Scanning electron microscopic observation of embryos and larvae of the carp (Cyprinus carpio) in the course of development was undertaken to further understanding of the relationship between larval behaviour and their chemo- and mechanoreceptors. Free neuromasts and olfactory pits equipped with mechano- and sensory-cilia were found on carp embryos. During larval development, the number of neuromasts and the olfactory mechano- and sensory cilia steadily increases and they become more morphologically differentiated. The olfactory features, presented in early larval stages, indicate that although the olfactory organ at this early life history has not yet attained its final configuration, it can perceive odour stimulation. The neuromasts found on the forehead of the newly hatched larvae seem to assist in the early behavioural selection, approach and attachment to a substratum, before the stage of filling of the gas bladder. The increasing number of neuromasts during development reflects the larvae's capability of capturing food items in the absence of visual stimulation. The rapid maturation of free neuromasts, the lateral-line system and the olfactory organ, is evidence of their post-metamorphosis position in the search for, location and successful capture of food.

Results are presented from laboratory-based experiments in which we investigated short-term responses of Mytilus edulis L. to experimental changes in the amount and composition of suspended seston. Working with large quantities of cultured algae, we have studied feeding behaviour over ranges of food availability and quality that extend well beyond earlier limits. Findings confirm the ability of mussels to selectively reject inorganic particles as pseudofaeces prior to ingestion, thereby enriching the organic content of ingested matter by 30% more than the organic content of natural filtered seston. Our findings also establish acclimation both of selective and absorptive processes, indicating that the extent to which growth of M. edulis can be stimulated in the short-term will depend upon prior nutritional history. Present maximal growth of 14.8% dry soft tissue d−1 in a standard M. edulis of 1 g dry soft tissue was much higher than has previously been documented for any adult mussel. Maximal growth was achieved when natural seston that had been enriched to more than about 60% organic content with a mixture of algal monocultures was available at concentrations above about 11.5 mg total particulates 1−1. Neither ingestion rate nor net energy balance were improved with further increases in food availability, associated with regulatory reductions in the rate of water filtration (clearance rate) that maintained organic ingestion rate independent of increases in both the amount and organic content of available seston. These findings suggest that digestive processes had become saturated when organic ingestion reached about 6.5 mg organics g−1 dry soft tissue h−1 in a standard M. edulis of 1 g dry soft tissue, representing as much as 20.6% of all soft tissue organic mass mussel−1 d−1. Such saturation of organic ingestion is consistent with previous conclusions based on comparative allometries showing that limitations to growth in M. edulis and other bivalves are associated with the rate of food processing.

The uptake and release of material by oysters (Crassostrea gigas) and mussels (Mytilus edulis) in Marennes-Oléron Bay (SW France) were measured in situ using benthic ecosystem tunnels (BEST). There was a large variation in seston concentrations, owing to high sedimentation/erosion dynamics. Organic matter was diluted by resuspended sediment, but had a low C/N ratio, i.e. a high quality, owing to resuspension of microphytobenthos. In many cases a significant decrease of chlorophyll concentrations was observed in tunnels with oysters and mussels. Average clearance rates per tidal cycle ranged from 4 to 7, and from 0.9 to 2.7 1.g−1AFDW.h−1 throughout the season, for oysters and mussels respectively. Calculation of the filtration pressure (the fraction of the primary production filtered per day), indicated that bivalves in the bay were able to filter much more than the daily pelagic primary production. It seemed obvious that the bivalve stocks depended not only on phytoplankton but also on microphytobenthos. Significant regeneration of nitrogen could be demonstrated from musselbeds on the sediment, at higher rates than direct excretion of ammonium. No significant release of nutrients was observed for the oysters or mussels, when they were put on tables, as practised by the oyster farmers. lt seemed likely that biodeposits were flushed away from the tunnels in this case, and mineralization occurred elsewhere. It was concluded that the carrying capacity of the bay for herbivores is extensively exploited by the shellfish. Mineralization of bivalve biodeposits represent a large potential for nitrogen regeneration.

Un modèle écophysiologique déterministe de la croissance, de la reproduction et des interactions entre le bivalve et son environnement estuarien a été élaboré pour l'huître japonaise Crassostrea gigas. Pour fonctionner, le modèle a besoin de la température de l'eau, des concentrations en matière en suspension, matière organique particulaire, matière minérale particulaire, chlorophylle-a, phéopigments, protéines, lipides et glucides particulaires. Les fonctions de filtration, consommation, ingestion, absorption et respiration ainsi que les efficacités de rétention, sélection et absorption sont modélisées à l'aide de variables endogènes (poids sec de l'animal, allocation d'énergie entre la croissance et la reproduction) et exogènes (température, quantité et qualité de la nourriture). Le modèle simule l'évolution temporelle de deux compartiments : somatique et réserves-gonades. L'énergie provenant de la nourriture absorbée est allouée au compartiment somatique jusqu'à un maximum de croissance somatique. Ce maximum décroît avec l'âge du bivalve et ne dépasse jamais 0,02 g.j−1.individu−1. L'énergie en excès est alors attribuée au compartiment réserves-gonades. Cette représentation permet d'identifier les périodes de stockage de réserves et de gamétogénèse ainsi qu'une bonne simulation de l'intensité des pontes.Le modèle tient compte d'une efficacité de sélection plus importante pour les microphytes que pour les particules détritiques. L'efficacité d'absorption a été représentée en fonction du pourcentage de matière organique dans la fraction ingérée afin de tenir compte de la dilution de la nourriture par les particules minérales. L'efficacité d'absorption des microalgues varie de 20 à 60 %, tandis que pour les particules organiques détritiques, le modèle simule des valeurs négatives pour tenir compte des pertes fécales métaboliques. Ces formulations permettent de suivre l'évolution temporelle des quantités de microphytes et de matières détritiques rejetées sous la forme de pseudofèces et de fèces.

Fluxes of nutrients (NH4+, NO3-, PO43- and Si(OH)4) were studied on an intertidal mudflat in Marennes-Oléron Bay, France, at two different seasons and at different times of the emersion period. Fluxes through the sediment-water interface were both calculated from vertical profiles of nutrient concentration in pore-water (diffusive fluxes, JD) and measured in light and dark benthic mini-chambers (measured fluxes, J0). Results indicate that ammonia was mainly released in summer while nitrate was mainly taken up in late winter. This uptake from the overlying water was probably due to the coupling of nitrification-denitrification within the sediment. The J0/JD ratio further indicates that bioturbation likely enhanced ammonia release in summer. Concerning phosphate, the comparison of diffusive and measured fluxes suggests that PO43- could be assimilated by the biofilm in winter while it was released in summer at a high rate due to both bioturbation and desorption because of the relative anoxic conditions in summer. Silica was always released by the sediment, but at a higher rate in summer. Statistically significant differences in measured fluxes were detected in dark chambers at different times of low tide, thus suggesting a short-term variability of fluxes. Microphytobenthos preferred ammonia to nitrate, but assimilated nitrate when ammonia was not available. It also turned out that benthic cells could be limited in nitrogen during low tide in late winter. In summer, ammonia was not limiting and microphytobenthic activity significantly decreased the measured flux of NH4+ in the middle of low tide when its photosynthetic capacity was highest.

A physiological model has been developed to provide a simulation of scope for growth responses of cockles Cerastoderma edule to environmental factors such as temperature and seston parameters. As in previous treatments, model structure consisted of a series of equations representing the vanous physiological processes which are integrated in the energy balance of filter-feeders. Unlike these treatments, however, the present model was designed to give account of the separate processing of phytoplankton and detritus, as components of suspended food that are submitted to differential selection and absorption. In this way, effect of quality indexes based on phytoplankton abundance on growth rates could be explored. Subsequently, we have applied this model to: (a) simulating scope for growth under conditions subjected to variation in the estuarine media, and (b) predicting growth variability as a function of food regime, by comparing two particular estuaries with contrasting seston characteristics.