and i n extreme cases cause smother ing (Dow and W a l l a c e 1 9 6 1 ) . M o s t c lams i n Chesapeake Bay t h r i v e i n a s u b s t r a t e t h a t i s l e s s t han 50% s i l t ( P f i t z e n - meyer 1972). Clams may con t i nue pumping when t o t a l suspended s o l i d s exceed 300 mg l l , bu t t h e p r o d u c t i o n of mucus and t h e l o s s of energy d u r i n g t h e e j e c t i o n o f pseudofeces s t r a i n t h e energy budget o f t h e c lam (Eaton 1983).
Coarse sediments and mats o f vege ta t i on h e l p p r o t e c t clams from most p reda to r s (Blundon and Kennedy 1982). D u r i n g w i n t e r months when phy top l ank ton popu la t i ons a r e lowes t ,
resuspended bo t tom sediments a r e an impo r t an t food source f o r s o f t s h e l l clams.
P o l l u t i o n
S o f t s h e l l clams i n p o l l u t e d wate r accumulate p e s t i c i d e s (Dow 19721, o i 1 (Mayo e t a l . 1975), heavy meta ls ( E i s l e r 1977), and sometimes t h e h a c t e r i a and v i r uses i n mun ic ipa l sewage. Tox i c m a t e r i a l s a re most damaging t o f e r t i l i z e d eggs and l a r vae , and l ess damaging as t h e clams grow l a r g e r . O i l p o l l u t a n t s can cause a r e d u c t i o n i n t h e c a r b o n f l u x o f clams ( G i l f i l l a n e t a l . 1976). More r e f i n e d pe t rochemica ls may have a g r e a t e r e f f e c t on t h e i n c i d e n c e o f p a t h o l o g i c a l tumors i n clams t han u n r e f i n e d o i l s and heavy meta ls (Walker e t a l . 1981). Clam l a r v a e a r e s e n s i t i v e t o c h l o r i ne-produced ox i dan t s bu t i n genera l adu 1 t clams a r e r e l a t i v e l y t o l e r a n t o f p o l l u t i o n (Brown e t a l . 1 9 7 7 ) . D e s p i t e t h e t o l e r a n c e o f s o f t s h e l l clams t o p o l l u t i o n , h a c t e r i a and v i ruses f r o m mun i c i pa l e f f l u e n t accumulate i n t h e c lam 's body t i s s u e s , and i f eaten, a r e a t h r e a t t o p u b l i c h e a l t h . U n t i l mun i c i pa l p o l l u t i o n i s adequate ly abated, clamming i n some wate rs w i l l con t i nue t o be p r o h i b i t e d .
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Newcombe, C.L. 1936. A corrparat ive s t udy of t h e abundance and r a t e o f g r o w t h o f Qa a r e n a r i a I-. i n t h e G u l f of S t . T a F e n C e a n d Bay o f Fundy reg ions . Ecology 17: 418-428.
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REPORT DOCUMENTAiiON 1. = E m R T NO.
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Car te r R . Newel 1 and Herbe r t Hi du 9. P a r l o n n ~ n ~ O ~ ~ a n # z a t l o n Nama and Addrass
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1 2 sponsor in^ Or~anlzatlon Nama and Addrass
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The s o f t s h e l l clam, M a a rena r ia , i s a commerc ia l l y and r e c r e a t i o n a l l y impor tan t i n v e r t e b r a t e t h a t i nha 3- i t m o t t o m sediments o f s u b t i d a l and i n t e r t i d a l waters o f moderate t o h i g h s a l i n i t y . I t s range i s 1 i m i t e d by water temperatures t o o low f o r r e p r o d u c t i o n i n t h e n o r t h and by l e t h a l warm temperatures i n t h e south. Clams feed by s iphon ing seawater and removing food p a r t i c l e s , e s p e c i a l l y phy top lank ton, w i t h t h e i r g i l l s . Clams a r e t h e r e f o r e s e n s i t i v e t o f a c t o r s a f f e c t i n g water qua1 i t y , i n c l u d i n g suspended sediments, s a l i n i t y , water temperature, oxygen, and waterborne p o l l u t a n t s . The c lam l i f e c y c l e c o n s i s t s o f mass spawning and e x t e r n a l f e r t i 1 i z a t i o n , t h e development o f p e l a g i c l a r v a e , s e t t l e m e n t and metamorphosis i n t o spat , and r a p i d j u v e n i l e growth t o m a t u r i t y . Clam r e c r u i t m e n t and t h e m i g r a t i o n o f spa t a r e dependent upon i nsho re c u r r e n t s . H igh m o r a l i t y o f eggs, l a r v a e , and spa t i s l a r g e l y o f f s e t by h i g h r e p r o d u c t i v e p o t e n t i a l . As t h e c lam grows, i t f i n d s r e f u g e f rom most p reda to rs deep i n t h e sediments, b u t i t a l s o l oses i t s a b i l i t y t o burrow and i s s u b j e c t t o s u f f o c a t i o n by s i l t a t i o n . Sediment types, c u r r e n t s , and t i d a l h e i g h t s a1 1 a f f e c t clam growth r a t e s .
E s t u a r i e s Growth Feeding h a b i t s F i s h e r i e s Sal i n i t y L i f e c y c l e s She1 1 f i s h Temperature Contaminants Sediments Suspended sediments Oxygen
b. IdenUlien/Oou+End*d Terms
S o f t s h e l l c lam %A a r e n a r i a H a b i t a t requ i rements Spawning
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REGION 1 Regional Director U.S. Fish and Wildlife Service Lloyd Five Hundred Building, Suite 1692 500 N.E. Multnornah Street Portland, Oregon 97232
REGION 4 Regional Director U.S. Fish and Wildlife Service Richard B. Russell Building 75 Spring Street, S.W. Atlanta, Georgia 30303
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As the Nation's principal conservation agency, the Department of the Interior has respon- sibility for most of our,nationally owned public lands and natural resources. This includes fostering the wisest use of our land and water resources, protecting our fish and wildlife, preserving theenvironmental and cultural values of our national parks and historical places, and providing for the enjoyment of life through outdoor recreation. The Department as- sesses our energy and mineral resources and works to assure that their development is in the best interests of all our people. The Department also has a major responsibility for American Indian reservation communities and for people who live in island territories under U.S. administration.
