Metabolic rate, temperature acclimation and resistance to high temperature of soft-shell clams, Mya arenaria, as affected by shore level

Camp Bmhem. Phym.. Vol 61 A. pp. 433 to 43X 0 Pwgomo,, Press Lfd 197X. Prmrd ,t, Grrur Brrtou,

METABOLIC AND

RATE,

RESISTANCE

SOFT-SHELL

TEMPERATURE TO

CLAMS,

HIGH

MYA

ACCLIMATION

TEMPERATURE

ARENARIA,

BY SHORE

OF

AS AFFECTED

LEVEL

GARY ANDERSON* Department

of Zoology,

University

of Maine,

Orono,

ME 04473, U.S.A

(Received 15 November 1977) Abstract-l. Metabolic rates, temperature acclimation patterns and mortality resistance at high temperature were determined for Mya arenaria collected in summer from four beach levels at Blue Hill Falls, ME. 2. For a clam of intermediate size, metabolic rate is highest at the high intertidal level and lowest at the low intertidal level; subtidal and mid intertidal clams exhibit intermediate rates. 3. Bivalves from the low intertidal level exhibited near-complete (Precht type 3-92.4”/,) metabolic compensation to temperature change while those from mid and high intertidal levels showed partial compensation (Precht type 3-61% and 32.5%. respectively) after 2 weeks. 4. Subtidal clams exhibited supraoptimal compensation (Precht type 1); warm acclimated clams from this level were metabolically temperature independent over the range of l&25 C. 5. Subtidal and high intertidal clams are better able to withstand high temperature (25 C) than clams from low and mid intertidal levels

INTRODUCTION At a given location, intertidal organisms are subjected daily and seasonally to dramatic changes in physicochemical characteristics of their surroundings. The nature and degree of environmental change to which

an organism may be exposed is largely dependent on its intertidal height. Although some species are able to modify emersion period lengths behaviorally by burrowing into the substrate or migrating down the beach face as the tide recedes (review by Newell, 1977), many intertidal organisms are sessile or live in restrictive burrows and are unable to evade adverse environmental conditions. The latter frequently rely on physiological strategies to withstand the environmental variations characteristic of the intertidal zone (reviews by Segal, 1961; Vernberg & Vernberg, 1972; Newell & Bayne, 1973; Bayne, 1976; Newell, 1977). Numerous interspecific (e.g. Southward, 195%; Read, 1962; Barnes et al., 1963; Davies, 1966; Micallef, 1967) and intraspecific comparisons (Southward, 1955b; Segal, 1956; Davies, 1965, 1966, 1967; Pickens, 1965; Markel, 1974) have established that various physiological phenomena may be influenced by organisms’ heights on tide-influenced shores. Generally, these have demonstrated depressed functional rates for high shore animals compared to rates for animals living at low shore levels. However, relatively few studies have evaluated the degree to which physiological compensation for temperature change may be influenced by an organism’s intertidal location. Among these are the studies of Segal (1956), Pickens (1965). Davies (1966, 1967), and Markel * Present address: Department of Biology, University of Southern Mississippi, Southern Station, Box 353, Hattiesburg, MS 39401, U.S.A. 433

(1974) which deal with acclimatization in high and low shore mollusc populations. Except for the latter study, in which metabolic rate-temperature response curves for high and low shore populations were simply compared, these investigations demonstrated that the differences noted between physiological rates of high and low shore populations were phenotypic, since transplantation of low shore animals to the high shore habitat (or vice versa) resulted in acclimatization. Although Markel (1974) demonstrated that heartbeat frequency of Acmaeu limatu/u shows partial temperature acclimation, he did not compare acclimation rates of high shore animals to those of low shore animals in the laboratory. Hence, to date no workers have attempted to determine under laboratory conditions if shore level affects the ability of sessile intertidal molluscs to alter physiological rates in response to changes in environmental temperature. The objectives of the present study are to determine if Mya arenaria from four shore levels vary from one another with respect to metabolic rate, temperature acclimation ability and resistance to high temperature stress.

MATERIALS Al\iD METHODS One hundred Mya urenaria were collected at each of four levels of a rocky beach at Blue Hill Falls. ME, in July 1975. The lowest (subtidal) collection level was 10 in. (25.4cm) below MLW and the vertical distance between each consecutive level (low intertidal, mid intertidal and high intertidal) was 2ft (61 cm): the mean tidal range at the collection site is 10.1 ft (3.08 m). The annual mean sea water temperature at this habitat averages 5-10-C; water temperatures higher than 2O’C are exceedingly rare. The sea water temperature and salinity were 2o’C and 30”(,,, S, respectively, at the time of collection.

434

GARY

Oxygen uptake rates of 27 bivalves from each shore level were measured at 20-C within 2 days of collection. Determinations were made over a 2 hr period using Warburg constant volume respirometers following established manometric procedures (Umbreit et (II.. 1964). Clams (size range = 0.06 1 1.038 g dry tissue weight) were placed singly in 140 ml reaction vessels containing 10 ml of Milliporea filtered 30”,,,, S sea water: 0.2 ml of 2011,, KOH was placed in each flask’s sidearm as a CO, absorbant. During the I-hr equilibration period preceding every run clams were generally observed to extend siphons and ventilate. which resulted in circulation of the water within the respirometer flasks. Hence. flasks were not shaken during determinations. After a run’s completion the tissue was carefully scraped from each clam‘s valves, dried to constant weight at 80 C and weighed to the nearest l.Omg. The oxygen consumption rate (VO,) was then calculated for each animal and expressed as ml OJhr/g dry flesh weight. Subsequently. all rates and weights were rectified (using logs) and linear regression analyses performed for each group to determine the u and h’ values in the allometric equation y’ _ o.yh

(Davies,

1Y66)

where I” is the V02, X is the body weight and a and h’ are fitted parameters. Each of the four sets of constants + 95”,, confidence Intervals were compared wtth one another. These comparisons indicated that the regression equations were not signi~cantly different from one another. Hence. the data for all animals were pooled and a common regression equation computed. Thirty-two of the animals remaining from each level were selected for acclimation. All were 3-4cm long (0.2-0.6 g dry tissue weight. .z = 0.35 g). Half were placed in a large bowl and maintained at 10 C for 2 weeks (CA) and half held at 20‘ C for 2 weeks (WA). During acclimation the sea water in the bowls was replaced daily wtth recently collected 30’:,,, S unfiltered sea water. Microflora present in the sea water served as the bivalves’ food during acclimation. Following acclimation. oxygen consumption rates were measured manometrically at five experimental temperatures (ET) for CA and WA clams from each level as follows: each acclimation group was divided into two subgroups of eight clams. Rates for clams in one subgroup were measured at 10, IS then 5’C (clams were held for 48 hr at the acclimation temperature between the latter two runs). Rates for bivalves from the second subgroup were measured at 20 and 25’C. Subsequent to oxygen uptake measurements dry weights were determined for ali animals. Since oxygen uptake rate had already been shown to be size dependent. all rates were corrected to correspond to values for clams weighing 0.35 g dry flesh wt. The regression equation derived as described previously was used to compute weight-adjusted values. Mean V02 and standard deviation were then computed for each treatment group (CA and WA) at each ET and beach level. Statistical comparisons of means were made using Duncan’s new muitiple-range test at the S’,, level (Steel & Torrie, 1960). Metabolic rate-temperature (R-T) curves for CA and WA animals from each level were plotted and thermal acclimation patterns classified using the schema of Precht (1958) and Prosser (1958). In addition, for animals exhibitTable

Shore

1. Regression

High intertidal Mid intertidal Low intertidal Subtidal

log tog tog log

Y’ Y’ Y’ Y’

= = = =

ing Precht’s type 3 pattern (partial capacity adaptation-all three intertidal groups), I determined the percentage compensation for clams from each level as follows: CA WA,, = --tox 100, WAz, - WA,0 where CAlo is the mean rate of oxygen consumption measured for the cold acclimated clams at 10°C. WAlo is the rate for the warm acclimated clams at 10°C. etc. Hence, I considered Precht’s type 4 capacity adaptation to be o”,, compensation and type 2 to be 100% compensation. Finally, median survival times were determined for small groups 0% = 20) of clams from each level during chronic exposure to high temperature. This experiment was begun the same day clams were collected. Bivalves of similar sizes were kept together in bowls containing sea water (25°C. 3O”,,,,S) which was replaced daily. Bowls were checked twice daily and dead clams removed. Gaping clams were considered to be dead when they had lost the ability to close the valves following tactile stimulation of the mantle and siphon. Median survival time was determined by graphical interpolation from the mortality-time curve plotted for each group. “,, compensation

RESUETS Regression equations showing the relationship of metabolic rate to body weight for MUJ a~~~~r~~ from each beach level sampled are given‘in Table I. The regression equation for pooled data is:

log Y’ = -0.98

-0.74 - 1.03 -0.92 - 1.23

log log log log

X x X X

- 0.78 - 1.04 - 1.03 - 1.08

log x - 0.98

(1)

Equation (I) was used to correct metabolic rates for CA and WA clams to correspond to expected values for clams weighing 0.35 g. Figures lL4 show metabolic rate-temperature (R-7’) curves for CA and WA M. arena& Curves for all intertidal CA groups exhibit translation to the left and clockwise rotation (Prosser’s Type IV A, 1958). However, only for clams from the low intertidal levei did cold acclimation result in a significant (P ~0.05) compensatory increase in oxygen uptake rate {Fig. 3 {ti, 5°C 10°C). Comparing acute and acelimated oxygen uptake rates of CA clams at 10 and 2O’C to rates for WA clams at the same temperatures, I have concluded that the high intertidal, mid intertidal and low intertidal groups exhibit 32.5, 61 and 92.4”;, capacity adaptation, respectively (Precht’s Type 3, 1958). The temperature acclimation pattern exhibited by subtidal MJU arenaria (Fig. 4) does not conform to any of those described by Prosser (1958). Nevertheless, one could conclude that Precht’s Type 1 (..supraoptimal”) compensation is exhibited because the metabolic rate for the CA group at 10°C is greater than the rate for the WA group at 20°C. However, Precht et (II. (1973) have warned against using this

equations showing relationship of VOz (ml 0,jg dry weightjhr) flesh weight) for Myu armaria from four beach levels Regression equation

level

ANDERSON

N

R

95% Cl of h’ (k)

20 23 22 22

- 0.86 -0.96 - 0.93 - 0.90

0.22 0.14 0.17 0.27

to body size (g dry

95”/, CI Predicted VO, for of log a( +) 0.35 g M. urenaria 0.15 0.11 0.12 0.19

0.36 0.27 0.25 0.30

Metabolic rate. temperature acclamation and resistance to high temperature scheme when CA and WA curves intersect between the two experimental temperatures. as they do in this case. As demonstrated in Fig. 4, the WA treatment group exhibits metabolic independence to increasing temperature over the range of lO-2SC: the CA group has a significantly lower rate of O2 consumption 5-C and a significantly higher rate of consumption at 15 C than the WA group (P < 0.05). Table 2 shows median survival times during exposure at 25 C for groups of M. urenaria from each level. Although the sample sizes tested were small (N = 20). the results indicate that clams from the high

435

intertidal and subtidat levels can withstand high temperature longer than clams from the two intermediate O-

o-

IOO-

k

0.50

-

; P

0” E

/ ;

v A

_--

,”

0.10

-

:

9

0.05

1’ 1’ ,’

o-

-

5-

---ct--

-

--

I

CA

I

I

5

IO

CA -C---

I

I

5

IO

WA

I

I

I

15 Temperature.

20

‘C

25

Fig. 3

WA

I

I

I

20 “C

Tempe:~+“re,

25

Fig. I.

0.05

0

CA

--C+--

WA

Temperature. Fig. 2.

-

1

1 5

‘C

_

CA

-Q--

WA

I

I

IO

15

I

I

20

25

Temperature, “C Fig. 4. Figs. 1-4. Relationship of V02 (in ml 0,/g dry flesh wtjhr) to temperature for cold and warm acclimated groups of Mya arenoria from high intertidal (Fig. l), mid intertidal (Fig. 2). low intertidal (Fig. 3) and subtidal (Fig. 4) levels of a rocky beach. Arrows in Figs. 3 and 4 indicate experimental temperatures at which the mean VOz of CA bivalves is significantly different from the mean VO, of WA bivalves. (Data expressed as mean YOz _+ SD.).

436

GARY ANDERSON times during exposure at 25 C mwuriu from four beach

water during metabolic rate determinations in the present study. Hence, rates recorded here may represent combined aerial and aquatic respiration. If high level M. arenaria are able to utilize atmospheric Median survical time oxygen more readily than low shore M. arenaria. then Level (hr f 12) the trend seen here is not surprising. The metabolic rate predicted for a subtidal M, High intertidal 32X urenariu of average size (Table 1) is substantially Mid intertidal X5 greater than that for either a low or mid intertidal 7X Low intertidal SubtIdal 25X clam and is not as low as one would predict from the trend seen for clams from intertidal levels. Such a rate cannot be explained in terms of aerial respiraDISCL!SStOi\ tory adaptation. However. a possible reason for the exceptional results observed for subtidal M. ~treffuri~ Of the metabolic rate-body weight regression equabecomes apparent when one considers the temperations in Table 1, none was significantly different than ture regimes to which bivalves at the study area are any other with respect to the constants log n and exposed in summer. For example, during low tide at b’. Nevertheless, based on the equations one would midday I measured air and substrate temperatures predict that an intertidal M_ru arenaria of average size (0.35 g dry tissue wt) living at the highest beach level as high as 32 and 26-C respectively: the water temwould have a higher metabolic rate than one from perature was only 2o’C. (Note: metabolic rates were measured at 20 C.) As the tide recedes. the body temthe middle level. The latter would have a higher rate than one from the low intertidal level. These concluperature of intertidal clams increases somewhat, sions are in contrast to those of previous workers although probably not as dramatically as the body who have noted in intraspecific comparisons that temperature of high shore limpets (Davies, 1970) physiological rates of organisms from low shore levels because M. areftaria remain several centimeters below tend to be greater than those of organisms from the surface of the substrate whereas Pare& spp, higher shore levels when rates are measured at a comremain on exposed surfaces. The length of exposure mon temperature. Such physiological differences have and maximum body temperature attained by bivalves been demonstrated in studies of heart beat frequency depend on height above MLW. In contrast, subtidal of Acmaea limutula (Segal, 1956: Markel. 1974) and bivalves are exposed to rather constant cool temperaMytilus ccrl@wiunus (Pickens, 1965) metabolic rate tures. Hence. intertidal bivalves are warm acclimated of Pate//a rulgrrta in summer (Davies, 1966): cirral relative to subtidal ones. Warm acclimated animals beat frequency of Elminius modestus. Balanus frequently exhibit lower metabolic rates than cold bu~u}~oif~e.~.and Chthl~mu1u.s steilatus (Southward, acclimated ones at a given temperature (review by 19556) and C. dolli (Southward, 1964) from several Prosser, 1973). Therefore the results for comparisons locales. Similar results have been reported for some between the subtidal and either the mid or low interinterspecific comparisons-species from low shore tidal group are not surprising. Although the metalevels often have higher physiological rates than bolic rate predicted for a bivalve from the high interspecies from higher shore levels [e.g. Barnes & Barnes .tidal level is higher than that for one from the sub(1959) and Southward (1955~) for barnacles; Micallef tidal level, this may be due to an enhanced aerial (1967) for aquatic respiration of trochids]. Of course, respiratory ability for the former, as discussed previously. the results of these interspecific comparisons may be due to genotypic physiological differences rather than As shown herein (Figs. l-3) intertidal M,ra arrnuria adaptation to habitat. respond to temperature change in a manner not unlike that demonstrated for many organisms (e.g. It is not clear why low shore organisms generally reviews by Vernberg 6t Vernberg. 1972: Precht et al.. have higher functional rates than high shore 1973: Wieser, 1973: Prosser, 1973). Rate temperature organisms. Southward (1955b) maintains that the dif(R-T) curves for cold acclimated (CA) clams are ference he noted between functional rates of high and low shore barnacles could be explained in terms of translated to the left and rotated clockwise relative to curves for warm acclimated (WA) clams (Presser relative growth rates of organisms from each habitat. Type IV A). This pattern was also exhibited by small Segal (1956) suggested that low level individuals appear to be “cold-adapted” relative to those from !$a arenaria from the Chesapeake Bay but not 250 mg M. arenaria from the same location (Kennedy higher levels. More recently, Davies (1967) proposed & Mihursky, 1972). The degree of physiological comthat such physiological differences may be due to variation in the amount of food available to high and pensation is least for clams from the high intertidal low shore organisms. level (32.5?;,), intermediate for clams from the mid inWhile the findings reported here for intertidal MJ’U tertidal level (617,), and most for clams from the low intertidal level (92.4’:.) (Precht Type 3). Of these three urenaria differ from those reported by many other workers, they are similar to the results of Micallef groups, significant differences between metabolic rates of CA ammals and rates of WA animals are shown (1967) who measured aerial respiration rates of only for clams from the low intertidal level (5 C, several species of trochids from various beach levels. l@C). Hence, the degree to which M. nrenaria metaHis study is also noteworthy because it demonstrated boiically compensated for reduction in temperature that although aquatic oxygen uptake rates for low is related to the shore level from which they were level trochids were higher than rates for high level collected: low shore forms adapted more readily than trochids, the opposite trend was seen for aerial respiration rates. Bivalves were not fully submersed in sea those living at higher shore levels.

Table 2. Median for small groups

survival

(N = 20) of MKI levels

Metabolic rate. temperature acclimation and resistance to high tem~rature Again, subtidal clams are exceptional in that the R-T curves exhibited by CA and WA groups (Fig. 4) are anomalous in comparison both to those of groups from other levels (Figs. l-3) and to typical thermal acclimation patterns described by Prosser (1958). The unusual pattern results from both a relatively low (15-C) temperature of maximum metabolic rate for the CA subgroup and a marked degree of metabolic temperature independence for the WA subgroup ilCr25 C). Such metabolic temperature independence is often suggestive of either standard metabolism or organismic starvation (review by Newell, 1973). Since WA subtidal bivalves as well as groups from the other levels exhibited pumping activity during oxygen uptake measurements, it seems unlikely that quiescence would account for the phenomenon noted. Furthermore. organismic starvation would also seem to be ruled out as a reason since starving animals would probably not survive as long as the subtidal ones did during the mortality study (Table 2). Regardless of the reason for the independence noted for WA subtidal M. arenaria it is apparent that bivalves living beneath MLW exhibit a different strategy for coping with high temperature than those from other levels. Nevertheless, with respect to compensatory ability. subtidal clams conform to the same generalizatjon as intertidal clams: bivalves from low shore levels acclimate more readily than those from higher shore levels. Although it is possible that canalization (Waddington. 1957) or genotypic physiological differences between high and low shore M. arenaria may account for the differences in acclimation abilities noted, many previous studies involving transplantation of intertidal animals from one level to another (e.g. Segal, 1956: Pickens, 1965; Davies, 1966, 1967) have shown such differences to be phenotypic. A further possibility is that shore level affects the rate of physiological compensation, rather than the extent to which M. are~7ar~a may respond to changes in temperature. Bivalves vary considerably with respect to acclimation rate. For example. Pickens (I 965) and Widdows (1973) demonstrated that physiological compensation occurs in as little as 14 days in Mq’rilus calr@nianus and M. edulis, respectively. However, Segal (1956) reported that acclimation takes 29 days in Acmaea limatula; Davies (1967) showed that Patella wlgata requires as long as 3 months to compensate physiologically. For the present study, the acclimation period of 14 days was selected arbitrarily. Hence, it is possible that compensation had not been completed by the time R-T curves were derived. Regardless of the underlying reason(s) for the differences noted, the adaptive value of variation between acclimation rates (or abilities) of high shore and low shore bivalves is apparent, A subtidal clam subjected to temperature change might be expected to begin physiological compensation quickly, since temperature fluctuations in the subtidal habitat tend to be gradual and long term ones. In contrast, an intertidal clam which could easily experience substantial variations in body temperature daily might be refractory in its acclimation response. The data recorded during the mortality study (Table 2) are unremarkable in light of the results of the acclimation study. The median survival time of

437

M. arenaria from the high intertidal level was greater than that for the mid or low intertidal clams. Similar results have been noted several times (review by Newell, 1977). Typically organismic thermal tolerance is correlated with the normal temperature range at the site at which the animal lives. In the summer, maximum substrate temperatures in the intertidal zone increase with increasing height above MLW. At first glance, the longevity of subtidal bivalves is surprising since the habitat temperature at the subtidal level is lowest. However, subtidal M. arenaria have a remarkable capacity to remain physiologically independent of temperature change (Fig. 4). Such an adaptive response enables bivalves from this level to reduce metabolic output and hence conserve energy during periods of thermal stress.

Acknowledgemenr--I am grateful to the University of Maine Faculty Research Fund for providing a grant which enabled me to conduct this study.

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