J.I.P. Iglesias

Some general relationships in comparing the feeding physiology of suspension-feeding bivalve molluscs

We have studied feeding behaviour in the cockle Cerastoderma edule (L.), the oyster Crassostrea gigas (Thunberg) and the mussel Mytilus edulis (L.) for input to a model predicting the environmental capacity of the bay of Marennes-Oleron, France for shellfish culture. We present a common set of equations that summarize functional interrelations observed between component processes of feeding physiology, whilst allowing predictions of feeding behaviour and net organic absorption rate on the basis of seston abundance and seston organic content alone. Selective processes, and the consequences of those processes, are an important feature of those interrelations. Each species was able selectively to enrich the organic content of ingested matter relative to filtered matter, preferentially rejecting inorganic matter prior to ingestion as pseudofaeces. When feeding upon natural seston, the efficiency of that selection varied positively with both the mass of seston filtered h−1 and the organic content of filtered matter. At the highest food availabilities, when the mass of seston filtered h−1 was greatest, more than 60% of the organic matter ingested h−1 by each species resulted from selective processes. Physiological consequences of that selection were amplified by positive exponential relations between the net absorption efficiency for ingested organics and the organic content of ingested matter. We show that our common set of equations satisfactorily predict net organic absorption rate measured directly in all three species feeding throughout the same natural tidal variations of food. Collective findings therefore establish that similar functional interrelations control feeding responses in each species, and identify key relations affecting selection and absorption for use in the future modelling of growth and environmental relations. By fitting a common set of equations to responses measured directly under the same natural conditions of seston availability, we have standardised the comparison of environmental influences upon rates and efficiencies of feeding behaviour for each species. The mass of seston filtered h−1 increased in similar positive relations with seston abundance in each species. However, there were significant behavioural differences in the processing of filtered particles. Compared with the epifaunal species C. gigas and M. edulis, C. edule is normally infaunal, and demonstrated a lower capacity to selectively ingest organic matter. Alternatively, compared with M. edulis, C. gigas was not as efficient either in the net selection of organic matter or in digesting and/or assimilating ingested organics, resulting in lower rates of net energy gain. These differences are discussed bearing in mind present experimental conditions, as well as the natural habitat and comparative morphology of each studied species.

Variability of feeding processes in the cockle Cerastoderma edule (L.) in response to changes in seston concentration and composition

Physiological processes controlling food acquisition by the filter feeding bivalve Cerastoderma edule (L.) were quantified under a broad range of seston concentrations and compositions. Experimental diets consisted of suspensions elaborated by adding variable amounts of microalgal cells of different species (or sediment particles in one case) to natural sea-water. Clearance rates exponentially decreased with seston concentration, but the rate of reduction was higher with suspensions of high organic content. Pseudofaeces production appeared as a positive function of the rate of particle filtration; however, for a given filtration rate, more pseudofaeces were rejected when filtered matter had a low organic content. As a consequence, after an initial elevation, ingestion rate remained almost constant across particle concentrations. Pre-ingestive food selection enhanced the rate of particulate organic matter ingestion and this organic enrichment of ingested matter became more pronounced for diets of low food value, where most filtered matter was being rejected as pseudofaeces. Selection of particles at the pre-ingestive level was more efficient in terms of chlorophyll, revealing preferential ingestion of algal particles compared with the whole organic matter. Stronger selection for algae, however, was not evident in terms of preferential nitrogen ingestion, as compared with carbon, which was probably due to similar low values of the CN ratios in all experimental conditions. Absorption efficiency depended on the organic content of ingested matter according to an exponential, saturating function. In general, feeding processes of cockles appear well adapted to cope with elevations in particle concentration and simultaneous reductions in the food value of available seston that occur when resuspended bottom sediments constitute a significant fraction of particulate materials of the water column. Under these conditions, high rates of seston filtration and pseudofaeces production, together with preferential organic ingestion act to compensate for the dilution of organic matter in suspension and its detrimental effect on the rate of food absorption. However, this compensatory behaviour is not so efficient as to make absorption rate independent of the organic value of available particles. The organic content of resuspensible sediments may thus become the main determinant of food acquisition in cockles.

Measuring feeding and absorption in suspension-feeding bivalves: an appraisal of the biodeposition method

Abstract In this paper we analyse methods that have been used to quantify the different physiological processes involved in food acquisition and absorption by suspension-feeding bivalves. In particular, we have considered those methods that have been used for determining food processing rates under ambient conditions of food availability. During the last few years, an increasing number of studies has been performed using the biodeposition method, which estimates feeding and absorption rates through measurements of suspended particles and biodeposit production. The main assumption of this method is that ingested particulate inorganic matter can be used as an inert tracer of feeding and digestive processes, i.e. that secretion or absorption of inorganic matter across the gut wall is negligible. There are several points that have to be taken into account when applying this method: a) the method assumes that sampled suspended particles and those retained by the gill are of similar organic content; b) faeces and pseudofaeces collection must be quantitative and separate and particle sedimentation must be controlled, if not avoided; c) where particulate food availability or composition is expected to fluctuate, the time lag for particles being captured and processed before they appear in the form of biodeposits must be evaluated to provide a proper reference value for available food. The biodeposition method and more conventional procedures provide equivalent results for estimating clearance rates. Moreover, predictions of scope for growth obtained with this technique also seem to be compatible with directly measured growth rates.

Infaunal Filter-Feeding Bivalves and the Physiological Response to Short-Term Fluctuations in Food Availability and Composition

An evaluation of nutritional determinants of growth in natural populations of bivalve molluscs involves two considerations: (a) The discontinuity of food environment, characterized by broad fluctuations in both the concentration and composition of the particulate complex that is accessible to filtration systems of animals.(b) The time-course of this discontinuity, ranging from seasonal to diurnal or tidal time scales.

Energetics of growth and reproduction in a high-tidal population of the clam Ruditapes decussatus from Urdaibai Estuary (Basque Country, N. Spain)

Abstract Energetics of growth and reproduction were studied in a high-tidal population of the clam Ruditapes decussatus living in the Mundaka Estuary in the Biosphere Reserve of Urdaibai (Basque Country, North Spain). The study included an analysis of growth rings on the shells to establish the growth curve as well as seasonal patterns of growth and body condition, and estimates of the breeding cycle including quantification of the reproductive output and reproductive effort. The simultaneous determination of the seasonal course of metabolism allowed estimates of assimilation, growth efficiency and reproductive costs. Growth rates were consistently lower in this population than in other populations from similar latitudes, and this effect is interpreted in terms of nutritional restrictions caused by the high tidal position of the population. Assimilation rapidly increased from March to July, as a consequence of optimal nutritional conditions and increasing water temperatures. Somatic growth (spring) and gonadal development (early summer) both took place during this period. Negative growth was restricted to the winter and late summer and was associated with poor nutritional conditions (winter) and high rates of metabolic expenditure induced by high temperatures (late summer). Net growth efficiencies (ranging from 27% in 1-y-old to 6% in 7-y-old individuals) ranked among the lowest recorded for populations of marine bivalves. Reduced reproductive-effort values were consistent with the poor growing conditions that appeared to characterise this population of clams.

Seasonal variation of digestive enzyme activities in the digestive gland and the crystalline style of the common cockle Cerastoderma edule

Abstract Seasonal variation of amylase, cellulase, laminarinase and protease activities of the main digestive organs (crystalline style and digestive gland) of the cockle Cerastoderma edule have been investigated. The simultaneous increment of specific enzyme activities (mg end product/mg protein per h), mass (mg), and protein content of digestive organs yield a remarkable enhancement of total enzyme activities (mg end product per organ/h) during Spring–Summer as compared with Autumn–Winter. Although extracellular enzymes of the crystalline style have typically received more attention than intracellular enzymes from the digestive gland, the present study shows the latter to release as much as ten times higher total activities than the crystalline style, highlighting the relevance of intracellular digestion in cockles. The sole effect of temperature on enzyme activity does not explain recorded seasonal variability. Additionally, the present study has shown that enzyme activities in the digestive gland increased rapidly (3 days) in the presence of food at any time of the year, suggesting that seasonal variation of enzyme activities would reflect a process of continuous adjustment of digestive enzyme levels to environmental food availability. Further shaping of the seasonal cycles may be influenced by requirements of specific biochemical components and past feeding history.

Acute and acclimated digestive responses of the cockle Cerastoderma edule (L.) to changes in the food quality and quantity: II. Enzymatic, cellular and tissular responses of the digestive gland

Cockles Cerastoderma edule were fed two different concentrations of two diets with different qualities which were achieved by mixing different proportions of ashed silt particles with cells of the microalgae Tetraselmis suecica. After 3 days (acute response) and 11 days (acclimated response) of exposure to the diets, we analysed the digestive activity of the digestive gland using cyto-histological and enzymatic techniques. We measured (i) the volumetric fraction of digestive and basophilic cells in digestive tubules, (ii) the diverticular radius and the thickness of digestive epithelia, (iii) the stereological parameters characterizing the lysosomal system and, (iv) dry weight, soluble protein content and specific and total amylase, cellulase, laminarinase, and protease activities of the digestive gland. In the conditions of the present study, specific cellulase and laminarinase activities in the digestive gland of cockles were correlated with the volumetric fraction of basophilic cells (r=0.672 and 0.642, respectively), whereas the specific protease was highly correlated (r=0.866) with lysosomal volume density. The implications of these correlations are discussed in relation to the feeding and absorptive parameters reported in the preceding publication. In the acute response, adjustments of the synthesis of constituents of the lysosomal/proteolytic system of the digestive cells seemed to be the only mechanism operating at the digestive level to respond to the changes in food availability. Lysosomal volume density increased with rising ingestion rate of organic matter, however, the occurrence of a limit in this short-term tissular response would account for the recorded trade-off between absorption efficiency and ingestion rate with different food qualities. With regard to acclimation, food quality determined the nature of the response of the digestive gland. With low quality diets, a time-dependent capability of the digestive gland for intensifying lysosomal/proteolytic production explains the increase of food absorption rates that result from higher filtration and ingestion rates. With high quality food, digestive acclimation differed with food particle concentration: with low rations, in spite of constant morphometrical and stereological parameters, the significant changes in the absorptive balance of biochemical components suggests the existence of an increased production of lysosomes that promotes an accelerated turn-over rate of the digestive epithelia. With high food concentrations, this response was coupled with increased activities of cellulase and laminarinase enzymes, probably as a consequence of higher rates of enzyme secretions from basophilic cells.

Temporal changes of feeding and absorption of biochemical components in Cerastoderma edule fed an algal diet

Temporal evolution of the ingestion and absorption rates of organic matter and biochemical components in the filter-feeding bivalve Cerastoderma edule (Mollusca: Bivalvia) fed for 12 d on the same ration (1.5 mm 3  l −1 ) of the microalgae Tetraselmis suecica (Chlorophyta: Prasinophiceae) having different biochemical composition, were measured. Cockles in group E were fed algal cells harvested at the exponential growth phase (55.5% of protein, 16.8% of carbohydrate and 27.6% of lipids) whereas cockles in group S were fed cells harvested at the stationary growth phase (45.3% of protein, 35.2% of carbohydrate and 20.0% of lipids). After the feeding period cockles were submitted to starvation and biochemical analysis of faeces produced during final steps of gut evacuation (after 24–96 h of starvation) was performed to estimate biochemical composition of metabolic faecal losses (endogenous matter lost during the process of digestion). Ingestion rate of organic matter was found to gradually increase during the feeding period in both groups. Absorption efficiency of organic matter decreased but reached an asymptotic minimum with rising ingestion rate. This relationship resulted in a gradual enhancement of absorption rate of organic matter over the time. Lipid fraction of food was found to be absorbed with significantly lower efficiency (even negative values were recorded in group S) than proteins and carbohydrates irrespective of the diet. Endogenous faecal losses presented high proportion of lipids (up to 66%) and lower proportions of proteins (∼25%) and carbohydrates (∼9%). On the other hand, increasing carbohydrate abundance in the diet brought about no apparent enhancement of carbohydrates absorption efficiency. Results are discussed in relation to the time-course of digestive enzyme activities.

Feeding behaviour of Cerastoderma edule in a turbid environment : physiological adaptations and derived benefit

Resuspension of bottom sediments by wind andtide-driven currents often occur in shallow waters,coastal embayments and estuaries. These processes maylead to dramatic variations in the concentration andorganic richness of suspended particle assemblages.Since resuspended matter is mainly inorganic,decreasing organic contents are usually associatedwith higher seston loads.Under this environmental context the feeding behaviourof bivalves shows a wide degree of plasticity, whichhas been interpreted as having high adaptive value. Inorder to evaluate benefits derived from thisbehaviour, we have used functional relationshipsobtained in previous studies, relating feedingparameters to characteristics of suspended food, topredict the effect that different feeding responseswould have exerted. In cockles, main processesdetermining energy acquisition are feeding rates andpreingestive food selection. Thus, the procedurefollowed in the present work consisted of simulatingrates of food absorption under alternative feedingbehaviours characterised by: (a) no preferentialingestion of filtered organic matter and (b) maintenance ofconstant clearance rates. In theabsence of selection of organic matter at thepreingestive level, ingestion rate of organics (OIR)would decline with increasing seston loads to 30% ofvalues predicted by functions fitted to experimentaldata; difference in absorption rate (AR) would be evengreater, falling to 10%, due to the strong effectthat the organic content of ingested matter exerts onabsorption efficiency. On the other hand, had theclearance rate (CR) kept constant despite theincreasing seston load, OIR and AR would have fallento values respectively 30% and 49% lower than actualvalues.From these results it is concluded that the ability ofsorting particles before ingestion and the capabilityof adjusting clearance rate are key elements in thefeeding behaviour that enable cockles to be welladapted to cope with changes in the water columncaused by resuspension events.