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AMER. ZOOL., 28:161-181 (1988) Calorimetric Studies of Behavior, Metabolism and Energetics of Sessile Intertidal Animals1 J. MALCOLM SHICK Department of Zoology, University of Maine, Orono, Maine 04469 JOHN WIDDOWS Institute for Marine Environmental Research, Prospect Place, Plymouth, England PL1 3DH ERICH GNAIGER Institul Zoophysiologie, Universitat Innsbruck, A-6020 Innsbruck, Austria SYNOPSIS. Behaviors to conserve water during intertidal exposure at the same time impair respiratory gas exchange, so that observed responses to emersion may reflect compromises between these incompatible needs. Behavioral isolation of the tissues from air results in the complete or partial reliance on anoxic energy metabolism, which is most reliably measured directly as heat dissipation. Combined direct calorimetry and indirect calorimetry (respirometry) enable the partitioning of total metabolic heat dissipation into its aerobic and anoxic components, which may vary according to physical and biological factors. The mussel Mytilus edulis is tolerant of anoxia and saves water and energy during aerial exposure in its rocky intertidal habitat by closing its shell valves and becoming largely anoxic. Like most suspension feeders in this habitat, its compensation for reduced feeding time involves energy conservation; there is little evidence for energy supplementation such as increases in feeding rate or absorption efficiency. Ammonia production continues during aerial exposure and is involved in acid-base balance in the hemolymph and mantle cavity fluid. Infaunal cockles (Cardium edule) and mussels (Geukensia demissa) gape their shell valves, remain largely aerobic and have high rates of heat dissipation during intertidal exposure, a response which appears related to the lower desiccation potential and exploitation of richer trophic resources in their soft-sediment habitats. The variable expansion of the symbiotic sea anemone Anthopleura elegantissima reflects interaction among the responses to desiccation, irradiance and continued photosynthesis by its zooxanthellae during exposure to air. "One could safely predict that all physiological processes correspondingly might be shown to be influenced by the tide, could we but read delicately enough the indices." F F R' k INTRODUCTION „, • •_, i , The intertidal zone presents a steep gra,• r L • i . ii .u dient of physical stresses, especially theri j •• / J • .• J »u mal and osmotic/desiccative, and the • „ , • c c . ,• potentially damaging force or breaking waves, as well as a decreased time available" for food capture by sessile suspension feedj - i i-r • u • -J i A ers. Accordingly, life in the intertidal zone ° ' , . . ,. , • r n , •, ' From the Symposium on Mechanisms of Physioloeical Compensation in Intertidal Animals presented at the imposes on them not only the challenge of holding their position, but also a restriction on their energy intake. Moreover, there is the difficulty of maintaining aerobic respiration in an environment where most marine invertebrates are morphologically ill-equipped to function and where behavioral responses to regulate water loss concomitantly impair gas exchange. 6 & " . _ . . ; r , Despite these rigors, large areas or the 6 & ' . ,, . r, , c intertidal zone often are dominated by ; ses, , , . ., r sue suspension feeders such as barnacles, v ,. . .. , ' T blvalve luSCS a ea TrT^'nQsm6 f ™° n ^* ™ w of Wolcott (1973) and G.llrnor (1980) the presence there or unexploited rood ^ . ,„ r .. l(1 resources may have lured motile grazers such as gastropods into the intertidal zone. This argument of expansionist exploitatiot , \ d o e s ° n l ° a I PP'y ^ -i • sessile suspension Annual Meeting of the American Society of Zoolo- feeding epifauna, which probably were gists, 27-30 December 1985, at Baltimore, Maryland. "bulldozed" Onto hard intertidal Sub161 162 J. M. SHICK ET AL. strates by bioturbators in soft subtidal sediments (Thayer, 1979), or "chased" there by predators and competitors: there are many examples of the lower intertidal limits of sessile fauna being set by the upper limits of predators or successful competitors {e.g., Connell, 1961; Paine, 1974; Ottaway, 1977). A critical point is not just that compensations and adaptations relevant to intertidal living exist, but that they can be shown to vary predictably among individuals and species occupying different shore heights. Discerning the interactions among these morphological, behavioral and physiological/metabolic characters provides a challenge for physiological ecologists and indeed defines the field of physiological ecology of intertidal animals. Our approach to understanding the compensations shown by sessile molluscs and sea anemones to the intertidal environment has been to compare the acute and the acclimatized responses to aerial exposure, and to consider metabolic responses in relation to other processes such as feeding, digestion, excretion and growth. An energetic approach can therefore provide an integration and a means of assessing the overall performance in terms of the "costs" and "benefits," and the effectiveness of various behavioral, physiological and metabolic responses to environmental change. anoides and Chthamalus stellatus but lacking in sublittoral B. crenatus (Barnes et al., 1963). Moreover, intertidal barnacles become quiescent during emersion, whereas sublittoral barnacles "struggled erratically and soon became desiccated" (Barnes and Barnes, 1957). Barnacles, like other crustaceans, utilize the lactate pathway of anoxic energy metabolism (Barnes et al., 1963). This pathway has a high power output but low energetic efficiency (Gnaiger, 1983a) and often involves an enhanced anoxic rate of glycolysis (Pasteur effect) compared to the aerobic rate. Even if there is a reduction in ATP turnover by quiescent barnacles during emersion (Barnes et al., 1963), it is unlikely to be greater in magnitude than the reduction in ATP yield from lactic acid production as compared with aerobic respiration, so that a requisite Pasteur effect to maintain ATP production would more rapidly deplete glycogen stores. Augenfeld (1967) observed that high shore species of barnacles indeed used glycogen more slowly than did low shore species and had higher maximum activities of cytochrome oxidase. Thus, the maintenance of aerobic respiration by intertidal barnacles (Barnes et al, 1963) appears favored, perhaps to save energy compared with shifting to lactogenic anoxibiosis. Unlike barnacles, bivalve molluscs employ pathways of anoxibiosis which have BEHAVIORAL AND BIOCHEMICAL a host of end products other than lactate. CORRELATES OF INTERTIDAL LIVING A common feature of these pathways is a Avoidance of desiccation during inter- greater biochemical efficiency than lactotidal exposure is the foremost constraint genic glycolysis (Gnaiger, 1983a; de Zwaan, on sessile invertebrates and involves not 1983), which when coupled with a diminonly morphological characters but also ished ATP demand and suppression of the behaviors which optimize their effective- Pasteur effect, spare glycogen reserves ness. Behaviors to minimize desiccation when the shell valves close to avoid desicsimultaneously impair respiratory gas cation during exposure to air. That some exchange, so that overt responses to emer- species do so and others do not seems ultision may reflect compromises between mately related to the desiccation potential these needs. In Balanus balanoides, the ten- and to the availability of food in their dency for the barnacles to maintain a respective habitats (see below), as well as micropylar opening that allowed aerobic to metabolic differences. respiration to continue during aerial expoAs might be expected in a carnivore, sure was positively correlated with ambient protein represents a major fuel for aerobic relative humidity (Grainger and Newell, respiration in the sea anemone Anthopleura 1965). Formation of the micropyle was elegantissima (Zamer and Shick, 1987). peculiar to intertidal species such as B. bal- Unlike bivalves, sea anemones appear to CALORIMETRY OF INTERTIDAL ANIMALS store relatively little glycogen but large amounts of lipid (Shick, in preparation). Lipid cannot be a fuel for anoxic energy metabolism, and the continuation of aerobic respiration by aerially-exposed anemones (Griffiths, 1977; Shick, 1981) may be related to this fact. Also unlike bivalves, intertidal sea anemones lack shells and so are directly exposed to the atmosphere, which facilitates aerobic respiration but poses a particular vulnerability to desiccation. Retention of a large volume of water in the coelenterons of intertidal actiniid sea anemones is made possible by their strong marginal sphincters (Stotz, 1979), coupled with initial postural adjustments and subsequent quiescence during exposure to air. That the behavior is associated with regular emersion can be seen by comparing intertidally acclimatized anemones with acutely exposed subtidal conspecifics: whereas the former are quiescent and maintain a hemispherical shape throughout exposure, the latter are continuously active and lose much of their coelenteric fluid (Griffiths, 1977; Shick, 1981). A controlled exudation of coelenteric fluid from the mouth, as well as evaporation across the body wall, provides evaporative cooling and allows Actinia equina to survive higher temperatures in air than in seawater (Griffiths, 1977). The attachment of gravel by the verrucae in A. elegantissima establishes a diffusive boundary layer, reduces desiccation and enhances survival under extreme conditions (Hart and Crowe, 1977). Again a difference between high and low shore individuals is seen: the former consistently attach more gravel and shell debris than do the latter (Dykens and Shick, 1984). The reflective shell debris may in addition help to keep body temperature from rising and so further reduce evaporation. Attached debris has an additional role, that of "sunscreen," an important consideration in soft bodied intertidal animals directly exposed to sunlight. In addition to affording protection from direct photodynamic effects of ultraviolet radiation on the anemone tissues, the debris may also reduce photosynthesis by the anemone's zooxanthellae in 163 very bright light and thus reduce photosensitized oxidations involving chlorophyll, as indicated by the direct relationship between the amount of gravel attached by anemones and the chlorophyll content of their algae (Dykens and Shick, 1984). Contraction behavior likewise has a role here: in low to moderate light (and high relative humidity), the oral disc remains expanded and allows photosynthesis to continue, but under very high irradiance, the marginal sphincter contracts, shading the algae and reducing photosynthesis (Shick and Dykens, 1984). Thus the nutritional benefit to the host anemone provided by the zooxanthellae must be behaviorally modulated to reduce potential detrimental effects inherent in this association. Biochemical and metabolic defenses such as carotenoid pigments, UVabsorbing substances, and enzymes which detoxify various forms of active oxygen are attracting increasing attention (see Shick and Dykens, 1985). Quiescence during intertidal exposure not only reduces water loss but also conserves energy, a central tenet of many studies. To some extent this conservation occurs automatically as suspension feeding activities cease in air, and when avoidance of desiccation restricts access to atmospheric oxygen and so reduces the level of aerobic metabolism, with a diminished energy demand often being met in part by anoxibiosis. Partial reliance on anoxic energy metabolism presents a problem to the experimenter using respirometry because a greater part of total metabolic rate is undetected, so that the true energy saving by quiescence during aerial exposure has scarcely been investigated. Direct calorimetry, the measurement of metabolic heat dissipation, is a more general method that detects all enthalpy changes and so provides the physiological ecologist with the total heat flux occurring in intertidal animals exposed to air. Whereas intertidallyacclimatized specimens of A. equina remained quiescent and gradually reduced their rate of heat dissipation during exposure, subtidal individuals acutely exposed to air became active and maintained a higher rate of heat dissipation (Fig. 1). The 164 J. M. SHICK ET AL. Actinia equina B A -1.5 intertidal 12 9 -0.45 -1.0 Subtidal g E -0.30 lntertidaT"~ -1.5 6 6 subtidal 12 1 -0.15 - I -1.0 baseline 2 3 4 Time in air (h) -0.5 K .- 4 3 4 Time in air (h) FIG. 1. (A) Continuous recordings of heat dissipation (tQ) in intertidally- and subtidally-acclimatized specimens of Actinia equina during exposure to air (acute exposure in the latter case). (After Shick, 1981.) (B) Massspecific aerial heat dissipation Qq) and oxygen consumption (rio2) in A. equina. Data are plotted on corresponding scales using a standard oxycaloric equivalent of 8.000 /rniol O2- h~'/mW. Measured heat dissipation in subtidal anemones significantly exceeds that calculated from Jio2 from 3.5 hr onward. (From data in Shick, 1981.) energy conserving effect of quiescence is highlighted in the power-time (PT) curve of an intertidally-acclimatized specimen of M. edulis (Fig. 2D). During the normal 5-hr period of exposure, the PT curve was smooth and the rate of heat dissipation low, but when the mussel was kept in the calorimeter beyond the normal time of reimmersion, it became active and increased its energy utilization about twofold, the rate approximating that seen in subtidal individuals. Although such close entrainment to the tidal cycle may not be the rule in M, edulis (Gnaiger, 1983c, did not observe this phenomenon, and Widdows and Shick, 1985, saw it in only one-third of the mussels tested), its occurrence in some individuals at least renders measurable the ofteninvoked but rarely quantified effect of quiescence on energy expenditure in intertidally acclimatized animals (Newell, 1979). METABOLISM DURING EXPOSURE TO AIR Since the seminal studies by Lavoisier and Laplace two centuries ago, much theoretical and empirical research has confirmed the concordance of heat dissipation measured directly with that predicted from the aerobic combustion of organic substrates. This enables the use of indjrect calorimetry (oxygen consumption, A'o2) as a valid predictor of enthalpy changes asso- ciated with fully aerobic catabolism through the use of dissipative (catabolic) oxycaloric equivalents. During exposure to air, however, bivalve molluscs accumulate end products of anoxibiosis while simultaneously consuming oxygen (albeit at a lower rate than occurs during immersion), so that measurement of oxygen consumption alone neglects anoxic enthalpy changes and underestimates total metabolic heat dissipation. Unlike aerobic respiration, the metabolic changes associated with anoxic energy metabolism in Mytilus edulis do not fully account for measured heat dissipation GQanox). so that there remains a biochemically unexplained "exothermic gap" which averages 50% of tQanox (Fig. 3). Thus the sum of enthalpy changes associated with aerobic and anoxic energy metabolism will estimate only about 64% of the actual heat dissipated during an 8 hr exposure to air in intertidal mussels (Fig. 3). The advent of calorimeters capable of measuring the heat fluxes in intertidal invertebrates enables the quantification of their total heat dissipation in air and in seawater (Gnaiger, 19836; Pamatmat, 1983a; Widdows, 1987). The coordinated measurement of both Aro2 and tQair enables the partitioning of the latter into its aerobic and anoxic components, for the difference between heat dissipation in air and in 165 CALORIMETRY OF INTERTIDAL ANIMALS Mytilus edulis -360 -300 ? . Most common behavior: "partial gaper" ® Natural "gaper" , (air; -240 3 "180 "120 -60 " baseline 0 " '^y-"-""1- ">- 4 Time (h) Mytilus edulis -300 •o -150 .baseline 6 4 Time (h) i-—t baseline 4 Time (h) 6 (winter) LSubtidal normal relmmersion Intertidal -150 4 5 Time (h) FIG. 2. (A-D) Heat dissipation in individual specimens of Mytilus edulis during exposure to air and to N2 gas. See text for discussion. (E) Heat dissipation in a specimen of Cardium edule during exposure to air. Sharp inflection in power-time curve is associated with initiation of air-gaping. (After Widdows and Shick, 1985; and Snick el al., 1986.) and in N2 gas is exactly matched by the caloric equivalent of aerial oxygen consumption (kQox, calculated as No2 times AvHo2, the oxycaloric equivalent) in M. edulis (Fig. 4). Application of direct and indirect calorimetry to intertidally acclimatized individuals of A. elegantissima confirms that they remain fully aerobic during exposure to air, despite their lower iVo2 in air than in water (Shick and Dykens, 1984). The latter is a consequence of retraction of the tentacles and contraction of the marginal sphincter, behaviors which not only conserve water but also diminish the respiratory surface area and increase diffusion distances (Shick et al., 1979). Fully aerobic respiration is also maintained in intertidal specimens of A. equina, but not in acutelyexposed subtidals, which become active and maintain a high rate of heat dissipation (Fig. 1A), which is increasingly supported by anoxic metabolism (Fig. IB) as they lose their coelenteric fluid and their hydrostatically supported tissues collapse. The aerobic function of weight-specific heat dissipation in air, $ air , in M. edulis is not fixed but may vary according to ration, acclimatization conditions (subtidal or intertidal), season, and temperature. The effect of ration is seen when comparing fed and fasted individuals (Fig. 5): in the former, the caloric equivalent of oxygen consumption (k<7olt) accounted for 40% of tq3ir, whereas oxygen consumption was not detectable in the latter. When the fasted mussels were again fed at a higher ration during immersion, they resumed oxygen consumption when subsequently emersed. These results suggest that the digestive process continues during exposure to air, and either that some aspect of the diges- 166 a> J. M. SHICK ET AL. reoxygenation of hemolymph, tissues, and mantle fluid anoxic metabolism during recovery from prolonged (24 hr) exposure, involving accumulation of strombine, alanine and lactate in '•.posterior adductor muscle 2a> Q.0) 3 O) •"exothermic gap" conservative, endothermic processes, including restoration of high-energy phosphates and glyconeogenesis, plus "endothermic gap" II (0 o .9-o 2 <3 a> Exposure to air Recovery in water Time (hours) FIG. 3. Schema of heat dissipation and metabolic phenomena in Mytilus edulis during exposure to air and subsequent recovery in oxygenated seawater during a simulated 12-hr tidal cycle. ,(? is total measured heat dissipation (heavy continuous line) and iVo, is oxygen uptake (broken line) plotted on the same scale, using a standard oxycaloric equivalent. ,Q minus the heat equivalent of oxygen consumption in air equals anoxic heat dissipation, lQmm. JQ_ is that fraction of lQjam that is explained by metabolic changes, the balance representing an unexplained "exothermic gap." Inset: fractions of mQ explained by the enthalpic equivalents of dissipative ATP turnover associated with the anoxic accumulation of succinate and alanine, and the net depletion of ATP and phosphoarginine stores. Anoxic metabolism may also occur during recovery from prolonged exposure or anoxia owing to increased activity of the posterior adductor muscle. See text for further discussion. (Based on data in de Zwaan et al, 1983; Gnaiger, 1983a; Shick el al., 1983; Shick et al., 1986; and Gnaiger et al., submitted.) tion, absorption or assimilation of food requires O2, or that the large specific dynamic effect of processing the ingested ration cannot be met by anoxibiosis alone. The heat increment associated with feeding and digestion is ~15% of no 2 during immersion (Widdows and Shick, 1985); this implies a large energy demand which could scarcely be met by anoxibiosis, which has an associated heat dissipation of from 4% to 13% of the aerobic rate in water in M. edulis (cf. Famme et al., 1981; Shick et al, 1983; Widdows and Shick, 1985). Since much of the heat increment of feeding is due to the mechanical activities of filtration, which ceases during aerial exposure, the fact that oxygen consumption in air is one-third of the increment in oxygen uptake associated with feeding and digestion strongly suggests that this aerobic component of tQlir supports the additional metabolic costs of digestion and absorption. Maintenance of some amount of aerobic respiration during exposure to air also depends on the mussels being acclimatized to such exposure. Subtidally-acclimatized mussels may not consume oxygen when acutely emersed (Fig. 5), likely owing to the tight closure of the shell valves as a stress response. However, this result varies seasonally: whereas "winter" mussels behave as just described, most "summer" mussels (intertidally and subtidally accli- 167 CALORIMETRY OF INTERTIDAL ANIMALS -0.25 Mytllus edulis k« O) 1 Air 4l 7 i 0L Air FIG. 4. Paired measurements of heat dissipation in four specimens of Mydlus edulis in air and N2 gas at 15°C. The caloric equivalent of oxygen consumption (k?ox) by mussels in air matches the difference between ,tj,ir and tqama (in N, gas). Combined direct and indirect calorimetry thus enables the partitioning of iq\ij into its aerobic and anoxic components. (Mean ± SE.) (Based on data in Shick et al., 1983.) Intertidal (lasted) Intertidal (re-fed) Subtldal FIG. 5. Effects of feeding, fasting, and resumed feeding on heat dissipation and oxygen consumption in Mytilus edulis during exposure to air. See text for discussion. (After Widdows and Shick, 1985.) heat dissipation shown by G. demissa in air as reflecting the inclusion of an air bubble matized) do gape in air and have an aerobic during a single gaping event, and gradual rate of heat dissipation that averages 40% consumption of the O2 in the bubble when of t<7ail. (Shick et al, 1986). This seasonal the shell valves remained closed. This was difference may be related to the need for confirmed in a specimen of M. edulis which evaporative cooling in summer, for the showed a single peak of ,<7air: assuming that aerobic component of t^air increases with at a respiratory quotient, RQ, of « 1 the acute exposure temperature (Fig. 6), again gas volume in the bubble remained conlikely owing to an increase in shell valve stant, consumption of all of the O 2 in that gaping. This behavior is pronounced in bubble was equivalent to 93% of the heat small individuals of M. edulis (Gnaiger, dissipation integrated above the minimum 1983c), which like small specimens of G. (anoxic) rate (Shick et al., 1986). demissa, may derive more benefit from Finally, to emphasize that variations in evaporative cooling than do large mussels tqair arose from individual differences in air(Lent, 1968). gaping behavior, mussels forced to gape by Variability in the PT curves of individual having their shell valves wedged open had mussels arises from behavioral differences values of tq3ir generally higher than paramong them. Whereas most individuals of tially gaping individuals (cf. Fig. 2A, C), tqair M. edulis apparently gaped only partially increasing with the volume of enclosed gas and had a rate of heat dissipation in air two (Shick et al., 1986). Moreover, while the to three times the anoxic rate in N2 gas PT curves of normal mussels in air are (Fig. 2A), occasionally the PT curve characteristically rough, likely due to variincreased steeply to a level some four or ation in shell valve activity, those of "forced five times the initial aerial rate (Fig. 2B), gears" are smooth and lack the peaks and presumably due to increased shell valve valleys seen in "natural gapers," although gaping. Volumetric measurement of the gas the maximum rates of heat dissipation are bubble released by mussels upon reimmer- similar (cf. Fig. 2B, C). Such an effect of sion revealed a positive relationship continuous oxygen availability was also seen between bubble size and t^air, with most by Pamatmat (19836) in G. demissa having individuals having rates of —0.1 to —0.5 their shell valves wedged open. mW/g dW (Fig. 7). A bubble volume of There is a direct relationship between zero corresponded to an anoxic tq of « — 0.1 ^ air and the steady-state rate of heat dissimW/g dW, in agreement with the rates pation in water, tqsl<.zdy, when recovery from measured in N2 gas (Shick et al., 1986). exposure is complete (Fig. 7, excluding Pamatmat (1984) interpreted the sharp extreme "gapers"; see below). Although peaks and subsequent gradual decline in the variability in t^air results from individual 168 J. M. SHICK ET AL. -320 - -3.0 -240 - -2.0 -160 •o- O £ 6 15 25 -1.0 - 30 Acute exposure temperature (°C) FIG. 6. Effect of exposure temperature on heat dissipation in Mytilus edulis in air and N2 gas. The increased aerobic component to heat dissipation (see Fig. 5) is due to increased air-gaping at the highest temperature. See text for discussion. (After Widdows, 1987.) differences in air-gaping behavior, this is not the case for variable t^steady It appears that some mussels have a higher energy demand than others, which is reflected both in a higher tqsleiAy when immersed and an increasing aerobic contribution to tqzir above the anoxic rate when exposed (Fig. 7). When constant difference between tqaaidy and t<7max (the maximum rate of heat dissipation associated with recovery: see below) again suggests that in mussels acclimatized to the same conditions at the same season, there is a minimum (anoxic) rate of heat dissipation during aerial exposure in all individuals which results in a fixed "cost of recovery" (oct^max - t^steady), and that higher rates of tqair are supported aerobically. Differences in air-gaping are also associated with behavioral differences following reimmersion. While "gapers" show very little shell valve movement during aquatic recovery, most individuals are more active (Shick et al., 1986). In both groups there is a suppression of the activity of the posterior adductor muscle (PAM) during the first 0.5 hr of reimmersion; together with the especially wide shell valve gape which occurs then (Coleman, 1974), this apparently serves to maximize the shell valve -0.1 anoxic -0.3 a j r (mW / g) -0.5 parllally aerobic 0-*Bubble volume (ml / g) FIG. 7. Relationship between specific rate of heat dissipation in air (tq,-,r) and that in oxygenated sea water during recovery. ,^mlx is the maximum rate of recovery heat dissipation associated with the oxygen debt payment, and ,^,«ady is the steady-state rate of heat dissipation when recovery is complete. Volume of air bubbles enclosed by mussels for which ,q,ir was measured was predicted from measurements of both ,q^r and bubble volumes released by other mussels upon reimmersion. See text for discussion. (After Shick et al, 1986.) aperture and so minimize resistance to ventilation of the mantle cavity, which promotes the rapid uptake of oxygen and removal of accumulated ammonia (see "Reimmersion and Recovery" section). Subsequent PAM activity may facilitate circulatory perfusion of the tissues, and the peak in PAM activity coincides with restoration of hemolymph pO2 to its preexposure level (Shick et al., 1986). If, as in G. demissa and C. edule, maximal air-gaping by M. edulis maintains hemolymph pO2 at nearly the same level as during immersion (Booth and Mangum, 1978; Shick, unpublished), the near absence of PAM activity in "gapers" during recovery supports the idea that PAM activity in nongapers is involved with enhanced perfusion of their tissues by newly reoxygenated hemolymph. Moreover, most individuals CALORIMETRY OF INTERTIDAL ANIMALS of M. edulis show a marked bradycardia during exposure to air (Coleman and Trueman, 1971; Shick et al, 1986), whereas maintenance of higher levels of cardiac activity by C. edule (Boyden, 1972) and G. demissa (Booth and Mangum, 1978) is associated with their pronounced airgaping behavior. Also, the metabolic overshoot associated with repayment of the oxygen debt is smaller in mussels which gape during prolonged exposure (Gnaiger et al., submitted), again suggesting a lower oxygen demand in "gapers" during early recovery. Maximal shell valve gaping and maintenance of high rates of aerial respiration are the exception in M. edulis, for most individuals of this species are predominantly anoxic during exposure to air. This is not the case in C. edule and G. demissa, which typically gape their shell valves and maintain high rates of oxygen consumption and heat dissipation (Fig. 2E) that are from 50% to ~90% of the aquatic rates (Kuenzler, 1961; Boyden, 1972; Pamatmat, 19836, 1984), with their metabolism remaining fully aerobic (Widdows and Shick, 1985). The question arises, what is the basis for such pronounced behavioral and concomitant metabolic and energetic differences between M. edulis on one hand and C. edule and G. demissa on the other? Based on experiments by Lent (1968) on G. demissa, air-gaping does not seem to be evoked by the need for evaporative cooling, at least in large mussels. Indeed, the semi-infaunal nature of this species and of C. edule affords a degree of protection from elevated air temperatures (Lent, 1968; McMahon and Wilson, 1981). That airgaping is associated with the successful occupation of an intertidal habitat is indicated by a close intrageneric comparison: the predominantly subtidal Cardium glaucum does not exhibit this behavior and moreover survives aerial exposure less well than does C. edule (Boyden, 1972). Since closing the shell valves, reducing the metabolic rate and relying on efficient anoxibiosis during exposure (and on aerobic recovery metabolism upon reimmersion: see below) provide mechanisms to save 169 energy (de Zwaan, 1977; Pamatmat, 1980; Widdows and Shick, 1985; Gnaiger et al., submitted), the question becomes more of why C. edule and G. demissa do not do this. Part of the answer may lie in the fact that these two species seem less well adapted to survive anoxia than does M. edulis, average anoxic survival at 10°C in C. edule being 4.3 days but 35 days in M. edulis at the same temperature (Theede et al., 1969). (We note that 35 days seems a very long survival time for M. edulis, as Wijsman [1976] found that mussels were already dying after 7 days of exposure at 13°C. Also, resistance to anoxia may not be a constant, for Gnaiger and Bitterlich [unpublished] observed Cornish mussels dying after 7 days at 15°C just after spawning in April, whereas mussels from the same population survive anoxia for approximately two weeks in autumn [Shick, unpublished].) Lent (1968) reports a median anoxic survival time of 5 days for G. demissa, apparently at 20-25°C, whereas this species survives for more than a month in moist air (Kuenzler, 1961). The shorter anoxic survival in C. edule may be a consequence of its heavy reliance on lactate production and maintenance of a relatively high rate of ATP utilization (Meinardus and Gade, 1981). The maintenance of a high rate of energy utilization under anoxia is also detected calorimetrically, since C. edule has an anoxic tq that is 42% of the aerobic ,q (Pamatmat, 1980). Although anoxic survival times on the order of days hardly seem relevant to intertidal exposures measured in hours, the differences in anoxic tolerance may be related to inherent differences in the rate and efficiency of anoxic energy metabolism and the readiness with which the various bivalves rely on it (perhaps involving differences in the speed of transition to the PEPCK route: Meinardus and Gade, 1981) when exposed at low tide. The occupation of an exposed rocky intertidal habitat by epifaunal Mytilus edulis exposes it not only to desiccating conditions but also to a low availability of food, both of which factors select for behaviors such as shell valve closure to conserve water and energy. The closely controlled air gap- 170 J. M. SHICK ET AL. ing behavior in this species may at times promote evaporative cooling, but also seems to regulate a variable aerobic component of total energy metabolism to support the costs of digestion and absorption when food is present in the digestive system. Occupation of trophically rich mudflats and salt marshes by Cardium edule and Geukensia demissa places them in a position to exploit abundant food, especially as feeding may continue when they filter tidal runoff or interstitial water after the tide has receded. Moreover, the costs of continued digestive processes during intertidal exposure may necessitate a major reliance on aerobic respiration, as argued for M. edulis. In short, the physical nature of the substrates where they occur is linked to trophic richness, and may allow C. edule and G. demissa to exploit this richness without the constraint of behaviors geared to avoiding desiccation. The interplay of behaviors to conserve water while at the same time enhancing energy acquisition is particularly apparent in the symbiotic sea anemone Anthopleura elegantissima: retraction of the tentacles and assumption of a hemispherical shape saves both water and energy during emersion, but the variable exposure of the oral disc depending on ambient humidity and irradiance allows photosynthesis and partial autotrophy to continue. Actinia equina lacks algal symbionts and remains fully contracted during exposure, there being no energetic advantage to partial expansion. REIMMERSION AND RECOVERY A major feature of recovery from aerial exposure shown by nearly all intertidal animals is the overshoot in oxygen uptake, known as the oxygen debt payment, immediately following reimmersion in oxygenated seawater. This occurs during recovery from both environmental hypoxia and physiological hypoxia, such as that caused by intense muscular activity. Respirometric studies have shown that the size of the oxygen debt payment by intertidal animals depends on body size, the duration of aerial exposure, behavior, ration and temperature. The relationship between the size of the oxygen debt payment and body mass in M. edulis is described by a mass exponent ("b") of 0.74, identical to the b value for the aquatic rate of oxygen uptake (Widdows and Salkeld, unpublished). In species and individuals that remain closed during aerial exposure, part of the total oxygen debt payment is the physical reoxygenation of body water, such as mantle cavity fluid in bivalves, and intra- and extracellular fluids (Fig. 3). The reoxygenation component forms approximately 15% of the total oxygen debt payment in M. edulis (Widdows and Shick, 1985; Gnaiger et al., submitted; Shick et al., 1986), the remainder representing the metabolic oxygen debt payment. Various authors have shown that the size of the oxygen debt payment increases with the duration of aerial exposure, at least over a period of 12-16 hr for mussels (Fig. 8). For the sea anemone Bunodosoma cavernata, Ellington (1982) has shown that the magnitude of the oxygen debt payment increases with the duration of exposure to anoxia in seawater, whereas intertidally acclimatized individuals of Anthopleura elegantissima remain fully aerobic during aerial exposure and show no oxygen debt payment (Shick, 1981). However, bivalves such as G. demissa and C. edule, which gape and consume large amounts of oxygen during aerial exposure, also show a distinct overshoot in oxygen uptake upon reimmersion (Widdows et al., 1979; Widdows and Shick, 1985). Ration not only affects the steady rate of aquatic oxygen consumption by M. edulis but also the maximum rate after reimmersion as well as the size of the metabolic oxygen debt payment in a similar proportional manner (Widdows and Shick, 1985). Widdows et al. (1979) have demonstrated an effect of air temperature on the size of the oxygen debt payment, with a marked increase in both M. edulis and C. edule when exposed to a 10°C increase in air temperature, probably as a consequence of the thermal dependence shown in the aerial rate of heat dissipation (Fig. 6). Although the oxygen debt payment associated with recovery from environmental anoxia and hypoxia has been described in many studies and reviewed by Herreid CALORIMETRY OF INTERTIDAL ANIMALS 171 32 24 • 0 O A A Bayne et a i d 976) -littoral M. callfornianus De Vooys & De Zwaan(1978) -sublittoral M. edulis De Vooys a De ZwaarK1978) -littoral M. edulis Widdows et al(1979) -littoral M. edulis Widdows & Shick(1985) -littoral M. edulis 16 o E s n o> •a 0 Amount of O 2 required to resaturate mantle cavity water & body fluids 36 ' ' 48 12 24 Hours of exposure to air FIG. 8. Relationship between the duration of aerial exposure and the size of the oxygen debt payment by Mytilus edulis and M. californianus. (1980), an adequate metabolic interpretation cannot be provided without a combination of direct calorimetry and indirect calorimetry, such as respirometric and biochemical measurements (Shick and Widdows, 1981; Shick et al., 1983; Gnaiger et al., submitted). The marked reduction in the anoxic rate of heat dissipation to approximately 4-7% of the aquatic aerobic rate, and the concomitant suppression of the Pasteur effect noted by de Zwaan and Wijsman (1976), represent aspects of the "energy saving" mechanism during aerial exposure or anoxia. But any evaluation of "energy saving" must take into account the "cost of recovery" (Shick and Widdows, 1981; Widdows and Shick, 1985). Furthermore, any energy saving associated with reduced heat dissipation during aerial exposure would be in vain if accumulated glycolytic end products were then excreted or completely burned during recovery and payment of the oxygen debt (Gnaiger et al., submitted). The use of simultaneous, or coordinated non-simultaneous, direct and indirect calorimetry under conditions such as aerial exposure, anoxia and recovery, not only provides a direct measurement of total metabolic heat dissipation but also quantifies the heat equivalent of oxygen uptake and thus indicates the nature of the metabolic processes, such as the degree of anoxibiosis or conservative anabolism. The heat or oxycaloric equivalent of aerobic catabolism ranges from —440 to —480 k j / mol O2 (Gnaiger, 1983a), and a dissipative oxycaloric equivalent of —450 kj/mol O 2 is commonly used to calculate metabolic heat loss in respiratory studies and the construction of energy budgets. However, calorimetric studies have shown that there are marked changes in the heat equivalent of oxygen uptake both during aerial exposure (see previous section) and aquatic recovery (Gnaiger et al., submitted). Figure 9 demonstrates that during aerial exposure the empirically derived heat equivalent exceeds the theoretical range, thus indicating anoxibiosis. The higher the 172 J. M. SHICK ET AL. -600 I -500 Theoretical range of oxycaloric equivalents for totally aerobic metabolism CD -400 o o 'c -300 CO > tT 01 -200 CO <u I -100 8 12 16 20 Hours (Recovery) FIG. 9. Effect of the duration of anoxia on the heat equivalent of oxygen uptake by Mytilus edulis during recovery in air-saturated seawater. See text for discussion. These long-term experiments were enabled by the development at IMER, Plymouth, of a perfusion system for an LKB microcalorimeter which allows simultaneous measurement of heat dissipation and oxygen consumption by animals of up to 1 g dry mass, using a flow rate of up to 120 ml/hr. (Gnaiger, 1983c; Widdows and Salkeld, unpublished). experimental heat equivalent of O2 uptake, the greater the reliance on anoxic metabolism. For example, intertidal individuals of M. edulis have a heat equivalent of -1,250 kj/mol O 2 (Widdows and Shick, 1985). The value of - 5 5 0 kj/mol O2 for subtidal Actinia equina suggests a smaller anoxic component (Fig. 1) , and —455 k j / mol O 2 for intertidal C. edule indicates fully aerobic metabolism (Widdows and Shick, 1985). Conversely, if the total heat equivalent of oxygen uptake falls below the theoretical range, such as during recovery, then net biosynthesis giving rise to endothermic anabolic heat changes is suggested. The extent to which the heat equivalent falls below —440 kj/mol O 2 increases with the duration of aerial exposure (Fig. 9). Consequently, the application of a standard oxycaloric equivalent of —450 kj/mol to convert the metabolic oxygen debt payment to metabolic heat loss will overestimate the energy expenditure during recovery perhaps by 20% during the initial hour of recovery (-360 kj/mol O2) and by 12% later in recovery ( — 400 kj/mol O 2 ), although this may be complicated by anoxic energy changes then (Gnaiger et al., submitted). Recently, we have used a combined direct and indirect calorimetric approach to advance the interpretation of metabolic events occurring during the anoxic—aerobic transition (recovery) in M. edulis (Gnaiger et al., submitted). Although the results are based on extended (24 hr) periods of aerial exposure, because the effects are more marked and readily detectable, more recent evidence confirms CALORIMETRY OF INTERTIDAL ANIMALS that the responses to shorter and more ecologically relevant periods of aerial exposure are of a similar nature, varying only in degree (Shick et al, 1986). To enable direct comparison of heat dissipation and oxygen uptake during recovery, they are plotted on corresponding scales based on the catabolic oxycaloric equivalent (Fig. 3). Recovery from 24 hr of aerial exposure can be divided into two distinct phases: early recovery in the first hour, characterized by the maximum rate and minimum caloric equivalent of oxygen uptake, and late recovery (1-3 hr), featuring a higher caloric quotient and a rate gradually declining toward the steady state. The functional complexities of recovery metabolism are most clearly seen in an analysis of net recovery processes, removing the variability of aerobic steady-state rates. After correction for the reoxygenation of mantle cavity water and body fluids, the caloric quotient of net recovery oxygen consumption is — 252 kj/mol O2 (total caloric quotient is —366 kj/mol O2) during early recovery, indicating conservation of heat in endothermic processes. After accounting for conservation of heat in the ergobolic debt payment {i.e., the restoration of high-energy phosphates with catabolic-ergobolic coupled metabolism: see Gnaiger, 1983a), the caloric quotient of residual net recovery oxygen consumption of —260 kj/mol O2 still indicates highly conservative processes. These can also explain previous discrepancies in biochemical studies by de Vooys and de Zwaan (1978) and Widdows et al. (1979), in which the size of the oxygen debt payment was less than the amount of O2 required for the complete oxidation of accumulated end products such as succinate, alanine and propionate. There is no evidence of excretion of succinate or alanine, but some propionate may be excreted or reabsorbed by mussels during recovery (Zurburg et al., 1982). On the basis of metabolic acid quotients (molar AQ, analogous to RQ), and stoichiometric and thermodynamic considerations, there is a postulated mechanism whereby six times more succinate can be discharged in catabolic-anabolic coupled 173 glyconeogenesis and in partial oxidation of succinate to malate and aspartate, as compared with the complete oxidation of succinate (Gnaiger et al., submitted). Furthermore, the theoretical conservative caloric equivalent of oxygen consumption in fully catabolic-anabolic coupled recovery is as low as —200 kj/mol O 2 on the basis of succinate. Although most of the observed changes in biochemistry, heat dissipation and oxygen uptake can be explained, the recovery oxygen consumption is higher than the oxygen debt expected on the biochemical basis of a highly conservative AQ. The excess conservative respiration during early recovery represents an "endothermic gap" (Fig. 3), the size of which is in close agreement with the size of the "exothermic gap," the excess heat observed during anoxia, which suggests some compensatory reversal during recovery of the biochemically unexplained exothermic gap under anoxia (Gnaiger et al., submitted). Here again are important similarities between environmental anoxia and recovery in mussels and physiological anoxia and recovery in vertebrate muscles (cf. Curtin and Woledge, 1978; Gnaiger, 1983a). In contrast to early recovery, during the late recovery period between 1 and 3 hr, there is a transient increase of total heat dissipation above the standard caloric equivalent of oxygen consumption (Fig. 3). This results in a high caloric quotient of — 584 kj/mol O2 of net recovery oxygen consumption (total caloric quotient is —503 kj/mol O2). Since the timing of this high oxycaloric quotient precisely matches the time course of strombine, alanine and lactate accumulation in the posterior adductor muscle ofMytilus edulis (de Zwaan et al, 1983), this high oxycaloric quotient suggests the occurrence of anoxic metabolism during late recovery, but the timing does not support the contention that anoxibiosis is invoked to compensate for a limited aerobic capacity in the oxygen debt payment (Fig. 3). The accumulation of strombine and lactate in the posterior adductor muscle correlates with the frequency of shell valve adductions which peak at ca. 12 hr after reimmersion (Shick et al., 1986). 174 J. M. SHICK ET AL. Reimmersion Exposure to air \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\v 12 18 30 Hours FIG. 10. Ammonia accumulation in hemolymph and mantle cavity water of Mytilus edulis during 24 hr of aerial exposure, and ammonia excretion rate before and after exposure. (Mean ± SE: n = 10.) (Widdows, unpublished.) Hence anoxic energy metabolism is required to meet the increased energy demands associated with muscular activity. Although the role of this muscular activity is unclear, it may be related to enhanced perfusion of the tissues with reoxygenated hemolymph, as discussed in the previous section. This anoxic component is not always apparent during recovery because ongoing endothermic reactions and individual variability in non-simultaneous mea- surement of heat dissipation and oxygen uptake may mask the exothermic heat changes. In contrast to bivalves, Ellington (1982) found that the overshoot in oxygen uptake by the anemone Bunodosoma cavernata, although not corrected for reoxygenation of body fluids following anoxia, appears to exceed that required to oxidize the accumulated end products. This difference is probably related to the nature of the end 175 CALORIMETRY OF INTERTIDAL ANIMALS T A B L E 1. Effect of aerial exposure on the rale of ammonia excretion by bivalves. Species Mytilus californianus Mytilus edulis Cardium edule Rate of ammonia accumulation in hemolymph and Rate of ammonia excremantle cavity water tion (subtidal aauatic during aerial exposure steady rate; /imoles N H 4 * g ' d W h r ' *imoles N H / g " 1 d W-hr' 0.07 0.11 0.213* 2.14 1.04 0.425 Aerial rate as % of aquatic rate 3-6% 11% 50% Reference Bayne et al. (1976) Widdows (unpublished) Widdows and Shick (1985) * Based on NH4+ released upon reimmersion. protein or free amino acids cannot be ruled out. It may also be that sea anemones rely heavily on protein as a substrate for anoxic energy metabolism, just as they do in aeroAMMONIA PRODUCTION bic respiration (Zamer and Shick, 1987). Ammonia is another metabolic end The former is indicated in studies by product which accumulates at a steady rate Ellington (1977, 1980, 1982) and Navarro within the hemolymph and mantle cavity and Ortega (1984), where predominantly fluid of mussels during aerial exposure nitrogenous end products such as alanine (Bayne et al, 1976; de Zwaan et al, 1983; and glutamate accumulate. In addition, Fig. 10). However, de Zwaan et al. (1982, Ellington (1982) has shown that the 1983) have shown that relatively little increase in ammonia excretion by B. cavammonia accumulates in the tissues of M. ernata during recovery from anoxia agrees edulis during 24 hr of exposure to air. closely with that predicted from the decline Ammonia accumulates in both the hemo- in alanine concentration in the tissues. lymph and mantle cavity fluid at a steady Unfortunately, the accumulation of rate of «0.1 ^mol-hr~'-g~' mussel dry ammonia during anoxia or aerial exposure mass, reaching concentrations of 200 nM has not been tested in sea anemones. (Fig. 10) to > 400 MM(de Zwaan <?/«/., 1983). If ammonia is excreted from the cells as The rate of ammonia excretion into the gas, it may play a role in acid-base balance two compartments (Table 1) is approxi- in the extracellular fluids of bivalves durmately 3 to 30% of the normoxic aquatic ing periods of hypoxia (perhaps reducing rate of ammonia excretion by M. edulis, the amount of shell decalcification that and therefore shows a proportional change occurs then), by the protonation of ammosimilar to that observed for total energy nia to the ammonium ion. Booth et al. metabolism. This relationship is also dem- (1984) showed that this may account for as onstrated in the air-gaping C. edule, which much as 30% of the combined Ca + + and has aerial rates of ammonia production and NH4+ buffering in the hemolymph of M. energy metabolism that are ~50% of the edulis during 8 hr of exposure to air, aquatic rates (Widdows and Shick, 1985). although increases in Ca + + and NH4+ Although it generally has been assumed accounted for only 50% of an observed base that the ammonia produced in bivalves excess. during exposure to air is derived from Upon reimmersion, typically there is an aerobic metabolism, Bitterlich and Gnaiger immediate release of ammonia by bivalves, (unpublished data) have shown that ammo- with the exchange of mantle cavity water, nia continues to accumulate in M. edulis and this is accompanied by a rapid loss of during 5 days of anoxia. Some of this ammonia from the hemolymph during the ammonia may be produced by deamination first hour (Fig. 10). Bayne et al. (1976) and of AMP, as the concentration of total ade- de Vooys and de Zwaan (1978) found that nylates in some tissues of M. edulis declines the amount of ammonia released by M. calslightly during prolonged anoxia (Wijs- ifornianus and M. edulis is proportional to man, 1976), although anoxic catabolism of the duration of aerial exposure, which is products, because anoxia does not produce significant changes in succinate, rather, mainly alanine and glutamate accumulate. 176 J. M. SHICK ET AL. in agreement with a steady rate of ammonia accumulation during exposure (Fig. 10). During the spring and summer, a time of abundant food and growth, aerial exposure results in a distinct overshoot in ammonia excretion byM. rfufo upon reimmersion, with ammonia accumulated in the mantle cavity and hemolymph accounting for 35 to 75% of ammonia excreted during the first hour. Conversely, during the autumn and winter, there is evidence of reduced ammonia excretion with no overshoot upon reimmersion. Specimens of M. edulis, both intertidally acclimatized and acutely exposed to 5 hr in air and 7 hr in water, reduce their rates of ammonia excretion over this 12 hr period to 14% and 65%, respectively, of the aquatic rates (Widdows and Shick, 1985). This suggests nitrogen conservation at this time of year (de Zwaan et al., 1983; Hawkins, 1985). This is in contrast to air-gaping bivalves such as C. edule (Widdows and Shick, 1985) and G. demissa (Jordan and Valiela, 1982), which maintain a daily rate of ammonia excretion independent of aerial exposure time. Furthermore, C. edule shows a distinct overshoot in ammonia excretion following reimmersion which reflects a flushing out of ammonia accumulated in the hemolymph and mantle cavity fluid during exposure. EFFECT OF TIDAL LEVEL ON THE ENERGETICS OF INTERTIDAL ANIMALS An energetic approach, based on the integration of rates of feeding, food absorption, total energy metabolism, excretion, growth and reproduction, provides a useful means of assessing the overall performance of an animal. Individual components of the energy budget should not be considered as isolated processes. The benefits derived from energy conservation during intertidal exposure have their costs in terms of aquatic recovery and the restrictions placed on feeding, digestion and biosynthesis, while any benefits derived from enhanced feeding activity have their associated metabolic costs; together they may be regulated to maximize or optimize the net energy gain. The net energy available for growth and reproduction, the "scope for growth," is the difference between energy acquired from food and the energy expended in metabolism and excretion. The relative contributions made to the intertidal growth performance of bivalves by "energy-conserving" and "energy-supplementing" capacity adaptations have been considered by Gillmor (1982). Energy conservation during aerial exposure and anoxia has been suggested by many authors (e.g., de Zwaan and Wijsman, 1976;Bayne?/a/., 1976; Pamatmat, 1979; Widdows et al., 1979) but the overall heat loss by intertidal bivalves during exposure and recovery has only recently been quantified (Widdows and Shick, 1985; Gnaiger et al., submitted; Shick et al., 1986). Mytilus edulis, which gapes only slightly during aerial exposure, reduces its daily rate of heat dissipation by 34% when acclimatized to a 42% air exposure regime in a tidal simulator, whereas subtidally acclimatized individuals acutely exposed to air reduce daily heat dissipation by only 17% compared to the steady state subtidal rate (Widdows and Shick, 1985). In contrast, C. edule, which typically gapes during exposure, reduces its daily energy expenditure by 29% when intertidally acclimatized and by only 9% when acutely exposed to air. Energy acquisition may be enhanced by increasing feeding and ingestion during immersion, and continuing digestion and absorption during emersion when feeding ceases. Increases in feeding rate, albeit transitory, in response to regular aerial exposure have been reported in three suspension feeders- the bivalve Lasaea rubra (Morton etal., 1957), the barnacle Balanus balanoides (Ritz and Crisp, 1970), and the sea anemone Anthopleura elegantissima (Zamer, 1986), and in the grazing gastropods Littorina littorea (Newell et al., 1971) and Melagraphia aethiops (Zeldis and Boyden, 1982). Only A. elegantissima has been studied with respect to multiple components of the energy budget. Anemones aerially exposed for 75% of the time exhibited both energy conserving and energy supplementing adaptations compared with subtidally acclimatized conspecifics (Table 2). Scope for 177 CALORIMETRY OF INTERTIDAL ANIMALS TABLE 2. Energy budgets ©/"Anthopleura elegantissima/rom high and low shore levels.* Tidal height High Low Dry weight (g) Daily immersion Daily ration (hr) (mgday 1 ) 6 22 6.35 7.24 Ingestion rate (kj-day-) Paniculate absorbed energy (kj-day-') CZAR* (kj-day-') 0.158 0.180 0.151 0.166 0.031 0.112 Energy Total assimi- respired and lated energy excreted (kJ-day-'J (kjday-) 0.182 0.278 0.075 0.140 SFG* (kjday-) 0.107 0.138 • From Zamer and Shick, 1987. b CZAR = contribution of zooxanthellae to animal's respiratory requirements. c SFG = scope for growth. growth was disproportionately large in the high shore anemones relative to their long exposure as a result of lower metabolic costs during aerial exposure (Shick, 1981; Dykens and Shick, 1984) combined with increased rates of prey capture during immersion and higher absorption efficiencies (Zamer, 1986). There were no differences in their feeding surface area, but high shore anemones appeared more receptive to prey. There is no evidence of major compensatory increases in the overall feeding rates or absorption efficiencies of most intertidal bivalves, including M. edulis (Jergensen, 1960; Widdows and Shick, 1985), Choromytilus meridionalis (Griffiths, 1981; Griffiths and Buffenstein, 1981), G. demissa (Jordan and Valiela, 1982) and C. edule (Widdows and Shick, 1985), with increasing height on the shore. However, intertidally acclimatized bivalves appear to pump more or less continuously when immersed compared with a more variable and intermittent pumping by subtidal bivalves (Brand and Taylor, 1974; Widdows and Shick, 1985). Further intraspecific compensations for a reduction in feeding time include a more rapid resumption in feeding activity by intertidally acclimatized individuals compared with subtidal specimens of C. edule, which results in a more rapid establishment of a positive scope for growth (Widdows and Shick, 1985). In addition, there are interspecific differences, with higher shore species such as L. rubra (Morton et al., 1957) and M. edulis resuming feeding more rapidly than lower shore species such as C. edule (Widdows and Shick, 1985). Part of any energy supplementation may also arise from a tidally synchronized diges- tive rhythm and from adaptations that maintain some digestive and absorptive activity during aerial exposure, thus enhancing daily food absorption (L. rubra, Morton, 1956;M. edulis, Langton and Gabbott, 1974; Langton, 1975, 1977; Robinson et al., 1981; M. californianus, Thompson et al., 1978; Crassostrea gigas, Morton, 1977). However, any extra food absorption is only apparent as a transient response immediately after reimmersion (Widdows, unpublished; Bayne et al., 1988), because Griffiths and Buffenstein (1981), Jordan and Valiela (1982) and Widdows and Shick (1985) found no evidence of a marked change in overall absorption efficiency in intertidally acclimatized cockles and mussels, compared with low shore or sublittoral individuals. The net effects of energy conservation and energy supplementation on the energetics of M. edulis and C. edule are summarized in Table 3. Although aerial exposure resulted in a reduction in scope for TABLE 3. Effect of 10 hrday' aerial exposure (i.e., 427c) on the daily scope for growth and net growth efficiency (K^j ofMytilus edulis and Cardium edule.* Scope for erowlh Species/condition Mytilus edulis (1 g) Intertidal Subtidal Acute air exposure Intermittently fed Continuously fed Cardium edule (0.3 g) Intertidal Subtidal Acute air exposure Intermittently fed Continuously fed * Widdows and Shick, 1985. (Jday-) K, + 65 0.50 +45 + 36 + 122 0.36 0.26 0.55 + 13 0.19 -14 -8 +64 — — 0.46 178 J. M. SHICK ET AL. growth compared to the continuously fed subtidal individuals, the intertidally acclimatized mussels had a higher scope for growth relative to the subtidal individuals acutely exposed, which had a higher value than the subtidal, intermittently fed individuals given the same daily ration as intertidals and acutely exposed subtidals. However, the net growth efficiency (K2) of the intertidal group was similar to the subtidal continuously fed group. In comparison, aerial exposure had a more adverse effect on C. edule, both in terms of scope for growth and growth efficiency, indicating little compensation in terms of either energy conservation or supplementation on the relatively low ration provided in that study. Aerial exposure affects not only the total energy available for production, but also the partitioning between somatic growth and reproduction. Griffiths (1981) has demonstrated that high shore C. meridionalis reduces the energy allocation to somatic growth and directs a greater proportion of the available energy to reproduction, thus maintaining a high reproductive output. A similar phenomenon was seen in M. edulis (Rodhouse et al., 1984). The high growth rates in high shore specimens of Anthopleura elegantissima (Zamer, 1986) may be directed toward the rapid achievement of the minimum size necessary for reproduction in this anemone, in spite of the reduced time available for feeding, and Sebens (1981) showed that high shore individuals had the same or greater gonad production than low shore anemones. Finally, the effect of aerial exposure on the growth rate of immature individuals of several intertidal bivalves species was assessed by Gillmor (1982) in both laboratory and field studies. In the case of lower shore species such as Modiolus modiolus and Ostrea edulis, the observed growth was less than the hypothetical growth strictly determined by the time available for feeding, due to the lack of energy conservation or supplementation, and the predicted zero growth level was at 24% and 52% air exposure, respectively. The growth rates of M. edulis and C. virginica both were slightly above the hypothetical growth curves due to energy conservation and some minor energy supplementation. The predicted upper limit of growth was at 80% to 85% aerial exposure. In the ribbed mussel, G. demissa, the growth rate was substantially higher in the intertidal zone compared to the subtidal, but the upper limit to growth could not be established from the data. Lent (1969) and Jordan and Valiela (1982) did not record any energy conservation or supplementation by high shore specimens. But continued growth of G. demissa in the high intertidal zone may be related to the gill structure in this species, which allows the efficient retention of bacteria-sized particles, thus increasing food availability in a rich salt marsh habitat (Wright et al., 1982). Further integrated studies on this species are clearly needed. Not surprisingly, these findings suggest that the observed growth rates of each species in relation to aerial exposure are in good agreement with the shore position they occupy in nature, and differences in growth can largely be explained in terms of behavioral, physiological and biochemical factors. However, the upper limit of growth and survival and the tolerance of aerial exposure are modified by environmental factors, particularly food resources and air temperatures. Productive soft-sediment habitats, such as marshes, increase food availability for mussels such as G. demissa, thereby enabling them to exploit higher shore levels despite shorter feeding periods. The adoption of an infaunal habit in muddy substrata by G. demissa and C. edule also ameliorate the effects of elevated air temperatures and minimize desiccation. 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