Brian L. Bayne

The physiology of suspension feeding by bivalve molluscs: an introduction to the Plymouth TROPHEE workshop

Abstract Suspension-feeding behaviour in bivalve molluscs is rich and varied, responsive in a variety of traits to changes in both the quantity and quality of available food. Recent research on a number of species documents this complexity with respect to both pre-ingestive and post-ingestive processing of food particles. An hypothesis of feeding as “automatized”, with no capacity for compensation to changes in the food environment, is not supported. In studies to understand the responsiveness of feeding behaviour to the environment, and its consequences for growth, an energy balance approach has proved useful. Similarly, formal statements of energy gains and losses are used to good effect in models of growth which may then be interfaced to ecological measures of carrying capacity. This interface between physiology and ecology was the main focus of a workshop held in September 1996, which provided a forum for discussion between physiologists, ecologists and modellers. This introduction to the workshop sketches some of the recent developments in our understanding of suspension feeding, argues for an approach that recognises the diversity of behaviours that are evident, and suggests some possibilities for future advances based on the application of novel technologies.

HYBRID VIGOR IN PACIFIC OYSTERS: AN EXPERIMENTAL APPROACH USING CROSSES AMONG INBRED LINES

Two competing genetic hypotheses for heterosis, dominance and overdominance, have been championed to explain positive correlations between allozyme heterozygosity and fitness-related traits for bivalve molluscs. To begin to test these hypotheses, we made controlled crosses among inbred lines of the Pacific oyster Crassostrea gigas. In such mating experiments, heterosis (hp) can be defined and quantified through ANOVA as QL > 1.0 or < − 1.0, where L is the trait-difference between the two parental inbred lines and Q is twice the deviation of the hybrid from the mid-parent value (Griffing, 1990). Inbred lines of the Pacific oyster were made by selfing hermaphrodites; brood stock pedigrees and a grand mean fixation index of F = 0.5 were confirmed by allozymes. In two separate experiments pairs of inbred lines were crossed in 2 X 2 fashion to produce two inbred and two hybrid progeny genotypes. Each genotype was replicated by pairwise matings and replicates were grown in multiple containers (8 1 plastic bags), from which data on larval mortality (proportional decrement in population number per day) and size (shell-height) were obtained. In the first experiment, the two inbred genotypes differed significantly from each other in daily larval mortality (−0.115 vs. −0.881) and both hybrids had lower mortalities than inbred genotypes (−0.034 and −0.078); mean hp, 1.15, was significantly greater than 1.0. By Day 14, larvae of one hybrid group were significantly smaller than those of the inbred genotypes (241.0 μm vs. 253.5 and 259.2 μm, respectively); only a very small part of this difference could be attributed to negative correlation between larval density and growth. Negative heterosis for larval size was highly significant (hp = −5.36, P < 0.001) and persisted to the juvenile stage (mean hp = −1.34, ns, for shell-height at Day 154 and hp = −7.77, P < 0.001, for live-weight at Day 163). In the second experiment, inbred genotypes again differed significantly in daily larval mortality (−0.341 vs. −0.654). One hybrid group had significantly less daily mortality than either parent (−0.110), but the other was similar to the best parent (−0.347), so that for one hybrid, hp = 2.47, P < 0.01, and for the other, hp = 0.96, ns). Variance among genotypes for larval shell-length was highly significant on Days 2, 7, and 14, with heterosis evident on Days 2 and 7 (hp = 1.04 and hp = 3.84, respectively). At 11 months of age, hybrid shell-height averaged 150% of the shell-height for the best inbred parent and hp ≈ 7.7. These measurements of heterosis, both positive and negative, implicate a third explanation of heterosis, epistasis. Intercrosses of F1 hybrids can be used to discriminate among dominance, overdominance and epistasis hypotheses for quantitative trait loci (QTL) causing hybrid vigor.

Feeding Physiology of Bivalves: Time-Dependence and Compensation for Changes in Food Availability

In spite of considerable experimental and observational study over many years, controversies and uncertainties still exist concerning fundamental features of feeding behaviour in suspension-feeding bivalves (Bayne and Newell, 1983; Griffiths and Griffiths, 1987; Jorgensen, 1991). These include uncertainties over the mechanisms of particle capture (Jorgensen, 1983; Silvester and Sleigh, 1984; Shimeta and Jumars, 1991; Ward et al., 1991) and controversy over the role of physiological processes in determining feeding behaviour (Bayne et al., 1988; Jorgensen, 1991; Iglesias et al., 1992). This paper addresses the second of these topics and suggests that physiological and behavioural compensations for changes in the food environment are important elements for a full understanding of suspension-feeding. This is not to deny the significance of physical constraints on feeding, such as the viscocity of the medium, or frictional forces influencing water flows within the mantle cavity, but rather to argue that behavioural and physiological flexibility in aspects of particle capture, sorting, digestion and absorption are also important. Since physiological traits are inherently timedependent, it follows that feeding in bivalves should be viewed as a linked series of processes with different time-constants, coupled to relevant time-scales of change in the environmental availability of food.

Nutrition of the giant clam Tridacna gigas (L.) I. Contribution of filter feeding and photosynthates to respiration and growth

The total carbon requirements (growth + respiration) of the host tissues of the giant clam Tridacna gigas from Davies Reef on the Great Barrier reef were measured, and compared with rates with which nutrients were acquired from the two potential sources, translocated photosynthates (estimated from oxygen flux-CZAR method) and filter feeding. Results showed that the giant clam is an efficient utilizer of particulate organic matter available in reef waters (Davies Reef mean: 97 μg C·1−1), retaining on average 75% of particles between 2 and 50 μm, and absorbing from them 54% of C. Clearance rates (CR, 1·h−1) of clams were size dependent as defined by the function: CR = 1.85 W0.58 (r2 = 0.85, n = 56). There are major differences between typical non-symbiotic bivalves and Tridacna gigas regarding the relative allocations of energy to respiration and growth. The proportion of carbon deposited in tissues relative to that respired is high in giant clams relative to completely heterotrophic bivalves. We conclude that autotrophy is the major source of carbon to this clam, potentially capable of satisfying all respiratory requirements of the host. However, the potential importance of heterotrophy to total energy needs of the host is also significant and changes with the size of clam. The spectacular rates of growth in this clam are such that filter feeding is able to provide 65% of the total carbon needed both for respiration and growth in small clams (100 mg dry tissue wt), whereas large clams (10 g) acquire only 34% of their carbon from this source.

Quantitative and molecular genetic analyses of heterosis in bivalve molluscs

Abstract Associations of allozyme-heterozygosity with growth and its physiological underpinnings have been well documented for bivalve molluscs. The associations are correlational, however, derived almost entirely from studies of wild-caught juveniles or adults. Such studies cannot resolve alternative genetic explanations of heterosis. Four experimental approaches have recently been made to this problem; (1) a correlational study contrasting allozyme and presumably selectively neutral nuclear DNA polymorphisms; (2) detailed studies of allozyme inheritance in families; (3) a study contrasting the performance of meiosis-I and meiosis-II triploids with diploids and (4) a classical quantitative genetic study of the performance of hybrids produced by crosses among inbred lines. The last approach has uncovered remarkable heterosis in growth and its physiological components, both for the larval and juvenile or adult stages, and has implicated epistasis as a significant cause of this heterosis. More importantly, this approach now permits dissection of heterosis into quantitative trait loci (QTL) mapped by the co-segregation of allozyme and nuclear DNA markers with growth phenotypes in the F2 hybrid and backcross generations.

Feeding behaviour and metabolic efficiency contribute to growth heterosis in Pacific oysters [Crassostrea gigas (Thunberg)]

Abstract Physiological measurements were made on Pacific oysters from controlled crosses between inbred lines. Hybrid individuals were expected to perform better than inbred oysters, for a variety of traits related to feeding behaviour. The oysters were offered a diet simulating natural suspended particulate matter. By quantifying the organic and inorganic fractions of food, faeces and pseudofaeces, various aspects of feeding were elucidated. The results agreed with expectation; on average, hybrid oysters had higher rates and efficiencies of feeding and growth than inbreds. In one experiment there were significant differences between hybrids and inbreds for seventeen out of twenty cases; in another experiment hybrids performed better than inbreds for eight out of sixteen cases. In both experiments, we find significant differences between the reciprocal hybrids, though heterosis for growth is evident for all hybrids. Our experiments therefore confirmed heterosis for growth and hybrid superiority for physiological traits, independent of ration level; emphasised the complexity of these relationships amongst genotypes; and demonstrated the segregation of physiological traits in the F 2 generation.

A genetic and metabolic basis for faster growth among triploids induced by blocking meiosis I but not meiosis II in the larviparous European flat oyster, Ostrea edulis L.

This study establishes a genetic and metabolic basis to faster triploid growth in the oyster Ostrea edulis. Triploidy was induced using cytochalasin B, and image analysis of biopsied tissue employed to ensure similar ploidy of all animals within each class. Results indicate that lifetime growth in total dry tissue weight over 15 months was more than 60% faster (p<0.001) in meiosis I triploids than in diploid siblings or meiosis II triploids, with no difference between meiosis II triploids and their diploid siblings. For six polymorphic enzyme loci, single-locus heterozygosity was consistently greatest in meiosis I triploids (p<0.001), so that average multiple-locus heterozygosity in meiosis I triploids was 49% higher than in normal diploids, and 55% higher than in meiosis II triploids (p<0.001). This suggests that faster growth resulted from increased allelic diversity, rather than the increased allelic quantity that results from the addition of one entire set of chromosomes among triploids generally. Results also confirm that the faster growth of meiosis I triploids resulted from reduced energy expenditure, associated with lower concentrations of RNA per unit total tissue protein, which infer reduced rates of whole-body protein turnover. Statistical analyses confirmed that differences in oxygen consumption and growth were associated with both ploidy class and average multiple-locus heterozygosity, indicating that performance in meiosis I triploids is not only improved as a result of reduced reproductive output, but also through the metabolic consequences associated with increased heterozygosity.

Nutrition Of Marine Mussels - Factors Influencing The Relative Utilizations Of Protein And Energy

Abstract Evidence is reviewed showing that biotic factors influencing the relative utilizations of absorbed protein and energy in marine mussels and other organisms may either be metabolic or dietary. Metabolic factors include genotype, size and nutritional/physiological state, which each influence the efficiencies with which absorbed ration is utilized for maintenance metabolism. Dietary factors comprise quantitative and qualitative variations in food intake, which influence the relative efficiencies with which absorbed protein and energy are utilized both for maintenance and for growth. Effects of metabolic factors on the relative utilizations of protein and energy for maintenance stem largely from their influence on whole-body protein turnover (WBPT), defined as the continuous breakdown and renewal of cellular proteins. WBPT incurs high energy losses due to the costs of protein synthesis, whereas up to 89% of the amino-N derived from protein breakdown may be recycled directly to protein synthesis. Consequences of different WBPT are therefore evidenced primarily in terms of maintenance requirements for energy, rather than protein. Further, regardless of dietary intake, significant WBPT results in consistent conservation of the absorbed protein relative to energy. On this basis, we suggest that mussels and other sedentary filter-feeders are more likely to be growth-limited by available energy than by protein. We also predict that genetically heterozygous individuals or populations characterized by reduced energy requirements for maintenance will be least affected by nutrient limitation, and will achieve faster growth on diets containing smaller proportions of non-protein energy. We suggest that metabolic requirements may vary according to genotype, dietary composition and nutritional/physiological status. This signifies that ranges of values, both in absolute and in qualitative terms, will prove most useful for defining nutrient requirements. Average protein requirements for maintenance in mussels (87.3 ± 29.06 mg protein N per kg 0.75 dry weight d −1 ) are nevertheless similar to those published for fish and mammals, so that absolute differences between the metabolic requirements of poikilothermic marine organisms and homeotherms appear restricted to energy.

The metabolic/physiological basis of genotype-dependent mortality during copper exposure in Mytilus edulis

Fitness consequences of interrelations between genotype, protein metabolism and physiology were examined in 88 individual Mytilus edulis exposed to 150 pbb dissolved copper until death. Electromorphs heterozygous for the enzyme phosphoglucose isomerase (Pgi) survived longest, complementing previous findings, and suggesting an adaptive nature of specific Pgi genotypes. Time to death varied between 4 and 28 days, and did not correlate with tissue concentrations of copper at death. Instead, multiple regressions confirm separate positive effects both of filtration rate and protein turnover upon rate of copper accumulation, as well as separate negative effects of these same two processes upon time to death. Rate of filtration was the primary physiological determinant of survival; we discuss underlying relations whereby the faster breakdown and renewal of proteins also accompanied reduced survival during stress induced by exposure to high levels of dissolved copper.