E. Navarro

Feeding And Digestion By The Mussel Mytilus-Edulis-L (Bivalvia-Mollusca) In Mixtures Of Silt And Algal Cells At Low Concentrations

Mussels Mytilus edulis L. from two populations were exposed for 2 days or 2 wk to mixtures of silt and the diatom Phaeodactylum tricomutum Bohlin before measuring rates of feeding, the passage time for food in the gut, absorption efficiency and metabolic rate. Experimental diets were set up to span the range of organic content in seston from the natural habitats (% organic matter by weight: 7–55%), but were less than the total levels of natural seston. Absorption efficiencies were adequately modelled in an exponential relationship to food quality and to gut passage time, although at high proportional silt concentrations metabolic faecal losses led to negative net rates of absorption. Over short-term exposures the scope for growth was a simple function of food quality, and the nutritional quality of the diet was best expressed as organic content per unit volume of particles. Over a period of 2 wk physiological acclimation occurred, across all levels of experimental food quality (which were as low as 10% organic matter by weight), resulting in positive growth potential. Relevant mechanisms of compensation include increased rates of ingestion, increased absorption efficiency and an apparent increase in digestive capacity, estimated here as gut fullness. In experiments in which natural diets are simulated by adding silt to phytoplankton cells, the consequences for net rates of absorption depend on the balance between mean particle size and organic content per unit volume. Calculations show how, in some circumstances, growth may be enhanced by the addition of small silt particles to living phytoplankton cells.

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.

The physiological energetics of mussels (Mytilus galloprovincialis Lmk) from different cultivation rafts in the Ria de Arosa (Galicia, N.W. Spain)

Abstract Physiological components of growth (clearance and ingestion rates, absorption efficiencies and metabolic rates) were determined in specimens of mussels ( Mytilus galloprovincialis ) from several cultivation rafts in the Ria de Arosa (Galicia, Spain). Choice of raft location within the estuary was intended to provide an oceanic-terrestrial gradient of conditions. Experiments were performed directly on the raft, under natural conditions of food availability. Water was pumped to feeding trays from two different points (front and back) to account for the variability in seston concentration associated with the filtering activity of the hanging culture. Food concentration was found to be highly variable (0.35 to 1.01 mg POM l −1 ), the main differences being accounted for by site characteristics. Conversely, food quality (here defined as mg POM per mm 3 of packed particles) experienced minor variations (0.59 to 0.71), with the sole exception of the innermost site studied (quality 0.43). Absorption efficiencies were clearly dependent on food quality and could be adequately modelled as an exponential function of that viriable. The scope for growth (SFG) measured at the front of the raft was consistently higher than at the back and confirmed empirical evidence on growth rates. This result does not merely reflect improved feeding conditions in the input area but associated effects of increased clearance rates and absorption efficiencies exhibited by mussels from the front of the raft. On the other hand, raft positioning within the estuary appears to induce noticeable differences in the feeding behaviour of mussels, as evidenced by higher clearance rates determined in more oceanic areas.

Feeding, particle selection and absorption in cockles Cerastoderma edule (L.) exposed to variable conditions of food concentration and quality

Abstract Cockles Cerastoderma edule (Linne) were exposed to different food suspensions consisting in mixtures of sediments and Tetraselmis suecica Butcher. Several parameters of feeding behaviour, including rates of filtration, pseudofaecal production, ingestion and absorption, were analysed as a function of both particle concentration and composition. We demonstrated particle selection at the preingestive level and resulting preferential organic ingestion. The relative energetic benefits derived from these processes were evaluated by computing different indices of sorting and food enrichment. Cockles fed organically rich diets were found to regulate rates of food ingestion mainly by adjusting clearance rates in order to maintain filtration of suspended matter virtually independent from changes in particle concentration. Conversely, with diets consisting of particles low in food value, clearance rates were kept nearly constant and rates of ingestion were regulated by increasing pseudofaeces production. Relative advantages derived from pseudofaeces formation have been evaluated for different dietary compositions by using a Benefit Ratio. Higher benefits were associated with preingestive food selection in the case of suspensions where particles of low organic content constituted the main dietary component, which may help to explain the maintenance of high rates of filtration coupled with extensive pseudofaecal deposition observed in cockles fed on low quality diets.

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.

Natural sediment as a food source for the cockle Cerastoderma edule (L.): effect of variable particle concentration on feeding, digestion and the scope for growth

Abstract Cockles Cerastoderma edule (L.) from a mud-flat estuarine population were exposed to variable concentrations of suspended bottom sediment to determine the physiological components of the energy budget and scope for growth, as a function of particle concentration. Associated processes of feeding and digestion were given particular attention, involving quantification of selective rejection of material in pseudofaeces and estimations of gut passage time of food and gut contents. Although clearance rate declined with increasing particle concentration, regulation of ingestion rate at high particle load was mainly achieved by pseudofaeces production. Two mechanisms contributed to maintain absorption efficiency of filtered material approximately constant throughout changing food concentration: (a) Preingestive selection of organically rich particles, resulting in adjustments of organic content in the ingested ration; and (b) reduced effects of ingestion on gut passage time of food, since changing gut contents provide an important mechanism for the adjustment of variations in ingestion rate. Scope for growth increased to reach an asymptotic value at food concentrations which correspond to a maximum of ingestion. Constraints on feeding rates thus appear as the main limiting factor for growth.

Comparative allometries of gut-passage time, gut content and metabolic faecal loss in Mytilus edulis and Cerastoderma edule.

Allometric relations were determined in bivalves collected from approximate mid-tide levels in Biscay, Spain, during March 1987. Species compared included the epifaunal suspension-feederMytilus edulis L. (9 to 1 108 mg dry soft-tissue weight) from a rocky-shore population at Meinakotz Beach, and the infaunal deposit-feederCerastoderma edule (L.) (1 to 192 mg dry soft-tissue weight) from the mudflats in Mundaka Ría. Relative to M. edulis, and compared per unit dry tissue weight,C. edule had similar palp areas but smaller gill areas. In addition, to help maximize absorption from organically-poor deposits,C. edule ingested three to four times as much food per hour, but had gut contents that were five to six times greater, so that gut-passage times available for the extraction of nutrients were 2.5 times longer. Metabolic faecal losses, which were comprised of endogenous materials lost from the bivalve into the gut, were two to three times greater inC. edule, but similar to those ofM. edulis when expressed per unit mass ingested by each species. These losses were very substantial, being equivalent to as much as 15% of the ingested mass, and represent a significant indirect cost that is presumably incurred largely by the intracellular digestion characteristic of bivalves. Weight exponents indicated that such metabolic “investment” represented an unchanging proportion of the total costs of growth. They also showed that age-related constraints on total production did not stem from decreasing gut content. Rather, associated exponents identified limitations to the production in each species as being linked with marked reductions of gutpassage time and ingestion rate, and indicated that these limitations do not derive from corresponding decreases in gill or palp areas.

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.

Interannual variation in the reproductive cycle and biochemical composition of the cockle Cerastoderma edule from Mundaca Estuary (Biscay, North Spain)

Seasonal determinants of body weight, biochemical composition and reproductive condition in the cockle Cerastoderma edule L. from the Mundaca Estuary, Spain, were performed from December 1983 to July 1985. Interannual differences in timing of gametogenesis and spawning between 1984 and 1985 were correlated with temporal variations in the cycles of storage and utilisation of carbohydrate reserves. Young individuals exhibited larger annual fluctuations in soft-body weight, displaying higher growth rates in spring and greater diminishment of energy reserves during autumn and winter than older individuals. Carbohydrate content displayed a similar tendency, levels being lower in young individuals throughout the year.

The effect of diets of phytoplankton and suspended bottom material on feeding and absorption of raft mussels (Mytilus galloprovincialis Lmk)

Abstract Measurements of feeding rate and absorption efficiency were performed to assess the nutritional value of sedimentary organics combined with phytoplankton for raft mussels ( Mytilus galloprovincialis Lmk) in Ria de Arosa (Galicia, N.W. Spain). Both clearance rate and absorption efficiency reached a maximum on mixed diets in which the proportions of phytoplankton and sediment particles were similar. Consequently, maximum absorption rates were reached on laboratory suspensions that closely resembled features of natural seston from the Ria. Values of absorption efficiency recorded with suspended sediments (around 13%) do not apply to the detrital component of ingestion in mixed diets, leading to the conclusion that a positive effect of phytoplankton on absorption efficiency of sedimentary organics is taking place. On the other hand, comparison of results for mixed and monoalgal diets also suggests that the occurrence of silt in the ingesta enhances absorption of microalgae, possibly through improving mechanical treatment within the stomach. It was concluded that mixing of bottom material with phytoplankton in the water filtered by raft mussels greatly increases growth potential, provided that no reduction of phytoplankton concentration below 40% of particulate volume takes place. Mussels with different locations in the Ria exhibit variable degrees of infection by the parasitic protozoa Marteilia refringens . Concomitant differences in physiological behaviour could be attributed to digestive impairment caused by parasitization.