Relations between variable rates of growth, metabolic costs and growth efficiencies in individual Sydney rock oysters (Saccostrea commercialis)

Rock oysters from a mass selection trial were compared with wild-caught (control) oysters of the same age to determine the physiological basis for faster growth rates amongst the selected individuals, and to describe the associated flexibility in phenotypic traits of feeding, metabolism and growth. In confirmation of earlier studies, fast growth was associated with faster rates of feeding, reduced metabolic rates and lower metabolic costs of growth. Selected individuals deposited more protein, at a lower metabolic cost, than the controls. Control oysters, however, deposited more lipid than the selected oysters, though the unit costs of lipid deposition did not differ between categories. The results indicated a wide plasticity of physiological rates and efficiencies and demonstrated how, by selection, interactions between physiological traits can serve to enhance growth. If differences in lipid deposition observed here were indicative of different rates of gametogenesis, then the results also suggest that selection alters the balance between growth and reproduction. Whether these differences can be termed compensatory with respect to the life history of the species remains to be determined, but the results indicate some of the ways in which physiological flexibility may be expressed to effect different patterns of energy allocation.     

Physiological components of growth differences between individual oysters (Crassostrea gigas) and a comparison with Saccostrea commercialis

Pacific oysters (Crassostrea gigas) of identical age from two genetically distinct lines, one fast growing and the other slow growing, were held at three levels of ration and analysed for physiological traits to explain differences in their rates of growth. The data supported three hypotheses; faster growth was associated with faster rates of consumption of food, reduced metabolic rate at maintenance (i.e., at zero growth), and reduced metabolic costs of growth. A comparison with the Sydney rock oyster, Saccostrea commercialis, based on similar experiments on the two species, indicated that faster growth of Pacific oysters depended on similar physiological differences; the mean metabolic costs of growth, however, were similar in the two species. It is suggested that a general model for genetically linked differences in the growth rate of bivalve molluscs will need to include the processes of metabolic control rather than relying solely on an analysis of the individual components of the energetics of growth.     

Multiple-Locus Heterozygosity and the Physiological Energetics of Growth in the Coot Clam, MULINIA LATERALIS, from a Natural Population

The relationship between individual energy budgets and multiple-locus heterozygosity at six polymorphic enzyme loci was examined in Mulinia lateralis. Energy budgets were determined by measuring growth rates, rates of oxygen consumption, ammonia excretion and clearance rates. Enzyme genotypes were determined using starch gel electrophoresis. Growth rate and net growth efficiency (the ratio of energy available for growth to total energy absorbed) increased with individual heterozygosity. The positive relationship between observed growth and multiple-locus heterozygosity was associated with a negative relationship between routine metabolic costs and increasing heterozygosity. Reduction in routine metabolic costs explained 60% of the observed increased growth of more heterozygous individuals. When routine metabolic costs were standardized for differences in feeding rates, these standard metabolic costs explained 97% of the differences in growth rate. Lower standard metabolic costs, associated with increasing heterozygosity, have been proposed as a physiological mechanism for the relationship between multiple-locus heterozygosity and growth rate that has been reported for a variety of organisms, ranging in diversity from aspens to humans. This study demonstrates that reduction of standard metabolic costs, at least in clams, accounts for virtually all of the differences in growth rate among individuals of differing heterozygosity.     

Influence of body size and food ration on the energy budget of rainbow trout Salmo gairdneri Richardson

Three size classes (3 g, 200 g, 1–3 kg) of rainbow trout, Salmo gairdneri Richardson, were each fed five rations ranging from zero to a 'maximum' ration and the relationships among body size, food ration, metabolism and growth examined. Water temperature was constant at 11–4°C. The gross growth efficiency in wet weight, K , increased asymptotically with increasing ration but decreased with increasing body size. Changes in fat and moisture content of the body, however, were such that Kin terms of energy utilization was independent of body size when the fish were feeding at the same level. This result was not apparent when wet weight values only were considered and use of a constant to convert wet weight offish to calorific values can lead to errors in interpretation of results. Assimilation efficiency averaged 71 -7 % over all feeding groups and was independent of body size and food ration. Total metabolic expenditure, however, increased with increasing body size and feeding level, but at any given feeding level a similar proportion of the energy of the assimilated ration was used for metabolism. At a single feeding level, therefore, the percentage utilization of energy in each category of the energy budget remained similar over all body sizes. Such a result can only be expected under laboratory conditions but it provides a base-line for comparison with the performance of other more complicated systems.     

Growing large in a low grade environment: size dependent foraging gain and niche shifts to cannibalism in Arctic char

Arctic char ( Salvelinus alpinus ) living in allopatry in lakes often show a distinct size dimorphism where the large char prey upon small char. We studied the relationship between size scaling of energy intake, total metabolic costs and levels of storage energy related to the niche shift from invertebrate feeding to cannibalism in an alpine Arctic char population. Gill-net samples consisted of two distinct size modes, small invertebrate feeding char (92–172 mm total length) and large cannibalistic char (265–606 mm total length). Food consumption was estimated for a one year period using a radioactive tracer (¹³⁷Cs). The energy intake followed a hump-shaped relationship with fish size for invertebrate feeding char and increased monotonically with size for cannibalistic char. Total metabolic costs increased exponentially with size for both groups. Char feeding in the invertebrate niche had a maximum net energy gain at 140 mm total length with an upper size limit for positive net energy gain at 180 mm. For cannibals the relationship between size and net energy gain formed a hump shaped function with maximum net energy gain at 400 mm and a lower size limit for positive net energy gain at 230 mm. The size at maturation for both small and large char was close to the size for maximum energy gain in their respective niches. Immature individuals close to the upper size limit for positive net energy gain in the invertebrate niche had low levels of storage lipids but tended to have high mass specific growth rates. This may imply a reallocation from reversible (stored) energy to structural (length) growth as a strategy to transform into the size-range where the cannibalistic niche successfully could be exploited.     

摄食水平对不同性别食蚊鱼仔幼鱼生长发育的影响

在实验室27℃水温下,研究了少食、中食和饱食三个摄食水平对0至25日龄雌雄食蚊鱼(Gambusia affinis)的生长发育特征和饵料利用效率的影响。26d饲养实验结束后,对试验鱼摄食和生长指标、臀鳍分化、性成熟及饵料转换效率进行分析。结果显示:到臀鳍开始分化时,饱食组雄鱼的累计摄食总能量和生长速度开始小于雌鱼,且随日龄的增加差异加大;随摄食水平的增加,0日龄仔鱼到臀鳍分化和性成熟的时间缩短。至实验结束,各摄食组的雄鱼均形成发育完善的生殖足,性腺都达到成熟状态;而雌鱼性成熟迟于雄鱼,且其性成熟更易受到摄食水平的影响,饱食组只有约50%的个体达到性成熟,少食组的卵母细胞则均处在小生长期。随着摄食水平的增加,雌雄鱼的体长、体重和干物质特定生长率均呈明显上升趋势,而干物质饵料转化效率则呈明显下降趋势;实验结束时,雌鱼的生长指标和干物质饵料转化率均大于雄鱼。以上结果表明,伴随臀鳍的分化,食蚊鱼在摄食、生长、发育、性成熟和应对食物丰度变化上表现出显著的性别差异。       

Separate effects of triploidy, parentage and genomic diversity upon feeding behaviour, metabolic efficiency and net energy balance in the Pacific oyster Crassostrea gigas

Triploid oysters were induced using cytochalasin B upon retention of either the first (meiosis I triploids) or the second (meiosis II triploids) polar body in embryos from a single cohort derived from mixed parentage. Allozyme and microsatellite assays enabled the confirmation of both parentage and triploidy status in each oyster. Comparison of meiosis I triploids, meiosis II triploids and diploid siblings established that improved physiological performance in triploids was associated with increased allelic variation, rather than with the quantitative dosage effects of ploidy status. An unidentified maternal influence also interacted with genotype. Among full sibs, allelic variation measured as multi-locus enzyme heterozygosity accounted for up to 42% of the variance in physiological performance; significant positive influences were identified upon feeding rate, absorption efficiency, net energy balance and growth efficiency (= net energy balance divided by energy absorbed). Whilst allelic variation was greater in both meiosis I and meiosis II triploids than in diploid siblings, both allelic variation and net energy balance were highest in triploids induced at meiosis I. This suggests that it may be preferable to induce triploidy by blocking meiosis I, rather than meiosis II as has traditionally been undertaken during commercial breeding programmes.     

Phenotypic flexibility and physiological tradeoffs in the feeding and growth of marine bivalve molluscs

Bivalve molluscs have a highly plastic feeding and growth physiology.The increasing availability of families artificially selectedfor faster growth has enabled physiological experiments to investigatethe genetic basis for variable rates of growth. Fast growthis achieved by a combination of increased rates of feeding,reduced metabolic rates and lower metabolic costs of growth.In at least one species there is a trade-off between growthin protein and the storage of lipids that are utilized in gametogenesis.Energy requirements for maintenance are also higher in slow-growingindividuals. Reduced costs of growth are due in part to increasedefficiencies of protein turnover. Nevertheless, high proteinturnover (and therefore high metabolic cost) may benefit fitnessin the later stages of gametogenesis. Faster feeding rates donot impair flexibility in feeding behavior which compensatesfor changes in the food environment. Both inter- and intra-speciesdifferences in feeding behavior are evident and suggest possibleconstraints imposed by faster feeding on the efficiency of selectionbetween food particles of different nutritional value.     

Energy partitioning in the Antarctic collembolan Cryptopygus antarcticus

Abstract. 1. Consumption, production and assimilation rates were determined for two age groups of Crypropygus antarcticus to give an estimate of energy utilization, and to investigate low temperature adaptation in its energy partitioning.
2. Feeding selectivity shown in laboratory preference tests was supported by gut analysis of field animals from contrasting sites. Although moulting rate was not significantly affected by food type, rates of growth were slowest and mortality highest when fed on a non-preferred substrate.
3. Both a radio labelling and a more direct method for measuring dry weight consumed gave similar results for Cvpropygus feeding on algae. The consuniption rate for animals when feeding on algae was lower than that on moss peat. The assimilation efficiency for immature animals feeding on algae was 46% and for mature animals was 19%; the values when feeding on moss peat were 7% and lo%, respectively, The net production efficiency ranged from 35%(inimatures) to 13% (matures) and was similar on both substrates.
4. Food consumption exceeded assimilation over the range 2.5–10°C, but the two converged from 2.5 to 0°C. Immature Cryptopygus maintained a net positive energy balance over 0–10°C, whilst below 1S°C respiration exceeded assimilation for mature individuals.
5. An estimate of the annual dry matter consumption (7 g m^-1 y^-1) by Ctypropygus in a moss turf at Signy Island agrees with one based on respiration data alone (Davis, 1981). The consumption at an alga-dominated site was c . 26 g m^-2 y^-l, and Crypropygus may have a locally limiting effect on net priniary production at such sites.     

Growth and tissue composition as a function of feeding history in juvenile cephalopods

We present the results of a series of experiments that examined the effect of feeding history on the growth and tissue composition of juveniles of two tropical cephalopods; the squid Sepioteuthis lessoniana and the cuttlefish Sepia elliptica. Juveniles were reared in individual containers for between 35 and 42 days at different ration levels, three ration levels for the squid and two levels for the cuttlefish. Although differences in ration were sufficient to cause different growth rates, both in body length and mass, the effects on tissue composition were less definitive. Sepioteuthis juveniles on the highest rations had higher concentrations of water, but no difference in lipid, carbohydrate or protein when compared with their lower ration siblings. In the case of juvenile cuttlefish no difference in tissue composition was detected between the two ration levels. RNA:protein ratios were also determined for the juveniles to provide an estimate of instantaneous growth. A significant correlation was found between body size and RNA:protein ratio in the squid; those juveniles that ate more had higher RNA:protein ratios than lower ration individuals. Significantly, the juvenile cuttlefish showed no relationship between growth rate and RNA:protein ratios, which means that we are unable to use this measure to estimate the growth rates of wild individuals. In conclusion, ration level did affect growth rates and food availability is an important factor in modifying growth rates of wild individuals. However, we could not find, at the individual level, an index or measure that could be used to explain the variability of observed differences in growth rates as a function of nutritional history.     

Aspects of feeding,including estimates of gut residence time,in three mytilid species (Bivalvia,Mollusca) at two contrasting sites in the Cape Peninsula,South Africa

Individuals of three mytilid species (Choromytilus meridionalis; Perna perna; Aulacomya ater) from two sites characterised by different qualities of ration available to these suspension feeders showed different rates in some components of the physiological energy budget. These differences included higher feeding (=clearance) and respiration rates, but lower absorption efficiencies, in individuals from the site with the higher quality ration. A novel technique was employed to estimate the residence time of food particles in the digestive gland. Individuals showing higher feeding rates had shorter residence times than those feeding more slowly and a significant positive correlation was demonstrated between residence time and absorption efficiency. These relationships, together with an exponential increase in rates of respiratory heat loss with an increase in ingested ration, are suggested to provide these animals with a physiological flexibility to compensate for reduced food quality in a way consistent with some theoretical predictions. Such compensations are made more effective if the total gut capacity can also change in response to the quality of the ration.     

Growth and body composition changes in mice selected for high post-weaning weight gain on two levels of feeding

Summary Two lines of mice were selected for high post-weaning weight gain (3 to 6 weeks) adjusted for 3 week weight. One line (F) was grown on freely available food and the other (S) on a feeding scale set at the same level for all mice. Food intake of the S line averaged 80% of the F line. The realised heritabilities after 6 generations of selection were 0.38±0.06 and 0.33±0.07 for the F and S lines, respectively. In generation 7, mice from the F and S lines and from an unselected control line (C) were compared on both free and set levels of feeding from 3 weeks to 9 weeks of age. Measurements taken were growth rate, appetite, food conversion efficiency (weight gain/food intake) and body composition (fat, protein, ash, water). The F and S lines grew more rapidly and efficiently than the C line on both levels of feeding, each line performing best on the level of feeding on which it was selected. The average genetic correlation between growth rates of the same line on the two feeding levels was 0.54±0.10. The F line grew 19% faster and was 9% more efficient than the S line on free feeding but the S line grew 15% faster and was 15% more efficient than the F line on set feeding. Relative to the C line, food intake per day on free feeding was 4% higher in the F line and 6% lower in the S line. There was no difference between the lines in food intake/g body weight. The rate of deposition of all body components increased in both selection lines. In the F, S and C lines respectively, efficiencies of gains in body components (10²x gain/food) were 1.79, 1.31 and 1.06 for fat, 1.53, 1.63 and 1.22 for protein and 5.88, 6.45 and 4.98 for protein + water. Apparently energy lost as heat was reduced in both the F and S lines. The partitioning of energy retained was altered in favour of more fat in the F line and more protein in the S line.     

Food consumption and growth of larvae and juveniles of three cyprinid species at different food levels

Synopsis The relationships between food availability, consumption and growth were analyzed from the onset of feeding to an age of 90 days in three cyprinid species. Fish were held at 20 ± 0.5° C and given two (three) constant rations of approximately 30, (40) or 100% dry body weight (dbw) ind^-1 day^-1. Food consisted of living zooplankton, the size of which correlated with fish size. At high food densities consumption rates decreased rapidly with fish size in all three species. At reduced rations, fish consumed most of the food offered until they were larger than 10 mg dbw. In all species and at each feeding level daily rations consumed increased allometrically with body size. Respiration rate, expressed as routine metabolic rate differed insignificantly between the three species. At high ration levels, growth rates of small bleak, Alburnus alburnus, were distinctly lower than those of roach, Rutilus rutilus, and blue bream, Abramis ballerus. At low food supply all three species grew at similar rates. Assimilation efficiency at low food conditions was approximately twice that of the well-fed groups. If the caloric equivalents of oxygen consumption as measured in well-fed fish are applied to fish fed at low rations their energy budgets do not balance. This indicates the limitations of fish larvae in the partitioning of energy for growth or activity at such conditions.     

Effects of obesity and sex on the energetic cost and preferred speed of walking.

The metabolic energy cost of walking is determined, to a large degree, by body mass, but it is not clear how body composition and mass distribution influence this cost. We tested the hypothesis that walking would be most expensive for obese women compared with obese men and normal-weight women and men. Furthermore, we hypothesized that for all groups, preferred walking speed would correspond to the speed that minimized the gross energy cost per distance. We measured body composition, maximal oxygen consumption, and preferred walking speed of 39 (19 class II obese, 20 normal weight) women and men. We also measured oxygen consumption and carbon dioxide production while the subjects walked on a level treadmill at six speeds (0.50-1.75 m/s). Both obesity and sex affected the net metabolic rate (W/kg) of walking. Net metabolic rates of obese subjects were only approximately 10% greater (per kg) than for normal-weight subjects, and net metabolic rates for women were approximately 10% greater than for men. The increase in net metabolic rate at faster walking speeds was greatest in obese women compared with the other groups. Preferred walking speed was not different across groups (1.42 m/s) and was near the speed that minimized gross energy cost per distance. Surprisingly, mass distribution (thigh mass/body mass) was not related to net metabolic rate, but body composition (% fat) was (r2= 0.43). Detailed biomechanical studies of walking are needed to investigate whether obese individuals adopt novel energy saving mechanisms during walking.     

Resting metabolic expenditure as a potential source of variation in growth rates of the sagebrush lizard

Along an elevational gradient on SW Utah, sagebrush lizards (Sceloporus graciosus) exhibit an unexpected pattern of growth. Lizards from a high elevation population grow faster than lizards from two populations at lower elevations despite shorter daily and seasonal activity. Results from a common environment study of growth suggest that the differences in growth are not due to adaptation to local environmental conditions. In this study, I test the hypothesis that higher growth rates in lizards from high elevation may be attributable to reduced resting metabolic expenditure compared to that of lizards from populations at two lower elevations. Resting metabolic rates were measured for individuals from each of the study populations across different times of day and over a broad range of temperatures. Under the same laboratory conditions, field-caught lizards from the high elevation population exhibited lower metabolic rates when compared to lizards from lower elevations. Daily resting metabolic expenditures were calculated using the observed metabolic rates coupled with estimates of daily activity. Daily resting metabolic expenditure was 50% greater for individuals from the two lower elevation populations, which could result in 12.5% more energy that could be potentially allocated to growth for lizards from high elevation. Such energetic savings may be able to explain differences in the patterns of growth observed in nature.     

Seasonal variability in feeding behaviour, metabolic rates and carbon and nitrogen balances in the Sydney oyster, Saccostrea glomerata (Gould)

To establish if nutrients limit the growth of bivalves requires information not only on the quality of food available, but also the animals' feeding behaviour and endogenous metabolic demands. We hypothesized that growth of the Sydney rock oyster (Saccostrea glomerata) would vary in response to seasonal changes in food quality rather than quantity. We also predicted that the oysters would show feeding preferences for nitrogen over carbon, and this behaviour would result in carbon/nitrogen (C/N) ratios for ingested and absorbed matter that would be lower than the C/N of both the seston and the oysters' estimated metabolic maintenance requirements. The experiments were done in two phases under natural conditions. In phase 1, feeding behaviour was assessed on a single occasion and the results used to pose hypotheses for testing in phase 2, which included measurements made on three occasions encompassing autumn, winter and spring conditions. Growth rates varied with changes in ambient food quality and not with the concentration of total suspended matter. Feeding behaviour responded to food quality and, in most cases, resulted in nitrogen enrichment. For example, when nitrogen was potentially limiting to growth and/or maintenance, due to high food C/N (July) or high nitrogen demand (March), pre-ingestive selection ensured nitrogen enrichment of ingested matter and C/N ratios of ingested matter which were below the maintenance requirement. However, in November, when endogenous demands indicated an increased requirement for carbon, feeding behaviour resulted in carbon enrichment, an increase in carbon conversion efficiency, and ingested C/N ratios greater than the maintenance requirement. The results support the assertion of variable feeding physiology in oysters, responsive to both exogenous (seasonal differences in carbon and nitrogen availability) and endogenous (cycles of reproduction and growth) factors.     

Direct effects of physical stress can be counteracted by indirect benefits: oyster growth on a tidal elevation gradient

The paradigmatic gradient for intertidal marine organisms of increasing physical stress from low to high elevation has long served as the basis for using direct effects of duration of water coverage to predict many biological patterns. Accordingly, changes in potential feeding time may predict the direction and magnitude of differences between elevations in individual growth rates of sessile marine invertebrates. Oysters (triploid Crassostrea ariakensis) experimentally introduced at intertidal (MLW+0.05 m) and subtidal (MLW–0.25 m) elevations in racks provided a test of the ability to use duration of water coverage to predict changes in growth. During early-to-mid winter, a depression of 38–47% in shell growth of intertidal oysters matched the 36% reduction in available feeding time relative to subtidal oysters. In late winter as solar heating of exposed oysters increased, growth differences of 52–55% departed only slightly from the predicted 39%. In spring, however, duration of water coverage failed to predict even the correct direction of growth change with elevation as intertidal oysters grew 34% faster despite 39% less feeding time. Intense seasonal development of shell fouling by other suspension feeders like ascidians, mussels, and barnacles on subtidal (94% incidence) but not on aerially exposed intertidal (21–38% incidence) oysters may explain why duration of water cover failed to predict spring growth differences. Less intense fouling develops on intertidal oysters due to the physiological stress of aerial exposure on settlers, especially during higher temperatures and longer solar exposures of spring. Fouling by suspension feeders is known to reduce growth of the host through localized competition for food and added energetic costs. Thus, in springtime, indirect effects of aerial exposure providing a partial refuge from biological enemies overwhelmed direct effects of reduced duration of water coverage to reverse the expected pattern of slower intertidal growth of a marine invertebrate.     

Correlated responses to selection for faster development and early reproduction in Drosophila: the evolution of larval traits

Studies on selection for faster development in Drosophila have typically focused on the trade-offs among development time, adult weight, and adult life span. Relatively less attention has been paid to the evolution of preadult life stages and behaviors in response to such selection. We have earlier reported that four laboratory populations of D. melanogaster selected for faster development and early reproduction, relative to control populations, showed considerably reduced preadult development time and survivorship, dry weight at eclosion, and larval growth rates. Here we study the larval phase of these populations in greater detail. We show here that the reduction in development time after about 50 generations of selection is due to reduced duration of the first and third larval instars and the pupal stage, whereas the duration of the second larval instar has not changed. About 90% of the preadult mortality in the selected populations is due to larval mortality. The third instar larvae, pupae, and freshly eclosed adults of the selected populations weigh significantly less than controls, and this difference appears during the third larval instar. Thereafter, percentage weight loss during the pupal stage does not differ between selected and control populations. The minimum amount of time a larva must feed to subsequently complete development is lower in the selected populations, which also exhibit a syndrome of reduced energy expenditure through reduction in larval feeding rate, larval digging and foraging activity, and pupation height. Comparison of these results with those observed earlier in populations selected for adaptation to larval crowding and faster development under a different protocol from ours reveal differences in the evolved traits that suggest that the responses to selection for faster development are greatly affected by the larval density at which selection acts and on details of the selection pressures acting on the timing of reproduction.     

Relationships between the synthesis and breakdown of protein,dietary absorption and turnovers of nitrogen and carbon in the blue mussel,Mytilus edulis L.

Summary Seasonal and nutritionally induced changes of whole body protein metabolism have been studied in 45 to 57 mm shell-length Mytilus edulis from Whitsand Bay, southwest England. The subtraction of measured net protein balances from coincident rates of protein synthesis, determined in vivo by supplying ¹⁵N-labelled alga and monitoring the enrichment of excreted ammonia, enabled computation of protein breakdown rates. Over the range of protein absorption from zero to 0.58% of total soft tissue protein 24 h^-1, fractional rates of protein breakdown decreased from 0.41 to 0.03%, whereas protein synthesis and net protein balance both increased from 0.25% to 0.39% and from-0.16% to 0.36%, respectively. The progressive reduction in fractional protein degradation with elevated net protein balance represented a protein sparing effect, whereby the efficiency of protein synthesis (defined as net synthesis/overall synthesis) confirmed theoretical predictions of as much as 92% during periods of maximal growth. In addition, 38% of breakdown products were recycled directly to synthesis under conditions of zero net balance, with an increasing contribution evident upon further decreases of protein absorption. The overall response was characterized by a consistently conservative elemental turnover of nitrogen relative to carbon, so that as a fraction of each element absorbed, between 1.2 and 1.9 times as much nitrogen was incorporated within structural materials. Such conservation was most pronounced among mussels starved prior to experimentation, indicating nutritionally related efficiencies in the utilization of resources for synthesis. The changing balance between individual processes also effected large alterations in proportional size of the metabolic pool of free amino acids (0.2 to 14.5% of total soft tissue nitrogen). Finally, it is suggested that adjustments of protein synthetic rate may be significant in the regulation of energy expenditure, accounting for at least 16% of basal energy requirements. Results throughout have been compared and contrasted with those for mammals, and whole-body measurements of both protein synthesis and breakdown proposed as a valuable index for environmental effects on instantaneous growth and metabolism.     

Feeding and Digestion in Suspension-Feeding Bivalve Molluscs: The Relevance of Physiological Compensations

SYNOPSIS. Intertidal suspension-feeding bivalves face reductionsin the time available for feeding proportional to the durationof aerial exposure. Such individuals do not compensate by increasingtheir rates of feeding during immersion. Simulations of feedingbehaviour, in terms of the energetic balance between the costsand the gains of changes in feeding rate, suggest that variationfrom the rates characteristic of the sublittoral condition wouldnot enhance net energy gain for intertidal individuals. Thesefindings are consistent with evidence of potential food limitationin the natural sublittoral habitat. Nevertheless, hydrodynamicfactors may enhance the supply of food on the shore, so amelioratingthe disadvantages of reduced feeding time. In addition, potentialphysiological compensations for reduced dietary quality includechanges in gut residence time, the volume of gut occupied fordigestion and (possibly) also in the provision of appropriatedigestive enzymes. However, these compensations require daysor weeks and are inappropriate on tidal time scales.