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Effects of Ternperature, Salinity and Oxygen on the Survivalof the Arnerican Lotrstert'2
Bv D. W. MclrnsrFisheries Research Board, of CarndaBi,ologi,cal Station, St. Andreus, N.8.
CONTENTS
INrnouucrroN
AcxNowrmclarNrs
Mercmer.s
Mnrnops
Acclimation tanks and controls
Lethal tanks and controls
Determination of death
Accr-rue'rroN
Temperature acclimation
Salinity acclimation
Oxygen acclimation
Pnnr,nvrrxeny CoNsmBnerroNs ron Lrrrr,lr, Lnl'rr,s
Size in relation to lethal levels of temperatue, salinity and oxygen
Resistance of lobsters from different areas to temperature, salinity and oxygen
Temperature, salinity and oxygen lethals of starved and fed lobsters
Moulting and resistance to temperature, salinity and oxygen
Lr,rrrar- Lnvnr-s or Trlapnnetrrnr, Ser-nrrry eNo OxyceN
Upper lethal temperatures
Lower lethal salinities
Lower lethal oxygen levels
Lower lethal temperaturesResistance times
Ultimate, maximum and minimum lethal levels
Bouvoany or Lntrrer. CoxorrroNs
Rnrrnr,Ncns
248
249
249
25025L25r
2522j.2253254
2542552562582,57
25826026r262262263264
269
271
rReceived for publication July 15, 1955.2Based on theses accepted by the University of Western Ontario and the University of
Toronto, in partial fulfflment of the requirements for the degrees of Master of Science andDoctor of Philosophy, respectively.
247
J. Frsrr. Rns. Bo. CeNane, 13(2), pp. 247-272, March 1956.Printed in Canada.
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ABSTRACT
Thermal acclimation for lobsters transferred from 14.5oC. to 23-0oC. is complete in 22
days. Substantial acclimation to low salinity and to low oxygen occurs within one week.
Lethal levels of t-hese three factors are not altered by difierences in size within the range
16-34 cm., by difierence in the areas where lobsters were caught' or_by starvation for_up
to 57 days. trlorrltirrg lobsters are less resistant to high temPerature, low salinity and low
oxygen conditions than hard-shelled lobsters.Upper letlal temperature Ievels and lower lethal salinity and oxygen levels were
investigated for hard-shelled lobsters acclimated to each of the 27 combinations of three
levels of temperature (5, 15 and 25oC.), salinity (20, 25 and 30%"), and oxygen (2.9, 4.3
and 6.4 mg,Zl-). The upper lethal temperature is raised by an increase in thermal acclimation,
and is lowered by a decrease in the salinity and oxygen acclimation levels. The lower lethal
salinity is raised by an increase in the level of thermal acclimation and a decrease in the
level of orrygen aiimation. It is lowered by acclimation to reduced salinity. The efiect oI
salinity acciimation is not always the same, but depends on the temperature acclimation. The
lower lethal oxygen is raised ty either an increase in the temperature acclimation Ievel or
a decrease in the salinity acclimation.The lower lethal temperature is 1.8oC. for 17o acclimated lobsters, and 5.0o for 27.50
acclimated lobsters.Ultimate and maximum or minimum lethal levels of temperature, salinity and oxygen-the
highest and lowest lethal levels that can be attained by acclimation-were interpolated fromthe results. These measures were used to integrate the lethal levels of the three factors ir'toa single three-dimensional graph which describes the boundary of lethal conclitions fotlobsters exposed to the three factors operating together (Fig. 7).
INTRODIJCTION
Wide seasonal and short-term variations in temperature, sporadic variation in
salinity, and reduction of dissolved oxygen are known to occur and either singly
or in combination may result in conditions that are unsuitable for lobster suryival'
Heavy mortalities occur at times among lobsters that are held in large numbers bythe industry. Sometimes the mortalities have been attributed to disdase, but in
general, adverse environmental conditions have been suspected.In this investigation the lethal effects of temperature, salinity and oxygen were
studied to determine the actual levels of these three factors that restrict lobstersurvival both in nature and in commercial holding units.
Fry (I9a7 ) provided a basis for understanding the relationships and inter-actions of various environmental influences on an organism. He stressed theidea that previous experience or acclimation modifies the physiology of anorganism and has a pronounced efiect on its response to environmental factors.Unless acclimation is considered in relation to the factor being studied, results willnot have a precise meaning. His concepts were developed from experimentalevidence derived from the study of fish. The present paper illustrates that the sameconcepts also apply to the invertebrate ffeld.
Physiological changes associated with moulting (Hollett, 1943; Lowndesand Panikkar, 1941; Donahue, 1953) may alter the response of lobsters to theirenvironment. In addition there are physiological differences associated withvariations in life-history among lobsters from different areas, (Templeman, 1936a;Wilder, 1953). Separate tests were performed to establish the effects of suchphysiological difierences on lobster survival.
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Some preliminary work on the lethal levels of temperature, salinity andoxygen for lobsters appears in the literature. Chaisson (1932) held lobsters suc-cessfully for several days in aerated sea water at 25"C. Lobster larvae werereared to the fourth stage at 24'C.by Templeman (f$6b).
Wood (1885) tested the survival of lobsters at low temperatures. Three outof five lobsters transferred from water at 2I"C. to water at the freezing point weredead by 24 hours.
Chaisson (1931) found that the respiration of 23-cm. lobsters stopped afterT)ihours at a salinity of LL.4%o and after 13 hours atLg.Sa/oo when the temperaturewas between 11o and 13'C. Templeman (1936b) reared lobster larvae to thefourth stage at salinities between IB/'o and 35"/oo at temperatures between 15oand 20'C. He concluded that ZIa/oo was almost as satisfactory for their survivalas the normal 3I%". Salinities lower than \9.41/oo, however, were definitely un-favourable and at L6.40/"0 larvae died before moulting into the third stage.
Some information on low oxygen tolerance was supplied by Chaisson (1932).Four glass iars of 3-quart capacity, each with one 23-cm. lobster, were filledwith water saturated with oxygen and sealed. Two of the lobsters were heldat 13.5' to 15.0'C. and died after 18 hours. The other two, which were heldat 0.5o to 1.0oC., were still living after 60 hours. He concluded that lobsters aremost resistant to low oxygen at low temperafures. The final oxygen contentsof the water were not recorded and a low rate of oxygen consumption at lowtemperatures could account for the observed differences'
ACKNOWLEDGMENTS
The author wishes to thank Dr. D. G. Wilder, Fisheries Research BoardBiological Station, St. Andrews, N.B., for suggesting the problem and for hiscontinued advice and criticism during the course of the work and the preparationof the manuscript. The assistance of Miss Marjorie Myers, Miss Anne Henry,Mr. C. J. McFarland and Mr. U. J. Walsh with the observations during theexperiment is gratefully acknowledged.
Thanks are extended to Dr. F. E. I. Fry, University of Toronto, for his aidwith the interpretation of parts of the data and to Dr. W. E. Ricker, FisheriesResearch Board Biological Station, Nanaimo, 8.C,, for his aid with the statisticalanalysis.
MATERIALS
Most of the lobsters were obtained from the sorrthwestern shore of NewBrunswick on the Bav of Fundv. Others came from eastern New Brunswick onthe Gulf of St. Lawrence, Prince Edward Island and Cape Breton, Nova Scotia.The stocks of lobsters used in the experiments are listed in Table I.
In addition to the lobsters listed in Table I, 280, 250 and 450 lobsters wereused during 1949, 1950 and 1951, respectively, in lethal temperature, salinityand oxygen experiments. The results of these preliminary experiments are notreported in this paper.
The lobsters from Prince Edward Island measured between 22 and 26 cm.Those from eastern New Brunswick ranged in size from 16 to 28 cm. total length
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Teern I.-Stocks of lobsters used in the experiments
Area Date and number
Prince Edward Island
Eastern. New Brunswick
Southwestern New Brunswick
(tip of rostrum to tip of telson). The lobsters from southwestern New Brunswickranged in size from 20 to 28 cm., but most ranged from 22 to 26 cm.
The inshore water of the Gulf of st. Lawrence is warmer in summer andcolder in winter than the water of the Bav of Fundv. Lobsters from the twoareas difier in life-histories (Templeman, 1936a). Fromhis observations, Temple-man ( 1944 ) estimated that half the female lobsters from the warm areas of theGulf of st, Lawrence have become mature at a length of 28.5 cm. and halfthose from southwestern New Brunswick have become mature at 88.0 cm.]udging from the sizes of the lobsters that were used in the experiments, it appearsthat most of those from the Gulf of St. Lawrence and virtuallv all those ]romsouthwestern New Brunswick were immature.
Some moulting occurs during the late spring and early summer months anda small number of the lobsters obtained for the experiments were preparing tomoult. These were segregated and special tests were performed, using the pre-moult lobsters and those that subsequently moulted. The remaining lobsterswere hard shelled and were not preparing to moult. These were used in all theother experiments.
During the period between the time lobsters were obtained and placedin conholled acclimation tanks, they were held in floating crates and a floatingcar at the end of the wharf at the Biological Station, St. Andrews, N.B. Theywere fed herring and flounder during these holding periods. They were not fed,however, during controlled acclimation because of the danger of water pollutionat high acclimation temperatures and reduced flows. Evidence is presentedin a later section to show that lack of food during acclimation had no appreciableeffect on the lethal temperature.
Accr,ruerroN Ta*rs ^,ro coNrnot, t"t*oot
Three wooden acclimation tanks were used in 1949 and 1950. Theymeasured 3 by 3 feet by 6 inches (91 X 9l X 15 cm.). These were replacedin 1951by nine wooden tanks measuringf feet by 4 feet by 8 inches (L22 X I22I 20 cm. ). In addition, three concrete tanks, 7 by 5.5 feet with 6 inches of water(213 X 107 X 15 cm.) were used for acclimation in 1952. Thirty lobsters at atime were held in the smallest tanks and 50 were held in the others.
During 1949 and 1950, temperatures were maintained in the three acclima-tion tanks by adding small flows of heated sea rvater from a constant level tankmaintained at a constant temperature. Final temperature control in the ac-
August, 1949 (120)
June, 1950 (200)
June, 1950 (474); June, 1951 (70);\1lay, 1952 (600); June, 1952 (r400);July, 1952 (400); Jan., 1953 (1550);May, 1953 (500); June, 1953 (1150).
To ta l :6464
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climation tanks was obtained by thermostatically controlled pyrex glass immersionheaters in each acclimation tank. Temperature fluctuated less than -+l.0oC.
During 1951, 1952 and 1953, water was heated as it passed through two
or three 400-watt, pyrex glass-lined heaters installed over each tank. The final
temperature control was obtained by thermostatically controlled pyrex-insulatedimmersion heaters. Temperature fluctuated less than -f I.0oC.
Reduced salinities were obtained by mixing fresh water with inflowing sea
water in the desired proportions. Salinities were determined by hydrometer and
reference to salinity tables, Knudsen (1946). Salinity fluctuated less than -+I'0%0.
Compressed air delivered through four or five aquarium-type air stones in
each tank kept the oxygen content over 4.3 mg. O2/L during 1949 to 1951, in-
clusive. During 1952 and 1953, 6.4 mg.Or/1. were maintained during te.mperature
and salinity acclimation. Reduced levels of oq/gen were obtained by reducing
the flow of air to the tanks. Oxygen was determined by the unmodified Winkler
method. The oxygen content fluctuated less than r-1.0 mg. O2l1.
LsrHAr Texrs AND CoNTRoLS
Two lethal tanks similar to the smallest acclimation tanks were used in
1949 and 1950. Fligh temperatures were obtained by adding small fows of heated
sea water from a constant level tank maintained at a constant temperature. Test
temperatures fluctuated less than f 0.3'C.Two lethal tanks measuring2by 2ieet by 6 inches (61 X 61 \ 15 cm.)
were used in l95l and 12 were used during 1952 and 1953. They were con-
shucted of unpainted l-inch (2.5-cm.) pine lumber. The method of heating was
the same as that used for the larger acclimation tanks. Temperature was con-
trolled within :+0.4oC. for lethal temperature tests and within -r1.0oC. for
lethal salinity and oxygen tests.Reduced salinities were obtained by mixing fresh water and sea water in
constant level buffer tanks, Salinity was controlled within t0.5%0.The required oxygen contents for the lethal temperature and salinity tests
were obtained by adjusting the amount of compressed air supplied to each tank.
Reduced oxygens for the lethal oxygen experiments were provided by apparatus
working on the principle described by Frv (1951). A glass cylinder 4fieet
(I22 cm.) long by 4 inches (10.1 cm. ) in diameter was filled with stones of
approximately'i-inch (1.3-cm.) diameter. Nitrogen that was bubbled up through
the column from beneath displaced oxygen from water entering the column
at the top. The oxygen content of the water leaving the column was regulated
by adjusting the flows of nitrogen and water until the desired oxygen level was
attained. Giass ;rlates placed over the surface of the test tanks reduced oxygen
exchange with the air to a minimum. Oxygen was controlled within -r0.3 mg.
Oz/1.Dnrpnlrtrr.ranroN oF DEATTT
Lobsters were considered dead when no movement of any part could be
detected upon close examination. "Dead" lobsters did not recover from tempera-
ture or salinity deaths when they were returned to the appropriate acclimation
conditions. Thorough inspection of each animal was necessary before death
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could be determined in lorv oxygen experiments since lobsters that are veryweak_ and appear to be dead from the efiects of low oxygen can recover com-pletely within I to 2 hours when returned to acclimation conditions.
ACCLIMATION
Any study of the lethal efiects imposed upon an organism by environmentalextremes must be based on a knowledge of its ability to acclimate, for then itsenvironmental history can be stabilized to yield comparable data. Temperature,salinity and oxygen acclimation for the lobiter are treated below.
Tnupnnerunr Accr,rtranroNAcclimation to a new level of temperature was followed using the same
method as Loeb and Wasteneys (1912) and Brett (1946). A test temperature atwhich average survival time is short at the start of acclimation buf reaches amaximum when acclimation is complete, was selected on the basis of preliminarytests. At completion of acclimation, the average survival time was taken as 72hours, the end point of the experiment, if
-no deaths occurred in the test
temperature. As well, in tests where less than I00% moftality occurred, theaverage survival time was calculated using 72 hours for each of the animalsliving past that time.
Lobsters that had been held at L4.5' C. were transferred to 23.0'C. foracclimation. After exposure from 1 to 31 days, groups of six to ten lobsters wereremoved and their average survival time at 30oC. rvas determined.
One hundred per cent mortality occurred within 8 to 16 hours for all groupsacclimated for 10 days or less. There was a slight increase in average survivaltime during this period. By 24 days, the average survival time had increasedto 72 hours and acclimation was considered complete. The gain in averagesurvival time is shown in Fig. 1.
o o
o 2 4 6 I r O 1 2 1 4 1 6 1 8 2 0 2 2 2 4 2 6 2 8 3 0 3 2 3 4ACCLIMATION TIME IN DAYS
Frc. l.-Gain in average survival time of lobsters hansferredfrom 14.5"C. to 23.0"C. for 1 to 31 days and then tested at30"c.
P e olor 7 07
F
r 5 0
d. aolU)
u.r 3oo
f f 2 0
t o
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Upward acclimation to high temperature (from 14'5' to.23'0"C') requires
u lorrg!, time for lobsters, ab]fi 21'days, than for those ffsh that have been
studied. The bullhea d (Ameiuru,s nebulosus) acclimates in one day when trans-
ferred from 20o to 28"C' (Brett, lg4)'The common mummichog (Funduhn
heteroclitus) reqrrires three days for approximately the same temperature -change(Loeb and Wasteneys, 1912). Fast rates of about one day were found for the
large mouth black bass (Huro salmoid'es) (Hathaway, -1927)'.and the marine
gof;y (Gittichthgs mirabili's) (Sumner and Doudorofi, 1938)' The goldffsh
iCirassl^ a*aius) requires about 6 days for acclimation when the temperature
is raised from 12o to 20'C. (Brett, 1946).The difierences in acclimation rates are partially a reflection of the latent
periods at the beginning of acclimation when there is little change -in tempera-'ture
resistance. flie lof,rter has a 10-day latent period. The goldftsh that ac-
climates in about one quarter the time, has a l7-hour latent period (Brett, 1946)'
The bullhead has no observed latent period (Brett, 1944)'
S er,rwrrv Accr-rnrLrroxAs a test for acclimation, lobsters surviving exposure to reduced salinity in
preliminary lethal experiments were subjected to an even lower salinif. Th"
iurvival of th"t" lobsters was compared with the survival of lobsters acclimatedin nature to 300/oo. The results are presented in Table II.
TasI-B II.-Acclimation of lobsters to reduced salinity.
Number dead at stated hours
HistoryTest No. of
conditions lobsters 1 3 6 9 7 2 2 4 4 8 7 2 9 6
From 30fi6 at 5' C. 7.5700 at 5"-C-.From 75%a at 5' C. {or 96 hours 7.5/66 at 5" C'
1 0 0 0 0 0 0 0 I 3 51 0 0 0 0 0 1 1 1 1 2
From 30fis at 5' C. 3.7/6 at 5" C.From \5f i6 at 5'C. for 96 hours
then 3.7/66 at 5" C.7.5/66 for 144 hours at 5' C.
l 0 0 0 0 0
8 0 0 0 0
5 1 0
o 2 5 -
From130/6 at 17'C. 11.4/66at l7-"-C.From 18.7/6s at 17' Cl. for 168 hours 1.1.4/66 at \7" C-.From 1.5/66-at 17' C. for 168 hours ll '7%o at 77" C.
r 0 0 0 1 - 8 1 0I 0 0 0 - 0 I8 0 0 0 - 0 0
The results are not strictly comparable, since some mortalities occurred
during the initial exposure of the acclimated grouPs. However, the mortalities
were iot greater fhan20% and the selection was not severe. It is concluded on the
basis of the difierent survival of the groups that substantial acclimation had
occurred within four to seven days. Whether or not acclimation was complete
in 4 to 7 days cannot be determined from these results, but in later experiments
acclimations of 7 to 14 days were used.A fast adjustment of iobster blood salinity to external salinity is indicated
by the work of smith (1951). He found that within the ffrst 12 hours of
"*porrrr" to low salinity the lobster blood often became more dilute than the
exlernal medium. This was followed by an increase in the internal concentration
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until at about 24 hours the two concentrations were nearly equal and furtheradjustments were usually small. If these changes are related. to^acclimation, thelatter may be substarrtially complete in the same time.
Other marine animals have been shown to have an increased resistance tolow salinities when acclimated to reduced salinities. These are the prawnPalaemonetes oarians (Panikkar, rg4l), the marine worm Nereis oirens (Toppingand Fuller, 1942), and the oyster ostrea airginica (Loosanofi and smith, ts+s).Loosanoff (1945) found no evidence of acclimation to reduced salinity in thestarfish Asterias forbesi.
OxxcrN AccLrMArroN
As a test for acclimation to reduced oxygen, lobsters were exposed toreduced oxygen for four days. Their survival time in low oxygen was comparedwith the survival times of lobsters that had been held at full oxygen. The resultsare presented in Table IIL
Tasr,B Ill.-Acclimation of lobsters to reduced oxygen.
Acclimationtemp.
Acclimation Testoxygen oxygen
Number dead at stated hours;ten animals in each test.
6 I 7 2 1 8 2 4 3 6 4 8 6 0 7 2 8 4 9 6
0 0 0 0 0 0 0 0 0 0 00 0 0 0 r 4 4 6 7 8 I
' c .ro - lo15-16
,nc.fi,
6.4
mc,./L0.80.8
0 0 0 0 3 3 3 30 0 0 0 0 2 6 6
3.66.4
1.21 . 1
24-2524-25
24-2524-25
2.9o-+
0 . 6 0 1 1 3 7 7 00 . 7 0 3 4 7 1 0 1 0
A definite increase in survival time is apparent for lobsters from reducedoxygen compared to the survival times of lobsters from high oxygen when placedin the same low oxygen test conditions. The exact rate of acclimation cannotbe determined from these results.
Acclimation to reduced oxygen has been shown for ffsh. Young salmon(salrno salar) were shown to acclimate to reduced levels of oxygen ( Nikiforof,1953 ). changes in the rate of respiration of animals reared in dlfierent oxygenconditions were used as an index of acclimation. Shepard (1955) found thatacclimation to reduced levels of oxygen in the speckled trout (Saloelinusfontinnlis) lowered the lethal oxygen level.
PRELIMINARY CONSIDERATIONS FOR LETHAL LEVELS
All the lobsters used in determining the lethal levels of temperature, salinityand oxygen were hard-shelled lobsters of a restricted size range and from onearea. This selection was made to rule out any possible relationships betweenresistance to environmental factors and size, geographic variation, and, themoult cycle. To interpret the scope of results obtained using a restricted stock,it is important to know if such relationships exist. The efiects of these items
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and_the possible efiect of starvation were given preliminary consideration. Theresults are discussed in the following sections.
srzB rN Rnr-erroN:ro Lnrrrer- Lrl'nrs or Trvrprnartmr, seuNrry AND oxycnN
TEMPERATURE' There is a diversity of response reported in the literafure con-cerning size in relation to order of death of animals at high temperattue.Huntsman and Sparks (L924) and B6lehr6dek ( 1935 ) claim thlat resiiance toheat diminishes as size increases. Hart (1952) found a size efiect in only threeof the fourteen species of freshwater fish that he studied. In two of these species,resistance diminished with increasing size but in the third, resistance increased.
sumner and r)oudorofi ( 1gs8 ), Doudorofi (rg4z) and Fry, Hart and walker(1946 ) concluded that there was no correlation between
' size of individual
specimens of the marine goby (Gi,lhchthys mirabilis), the greenfish (Glrailnnigricans), and the speckled trout (salaelinus fontinatls) aid order of deathat high temperature.
^^Tests wer_e performed with lobsters of two size groups, 2L to 22 cm. and 26 to28 cm., to determine if their size is related to death ut ttigt temperatures. Thelobsters were acclimated at goc. and tested by placing te]n of eich size groupin constant temperature baths at 25",26",27" and 2g.C.
For both size groups the temperature that killed 50% of the animals in 48 hourswas 26,5oc. It was concluded that small (21 to 22 cm.) and large (26 to 26 cm.)lobsters frorn the same area and acclimated to the same tem*perature respondidentically to high temperature. Within the range tested, size doel not appreciablyaffect tlie resistance of lobsters to high temperatures.
_ sArrNrry. Bert (1871) concluded that larger ffsh were more resistant toreduced salinity than smaller fish of the same species. on the other hand,Topping and Fuller (L942) concluded from their itudy of fourteen species ofmarine invertebrates that, in general, smaller animals of a particulai specieswere better able to survive in reduced salinities. Quigley (1928) reported n-o sizerelationship in the resistance of dogfish (Squalus suiklegl) to reduced salinities.Experiments were performed to test the efiect of size bn the lethal salinity oflobsters. Lobsters measurinq 16 to 18 and 21 to 28 cm. were acclimated at 15.C.and others measuring 16 t;m and 2g to s4 cm. were acclimated at 25'c.
The data are available in Tables IX and X of Mcleese (1950). In general,they suggest but are not quite sufficient to prove that small lobsters ur-" rrror"resistant to reduced salinities than large lobsters, over the range 16 to 34 centi-metres. At 13'''C., ihe salinity that killed 50% of the lobsters in 48 hours was inter-polated as r2.3/'o for the small and r3.2%o for the large lobsters. At 25oc., thesmaller lobsters were more resistant to 15/oo salinity, but less resistant to Lg/o,salinity: the interpolatedS0% lethal level was 200/oo for the small and lg/.o tor thelarge lobsters. If a size efiect is present, it is small in comparison with most o{the effects to be tested. In any event, in the main exp&iments the sizes oflobsters used were restricted to 23 to 26 centimetres, and efiects of size withinthese limits are negligible.
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oxycEN, size in relation to order of death in low oxygen was not studied.
Thomas (1954) working on the European lobster (Homarus oulgaris)-found
that the oxygen uptake Jf small lobsteri is greater per-unit weight than that for
large lobste.i. fnit" is a possibility then ^th-at
size could be an important factor
when lobsters are exposed^to low o*yg"tt. Size was eliminated as a variable factor
in the lethal oxygeri experiments by"using lobsters of a limited size range (23
to 26 cm. ).
Rasrsr.lNce or LossTEns rnorw DrFnnnrNr Anres ro TEnapBnertlnr, salrNITY AND
OxvcBN
TEMPERATuRE. As discussed previously, lobsters from the Gulf of st. Lawrence
and those from southwestern New Brunswick difier in their life-histories' Most
of the difierences are thought to result from difierent temperature characteristics
of the water in th"r" *eur. Lobsters from both urea, w"i" acclimated at 10oC'
and 14.5"C. The temperature that killed 5070 0f. the lobsters in 48 hours was
28.C. for both groups'acclimated at 10'C. At 14.5'C. acclimation, lobsters from
both areas died in 24 hours when tested at 30'C. The times to death correspond
almost exactly. It was concluded that the thermal resistance of both groups is
identical when they are acclimated at the same tempel"t"t:'-^-
Out of ten species of freshwater fish studied by Hart (1952) only three in
which sub-species u."-r""ogrri"ed showed appreciable geographic differences in
temperature resistance.sALrNrTy AND oxycEN. The resistance of lobsters from difierent areas to low
salinities and oxygens was not studied. In nature, the various areas where lobsters
o"".r1. g"rr"rally'iifier more in water temperature than they -d-o in salinitv and
o"yg"rr-l"u"ls. Sirrce no difierence in temperature resistance could be demonstrated
for lobsters from difierent areas, potSbl" difierences in salinity and oxygen
resistance have been disregarded.
Truprnerunr, ser-rNtry eNn oxycnN Lrrrrer-s or sranwl en'p Fro LossTEns
The standard experimental procedure was to starve lobsters for periods up
to 45 days during conirolled accliimation. There is a possibility that the physiology
of animals givei this treatment would be modified in some wty'^ {or. example'
Hoar and Dirchester (1949) found that the heat tolerance of-goldffsh (Carassius
auratus) was modified when their body fats were altered by feeding the ffsh
special foods.TEMPERATURE. A lethal temperature experiment using starved and fed
lobsters was conducted to deteimine whether starvation altered the lethal
temperature (Mcleese, 1954, Table I). Lobsters were acclimated to 15"C' for
p"riid, up to 57 days. During this time half the lobsters were fed at the rate of
in" porrrr-d of herring per hundred pounds of lobsters per day. The other half
were not fed.During the acclimation period unexplained mortality- reduced the numbers
of animals-in both groups by about one half. Presumably because of residual
debility from this episode, lethal temperatures determined for the survivors were
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less than for other similar groups. However the relative performance of the starvedand the fed is assumed to be indicative of the normal situation.
Ten starved and ten fed lobsters were tested at each temperature of. 26",27",28" and 29"C. The temperatures that killed half the lobsters it 48 ho.trs weteestimated by interpolation to be 27.7" and 27"C. for starved and fed animals,respectively, but the differences rvere not consistent and were not "significant".I concluded that starvation during acclimation in the experiments below couldhave little or no efiect on the lethal temperature level, especially since theacclimation periods used were on the whole considerably less than 57 days(35 to 49 days).
Brett (1944 ) studied the efiect of starvation on the lethal temperature ofthe bullhead Ameiurus nebulosus. There was no appreciable difierence in lethaltemperature after 40 days' starvation, even though the weight-length relationshipsof the ffsh charLged. In tests with lobsters, Wilder (personal communication)found that starvation for periods up to 5 weeks had no signiftcant efiect on theweight of lobster muscle.
sALINrry AND oxycEN. Experiments were started to determine the lethallevels of salinity and oxygen for starved and fed lobsters. Unfortunately, however,unexplained mortality during the acclimation period killed the experimentalstock. Considering that the lethal temperature is the same for starved and fedlobsters, it is assumed in this paper that starvation during acclimation does nothave an effect on the salinity and oxygen lethals.
Mour-rrNc exp RnsrsreNcE To TnupnnarunE, SALrNry eNr OxycsN
TEMpERArTme. The moult period has long been recognized as a critical periodin the life-history of the lobster because of the susceptibility of soft-shelledlobsters to physical damage. As well, the lobster's physiology undergoes changesat this time. These changes could be extensive enough to produce signiffcantchanges in the tolerance of the lobster to environmental extremes.
Five high temperature tests were per{ormed with lobsters shortly afterthey had moulted. Hard-shelled lobsters were used as controls. The results arepresented in Table IV. The average survival time of moulting lobsters is lessthan that for hard-shelled lobsters.
sAr.rNrry. Four low salinity tests were performed with lobsters shortly beforeor after they had moulted. Six of the lobsters were in the premoult condition
Tasl-p IV.--Average time to death at high temperatures of moulting and non-moulting lobsters, acclimated to 15' C. and various levels of salinity and oxygen.
Acclimation level Average time to death
Salinity OxrrgenTest Number of
temperatrlre lobsters l \Toul t ing Non-moul t ing
o c .2828toq a
28
T*3030252520
mg./1.
9 q
o . +2.52.9
mTnutes102219380
400l J i )
nnlnutes> 1860>1753
580640
1080
A
t)
r)
2
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and two had recently moulted. The results are presented in Table V. The moultinglobsters died in a shorter average time than the control lobsters with theexception of the two premoult lobsters acclimated at 20"1o salinity and 2.9m8'Oz/l '
TanrB V.-Average time to death in low salinities of moulting and non-moultinglobsters, accl imated at 15'C. and var iorrs levels of sal in i ty and oxygen.
Acclimation level Average time to death
Salinity OxygcnTest
sal initvNumber of
lobsters Nloulting Non-moulting
mtnutes2 3602 4802 7402 2390
mLnTttes1980630720
> 2880
oxycEN. A low oxygen experiment was performed with 20 lobsters shortlybefore or after they had moulted. They were acclimated at 15oC., 30/o, salinityand 6.4 mg. Oz/1. and tested at 0.3 mg. Oz/1. The average survival time of themoulting lobsters was 750 minutes and that of the controls was greater than1040 minutes.
These tests have shown that moulting lobsters are less resistant than hard-shelled lobsters to high levels of tempeiature and low levels of salinity andoxygen.
In summary; when acclimation histories are similar, lethal levels of tempera-ture, salinity and oxygen are not influenced by size differences, variation inlife-history or short-term starvation. One possible exception is starvation andlethal oxygen levels. The resistance of moulting lobsters to all three factors isless than that for non-moulting lobsters. Consequently, the selection of a re-stricted stock of lobsters for the experiments has limited the possibility of in-terpretation of the results to hard-shelled lobsters.
LETHAL LEVELS OF TEMPERATURE, SALINITY AND OXYGEN
Preliminary experiments (Mcleese, 1950) showed that lobsters can be ac-climated to withstand high temperatures under favourable conditions of salinityand oxygen. Further, the lethal levels of salinity and oxygen vary with tJletemperature.
To understand the efiects of temperature, salinity and oxygen on lobstersurvival as they operate together, the three factors were varied during acclimation
according to a pattern. Lobsters were acclimated to the 27 combinations of threelevels of temperature (5, 15, 25'C.) with three levels of salinity (20,25,30%")
with three levels of oxygen (2.9,4.3,6.4 mg./L). Enough lobsters were ac-climated to each of the 27 combinations to allow for the determination of lethal
temperature, lethal salinity and lethal oxygen levels,All the lobsters acclimated to the 5oC. temperature were caught in the
winter months at temperatures of approximately 3 to 4'C. They were held atleast a week at 5oC., when it was assumed that acclimation was complete. TheIobsters for the l5oC. acclimation were taken in the spring from water which
o,6o n c. /l . 7oo25 2 .9 1125 2 .9 I20 2.9 030 6 .4 13
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varied from 7 to l0oC. and were held at 15oC. for two to three weeks, T]rose forthe 25'C. acclimation were caught in late spring when the water teml,eratureranged from 10 to lI'C. They were held at the surface for two to foul weekswhere the water gradually warmed to 12 to l4oC. After this they were l.eld forthree weeks at2{"C. to complete their acclimation.
After temperature acclimation was complete, at least two weeks were allowedfor salinity acclimation. When salinity acclimation was complete, one week wasallowed for oxygen acclimation. During salinity and oxygen acclimation, tempera-ture was maintained at the acclimation level. The salinity acclimation levelswere maintained during the oxygen acclimation.
The method for determining lethal levels was derived from the constanttemperature tech:niques of Hathaway (L927), Doudorofi (1942), Brett (1944)and Fry et al. (1946). For the lobster experiments the level of each of the threefactors that kills 50% of the animals with 48 hours' exposure was taken as thelethal level. Fifty per cent lethal levels can be measured precisely whereas thedetermination of the highest temperature (or lowest salinity or oxygen) thatjust fails to kill any animals is dificult.
The end point of 48 hours was chosen on the basis of preliminary experiments.These indicated that mortality caused by 48 hours' exposure to high temperaturewas not significantly less than mortality caused by 72 hours. The same end pointwas chosen for salinity and oxygen experiments to correspond with the lethaltemperature experiments. The suitability of this end point will be discussed inthe section on resistance times.
The 507o lethal levels of each of the three factors were derived from thedata in similar ways. To illustrate the method, a sample of the lethal temperatureresults is shown graphically in Fig. 2. The curved line is drawn through points
o lz 25 26 27 2 9 3 0 3 l
TEST TEMPERATURE OC.
Frc. 2.-Percentage mortality at 48 hours for lobsters acclimated at5"C., 15"C. and 25'C. each at 30%o sallnity and 6.4 mg. Or/1. Fifty percent lethal temperatures are shown on each curve.
roo
E B oJ
Ftr9 o o2
-zU+oUJo
2 0
4
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representing percentage mortality in the test temperatures at 48 hours. Thetemperature that would cause 50% mortality can be read directly from the graph.The complete tables from which the lethal levels were determined may bereferred to in manuscript form (Mcleese, 1954).
Uppnn Lnrrrar- TpwBnanrnnsUpper lethal temperatures were determined in four constant temperature
tanks maintained at loC. intervals with salinity and oxygen maintained at theacclimation level. In each case, the four test temperatures usually caused nomortality at the lowest temperature and 100% at the highest. Ten lobsters wereused for each test temperature and mortalities were observed for 48 hours.Observations were taken at hourly intervals during the first 24 hours and everyhour and a half or two hours during the second 24 hours. There were minordeviations from this plan when the condition of the lobsters warranted. Tempera-ture, salinity and oxygen levels were determined at the same intervals.
The upper lethal levels of temperature for each of the 27 acclimation com-binations are presented in the fourth column of Table VL
The27.5"C.lethal temperature for lobsters acclimated to 15oC., 25alo salinityand 2.9 mE.Oz/1. replaces dre original determination of 25.6"C, which is aberranl.
T-lero VI.-Upper lethal temperature levels and lorver lethal salinity andoxygen levels for lobsters acclimated to 27 different combinations oftemperature, salinity and oxygen.
Acclimation conditions
Temp. Salinity OxygenLethal
temperatureLethalsalinity
Lethaloxygen
7aa
20
c)<
30
mc,./l-2.9
6 .42 .9A 2
6.42.94.36.4
' c .20.622.O23.7,.2 422.124.624.025.225.7
a/@
1 1 . 09 .09 .0
t2 .012.49 .2
10.81 1 . 56 .0
mc./1.0.720.770.720.570.510.240.290.330.20
20
otr
30
15
1 D
l,i)
2.94.3o . +2.94.36.42.94.36 .4
27.327.727.827.5"28.228.027.828-228.1
9 .09 .08.2
t0.710.79 .5
10.61 1 . 011.2
0.860.79t .200.800.901.000.660.830.83
20
25
30
25
25
25
, a4.36 .42 .94.36 . 42 .94 .36 . 4
28.525.029.329.025.529.628.729-530.5
I l . D
I t . J
1 1 . I14 .314.814.075.416.016.4
1 7 9
1.581.267 . 1 71.201.601.30r .25t . l 7
" Adjusted level, see text.
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The low reading is accounted for by irregularities in the 26o and 27oC, tests that
were used in its determination. The adjusted value is the average of the lethallevels at 15oC. for 20o/oo and 30o/oo salinity both at 2.9 mg. O2/1.
The whole range of lethal temperatures is not presented in the table. At
acclimation to temperatures below 5"C. and to salinity and oxygen levels below20o/oo and,2.9 mg. O2/I., the lethal temperature would be depressed below 20.6o'C'The latter
"orrditions, however, are probably going beyond the bounds where
temperature alone is operating as a lethal factor into the region where salinityand oxygen may operate as lethal factors.
The-lethal levels of temperature listed in Table VI were analysed statisticallywith the results shown in Table VII.
Tesrp VII.-Results from analysis of variance of lethal temperature levels for,variouslyacclimated lobsters. Double asterisl<s indicate significance at the one per cent level.
Source of variauce Sum of sqLlares Degrees freedom Variance
TotalTemperature acclimationSalinity acclimationOxygen acclimationDiscrepancyTemperature by salinity interactionTemperature by oxygen interactionSalinity by oxygen interactionError
19,60316,326
737936
1,00444320920
332
26222
20/A
48
8163++368**468+*bU
1 1 052
4I
Temperature, salinity and oxygen acclimation all have a highly significant
efiect on the lethal temperature level. By reference to Table VI it can be seen
that the lethal temperature is raised when the acclimation temperature is raised.
It is lowered when the salinity and oxygen acclimation levels are lowered.
Lownn Lrrrnr- S erntrrrrs
Lower lethal salinities were determined in four constant salinity baths,maintained at 2a/oo intervals. The salinity range of 6/oo was chosen to includelow salinities that would cause from 0% to I00% mortality with 48 hours' exposure.The lethal levels of salinity for each of the 27 acclimation groups are listed in
the fffth column of Table VI. These were analvsed statisticallv with the results
shown in Table VIII.
TasrB VIII.-Results from analysis of variance of lethal salinity levels for variouslyacclimated lobsters. Double asterisks indicate significance at the one per cent level,s insle at the hve per cent level .
Source of variance Sum of squares I)esrees freedom Variance
TotalTemperature acclimationSalinity acc!imationOxygen acclimationDiscrepancyTemperature by salinity interactionTemperature by oxygen interactionSalinity by oxygen interactionError
16,3348,8472,682
9063,899r,8771,,052
204766
t r ADa**
134148453*195469*2635 196
26222
204448
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_ Jemperature and salinity acclimations have highly significant efiects on thelethal salinity level. In addition, oxygen acclimation arrd a temperature bysalinity interaction are signiffcant.
The directions of these efiects can be seen by reference to Table vI. Thelethal salinity level is raised by an increase in the level of temperature acclima-tion and by a decrease in the level of oxygen acclimation. A decrease in thelevel of salinity acclimation lowers the lethal salinity level. The significantinteraction between temperature and salinity means that the efiect of salinityacclimation is not always the same, but depends on the temperature at whichlobsters are acclimated.
Lownn Lerrrar, Oxycpx Lrvpr-s
Lower lethal oxygen levels were determined in four constant oxygen bathsmaintained at intervals of approximately 0.3 mg. 0z/1. The range of oxygen,about 1.0 mg. O2/1., was chosen to include low oxygens which would cause from07" to I00% mortality with 48 hours' exposure.
The lethal oxygen levels for each of the 27 acclimation groups are listed inthe sixth column of Table VI. Tlhese were analvsed statisticallv with the resultslisted in Table IX.
TeerE IX.-Results from analysis of variance of lethal oxygen ler.els for variously accli-rnated lobsters. Double asterisks indicate signilicance at the one per cent level.
Source of variance Sum of squares Degrees freedom Variance
TotalTemperature acclimationSalinity acclimationOxygen acclimationDiscrepancyTemperature by sal in i ty interact ionI empera. ture by oxygen interact ion
5alrnr ty by oxygen interact ionError
22,00516,9892,809
A A
2,r33r + l679153
1 , 1 5 4
26222
204148
8494**1404xr
J O
707J O
77038
711
Oxygen acclimation does not have a significant efiect on the lethal oxygenlevel. Temperature and salinity a.cclimation both have a highly signiffcant efiecton the lethal oxygen level. By reference to Table VI, it can be seen that thelethal oxygen level is raised by an increase in the level of temperature acclimationand by a decrease in the level of salinity acclimation.
Unless precautions are taken there is danger of carbon dioxide accumulationin experiments involving low levels of oxygen. This was avoided in the presentexperiment by driving ofi excess COz in acclimation, lethal temperature andsalinity tanks, using the method of Thomas (1954). For lethal oxygen tests, lowlevels of oxygen were obtained try passing the water through a nitrogen atmo-sphere, which prevents increase in COz content.
Lownn Lrrrrer- Telrppnerunns
Resistance to cold is lost to some extent as animals show a gain in heattolerance with acclimation to high temperature (Fry et al., L946). Approximate
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lower lethal temperatures have been established for lobsters acclimated at 17.0'and 27.5'C. The results are presented in Table X.
All animals died within 24 hours in the I7.0"C. acclimation group. In upperlethal experiments, temperatures that killed all the animals in 24 hours wereabout 1.5"C. higher than those which killed half the lobsters in 48 hours. Using1.5"C. as a correction factor, the lower lethal temperature for 17.0'C. acclimated
Tanr-o X.-Mortalities of lobsters acclimated to 17.0' and 27.5" C., when tested at lowtemperatures.
Number dead at stated hoursAcclimationtemperature
Test Number oftemperature specimens q l1 8
" c .77.027.527.5
" c .0.34.86.0
7 0 0 0 6 71 0 0 1 2 4 5 6 67 0 0 0 0 0 0 0
lobsters was estimated to be 1.8'C. The 50% lethal temperature in 48 hours forlobsters acclimated to 27.5"C. was estimated to be 5"C. Although these lowerlethal temperatures are estimates, they indicate the approximate lower limit ofthermal tolerance.
RrsrsreNcn Trurs
An animal can withstand exposure to levels of an environmental factorwithin the lethal zone provided the exposure does not exceed certain finite periodswhich depend on the level of the factor. The time taken to catse 50% mortalityat a lethal level of the factor is usually taken as the resistance time. When thelogarithm of resistance times in a series of lethal levels of the factor is plottedagainst the level of the factor, straight lines can be drawn through the resistancetimes.
Fry et al. Q9a6) discovered a break in the slope of lines relating medianresistance times and upper lethal temperatures. The temperature at which thebreak occurred was iust below the level at which pst 50% mortality occurred( the "incipient lethal level" ) . At temperatures below this, median resistance timesrapidly approached infinity. As Brett (1952) points out, the discovery of thisbreak allows the selection of the proper duration of experiments if precisemeasurement of the incipient lethal level is desired.
Some of the data for the lobster lethal experiments have been plotted in asimilar fashion to determine the relationship between the 48-hour lethal levelobtained from the experiments and the incipient lethal levels ( Mcleese, Lg54).As an example, thermal resistance lines for lobsters acclimated to three levelsof temperature (5o, 15o and 25'C.) each at three levels of salinity (20,25 and30%o) and one level of oxygen $.a mg./|.) are shovvn in Fig. 3,-together with thecorresponding lethal levels of temperature from Table VI. The lethal levels canbe used here since bv deffnition thev cause 50% mortalitv in 48 hours (2880
minutes ).
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Two of the resistance lines at soc., and probably the third, appear to havebreaks after which the slope becomes zero. For these, longer exposure to theexperimental conditions would not lower the lethal level because the slope ofthe lines is zero and the observed lethal level probably approximates the trueincipient lethal level. The lines at l5o and 25oC., however, do not have breaksby 48 hours and exposure for longer times would possibly lower the lethal level.
(J
trJ Z O
F
EU
9, ,U
s ' c
2 4
200 300 400 500 1000T rME ro so% von re l t rY (MTNUTES)
Frc. 3.-Temperature resistance at various lethal levels of temperaturefor lobsters acclimated to 5, 15 and 25"C. each at three levels of salinity(20, 25 and 3O%") at 6.4 mg. O2,/1. Black circles are lethal levels fromTable VL
A similar situation rvas found with salinity and oxygen resistance lines.
Incipient lethal levels were obtained for some of the acclimation groups butnot all.
The results of the experiments do not apply to periods longer than 48 hours
since incipient lethal levels with the understood indefinite exposure times were
not obtained in all cases.
lfr-rrvere, Maxrvuv eNp Mrxrlrurr Lrrrrer- LBvrrs
To determine the extremes of temperature, salinity and oxygen levels which
can be tolerated by the species, the ultimate and maximum lethal temperature
levels and the minimum lethal levels for salinity and oxygen that could be at-
tained through acclimation were estimated.
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Fry et al. (I9aZ) defined the ultimate upper incipi,ent lethal temperatureas the temperafure beyond which no increase in the lethal temperature resultsfrom further increase in acclimation temperature. They were working with afreshwater fish which was not exposed to varying levels of salinity. For thelobster, acclimation to reduced levels of salinity and oxygen both lower thelethal level of temperature. Under such conditions the ultimate lethal temperaturecannot be attained but the lethal temperature does reach a slightly lower maxi-mum value depending on the salinity and oxygen conditions. These are termedmaxirnumlethalleuels in this paper. Similarly, for lower lethal salinity and oxygenlevels, theminimumlethalleael is used as the name of the lowest lethal in caseswhere the ultimate lower lethal level cannot be attained.
TEMrERATURE. Fry et aI. (L942) developed a method for describing thethermal tolerance of fish as an area instead of expressing it in terms of points.The ultimate upper lethal temperature can be determined from this method ofplotting thermal tolerance.
The tolerance area is bounded above and below by lines relating the upperand lower lethal temperature to the acclimation temperature; it is boundedlaterally by two perpendicular lines, one erected at 0oC. acclimation and theother at that acclimation temperature which is equal to the ultimate upperlethal temperature.
In their work with freshwater animals, Fry et al. (1942) did not study thetolerance of animals below 0oC., the freezing point of fresh water. Similarly, itis not practical to study the tolerance of marine species below the freezing pointof sea water. The main interest in a tolerance diagram for the lobster in this paperis its contribution to the determination of the highest levels to which the upperlethal temperature can be raised for the different acclimation groups. For thisreason, no attempt has been made to project the tolerance diagram beyond 0'C.
Figure 4 illustrates the data for lobsters acclimated at 300/oo salinity. Thelower lethal temperatures were taken from Table VII. The ultimate and maximumlethal temperature levels are indicated by arrows.
Similar diagrams have been drawn for lobsters acclimated at 250/oo and 200/"osalinity. They are not reproduced here but the maximum lethal temperaturesderived from them are shown in Table XI.
The area within the tolerance diagam (tolerance zone) can be expressednumerically as degrees centigrade squared and is a relative measure of thetolerance of the animal. The tolerance zone for lobsters acclimated to 30%osalinity and 6.4 mg. Oz/1. has an area of 830 in this unit. Areas for four ffshesare computedby Brett (1944) and Fry et al. (L946) as follows:
Species
Goldfi sh (Caras s,ius auratus)Bullhead (Ameiurus nebulosus)Greenfi sh (G irella nigrican s)Speckled trout (Saltil i nus lont i nali s\Lobster
Tolerance Originaldata
l22O Fry et a1.,79421162 Brett, 1944800 Doudoroff,1942625 Fry et o1,.,1946830 This paPer
The thermal tolerance of the lobster, in this area unit, is intermediate among
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tfF
!
r
:
'o o.. . ,uo-,o*
'?."r."^.u*. "t l
25 30
Frc. 4.-Thermal tolerance of lobsters acclimated to 30%o salinity and 2.9, 4.3 and 6.4 mg. Or/LArrows show ultimate and maximum lethal temperatures.
those for freshwater ffshes, and is approximately equal to that of the marine
greenffsh.Comparisons of the numerical values for tolerance areas can be misleading
because they do not describe the shape of the diagram or the relationshipsbetween acclimation and lethal levels.
The zone of tolerance as shown for the lobsters includes an area that is
lethal to 50% of the animals in 48 hours by virtue of its derivation from lethal
levels. The zone in which no animals will die is somewhat smaller.sArrNrry. The lethal salinity data can be plotted as salinity tolerance diagrams
similar to that for temperature although there are some major differences between
the two. In a marine environment, reduced rather than increased salinity is most
likely to operate as a lethal factor. Therefore, upper lethal salinity levels are not
-x - - - ' - -
o- 6.1 mqh/t . oxv4cno- t.t nlm/t. o'jg"cn\- 2.9 mgm/{. oxytcna - p a e L r ' u r N n q i - E X e E R T M E N T S
( t 9 4 9 - r g s c J
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shown on the tolerance diagram. As well, the data are somewhat restrictedbecause acclimations between 20fu0 and 30/'o only were studied. Dificulties areexperienced in acclimating lobsters to salinities much below 20a/oo, especially atreduced levels of oxygen and high temperatures. Salinities much above 30%ocannot be obtained unless the salinity is artiffcially increased.
The tolerance diagram for 15oC. lobsters is shown in Fig. 5. If the lethallines are projected back to meet the diagonal construction line, they meet the linebetween 2 and 3o/oo salinity. Lobsters cannot be acclimated to these low salinitieswhich are lethal and the projected line must have a point of inflection at whichthe line becomes parallel to the abscissa. Further acclimation to reduced salinitybeyond the point of inflection would not alter the lethal level. Since the actualinfection points are not known, the Iethal level for lobsters acclimated to 20%osalinity has arbitrarily been chosen as the inflection point. In other words, it isconsidered that the lethal levels lor 20%o saliniW acclimation are the minimumlethal levels.
a-6.4 ngm/. oxlg.hO-4.3 hgh/l. oxy7en.x-2.9 mfn/. orltai-
o 5 'o ,o.,*, ' -1 ^.. . ,ui lon o/oo
25
Frc. 5.-Salinity tolerance of lobsters acclimated to l5'C. and 2.9,rng, Or/1. Arrows show minimum lethal levels.
:i zo
Iz).{). L l 5I
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4.8 and 6,4
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The figures for 5o and 25oC. acclimation are not reproduced since theminimum lethal salinities can be obtained directly from the 20a/ao acclimationsin Table VI.
oxycEN. The lethal oxygen results are presented as tolerance diagrams inFig. 6. The slopes of the lethal lines are not consistent in direction and statisticalanalysis of the data has shown that oxygen acclimation does not have a significant
- : o % " s a r r r v
as oc-x - . - . - . x - - ._ ._ - ._ r
- , - - - -4 - - - - - - - - O
5
Frc. 6.-Oxygen tolerance of lobsters acclimated to 5o, l5o and 25'C. each at 2.9, 4.3 anil6.4 mg. Or/1.
effect on the lethal oxygen levels. Therefore, the points of intersection where thelethal lines meet the diagonal construction line will not give saUsfactory minimumlethal oxygen levels. In this case, the best estimate of these levels is obtainedby using the average of the data contributing to each line.
Oxygen acclimation was shown to exert a signiffcant effect on the lethaltemperature and salinity levels, but not on the lethal oxygen level. It is possiblethat the efiect in the two former cases was not an acclimation efiect but simplythe result of the low levels of oxygen prevailing at the time of the tests.
In summary, the maximum lethal temperatures and the minimum lethalsalinity and oxygen levels have been assembled in Table XI. The only "ultimate"
Tenr,p XI.-X4aximum upper lethal levels of temperature, and minimum lower lethal levels ofsalinity and of oxygen, for lobsters acclimated to 27 combinations of temperature, salinity andoxygen. The temperature indicated by an asterisk is the ultimate lethal level.
Temperature Salinity Oxygen
Acclimation Ultimate and Acclimation Minimum Acclimation MinimumSalinnt-o-yge" maximum levels Temp:-O-yge" level Te-pl$lfiitt level
7aa303030252525202020
" c .32.0*30.229.030.530.129.830.129.629.0
hot7.211 .5rt.78.09.09.0q n
1 1 . 5I I . 5
o c .252525I t
I O
15
E
mc./1.6.4
t oo .+A '
2.96.4A D
2.9
nne./1.6 .44.32 .96 .44 .32 .96 .4n t
2.9
" c .252525151515
o5t
7* tnC./I.30 r.2425 1.3220 r.5230 0.7725 0.9020 0.9530 0.2725 0.4420 0.74
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level which could be estimated was that for temperature, which is 32.0'C. underoptimum conditions of 3}/oo salinity and 6.4 mg./|. of oxygen.
BOUNDARY OF LETHAL CONDITIONS
A three-dimensional diagram relating the lethal levels of the three factors
has been prepared (Fig. 7) using the ultimate, maximum and minimum lethal
levels shown in Table XI.
.-2a
t \ 2 4
\ 2 n
st-..-t? z
l - - . #\ a
,u .,)A- /
, t u . ,t - /z 1 2 ,1 /
A 6,4 / ,(,Q2j\ l-=---'* :' ,6 ,( 7->---r--._o"**oi,"f ( ,/ 2
68
l / " i , / : o
{ i ,,(;:"+"| ./--o.+*
lZ]:+'
Frc, 7.-Diagram of the boundary of lethal conditions for lobsters forvarious combinations of temperature, salinity and oxygen.
T-region in which temperature alone acts as a lethal factorS-region in which salinity alone acts as a lethal factorO-region in which oxvgen alone acts as a lethal factor
Temperature and oxygen were plotted against each other on the horizontal
plane and provided the base for the model. Templates, scaled to represent theminimum lethal salinities were erected on this base. In all, six templates wereerected, three on the oxygen lines at 2.9, 4.3 and 6.4 mg. Oz/1. and three on thetemperature lines at 5, 15 and 25"C. The smooth curves of the templates formthe boundary between the lethal zone below and the tolerance zone above.
The diagram deffnes a segment of the complete zone of tolerance for thelobster to combinations of temperature, salinity and oxygen. It is bounded onthe top and on the front by the salinity and oxygen contents of sea watet, te-
spectively. Experiments with concentrated sea water and supersaturated oxygencontent would be necessary to extend the scope of the diagram into these regions.
The diagram could also be projected toward the right to the freezing point
of sea water.
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The diagram provides a method for assessing the lethal efiects of any one oftemperature, salinity or oxygen with respect to the other two factors. Thesteep slope of the boundary surface from 25oC. up to 32'C. and from approxi-mately 1.4 mg. Oz/7. to lower oxygen values is perhaps the most striking featureof the diagram. This indicates that high temperature and low oxygen eachaffect the survival of the lobster within a limited range only. The flat areas inthe diagram are areas where one of the factors only is operating as a lethalfactor. The flat area S, for oxygen values between 6.4 and 2.9 mg. O2/1, attemperafures between 5o and 22"C., probably represents an area where tempera-ture and oxygen conditions exert little stress on the organism and death isprimarily due to low salinity. A similar situation prevails for oxygen values of1.1 mg. Oz/7. and less between 5 and 21'C. (area O). Here the salinity valuesare between 16 and 300/oo and death is caused by low oxygen. The area T, foroxygen values between 6.4 and 2.9 mgm. Oz/1., at temperatures between 28 and30oC,, probably represents an area where high temperature is the lethal agent.
The area where the temperature and oxygen surfaces meet depicts an areaof mixed lethal effects of high temperature and low oxygen. In addition, thereis a mixed lethal effect of temperature and salinity at approximately 25'C. athigh levels of oxygen, and a mixed lethal efiect of salinity and oxygen atapproximately 2.9 mg. Oz/1. at low temperatures. The picture of mixed lethaleffects is further complicated in the region where the areas T, S and O cometogether. This is an area where temperature, salinity and oxygen all combine toexert a lethal effect.
The diagram was constructed from ultimate, maximum and minimum lethallevels and as such defines the zone of tolerance for lobsters fully acclimatedto the conditions of the experiment. The tolerance of lobsters not fully acclimatedto the experimental conditions would be somewhat less than shown by thediagram. In addition, the surface is based on 50% lethal levels and thereforeactually includes a small part of the lethal range.
The information shown in Fig. 7 can be recorded in a more convenienttabular form. The salinities at whiJh half the lobsters will die in 48 hours havebeen read from the model for particular combinations of temperature and oxygenand are listed in Table XII.
TesI-B XII.-Lethal salinities of lobsters for various combinations oftemperature and oxygen. When lobsters are grad'ually subjected toeach of the combinations indicated, their average mortality is 50 percent .
Temperature " C.
Oxygen
mc./1.1.02.03.04.05.06.0
18.0
1 1 . 6tt.41 1 . 09 .8
2t .o72.610.4
0 7y. l )
8.8
20.112.o9 .69 . 48 .88 .4
15.0tt.2tI.277.217.2
30.b20.417.816.4
Salinity, %o22.01,7.29.09.08.88.4
30.012.09.39.39.39.3
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The table is useful in determining the cause of death of lobsters in com-mercial holding units. Such informatin is of value in providing a basis foradvice toward the improvement of holding conditions.
A complete description of the lethal effects of an environmental factor re-quires a statement of the acclimation histories of the animals since the lethallevel may vary with acclimation. Where three factors of the environment areat extreme levels at the same time, the lethal levels for each factor are relatedto the various acclimation levels of all three {actors. It is impossible. therefore. todepict the combined lethal efiects of the three factor, .rri.rg the data in theiroriginal form. The problem has been resolved by deriving ultimate and maximumor minimum lethal levels. At these points the acclimation levels are equal to thelethal levels. The acclimation levels can, therefore, be eliminated from a graphicpresentation and the combined lethal effects of the three factors can be illus-trated in a single figure. Such an illustration also represents the extreme con-ditions to which the animal can be acclimated.
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