Comparative physiological energetics of two suspension feeders: polychaete annelid Lanice conchilega (Pallas 1766) and Pacific cupped oyster Crassostrea gigas (Thunberg 1795)
Introduction
Many benthic marine species have either exclusively deposit or suspension feeding strategies. Certain species are capable of both however, obtaining food at the sediment surface or from the water column depending on the environmental conditions (Miller et al., 1992). It was long thought that the induction of suspension feeding behaviour was solely due to the presence of particles in suspension (Taghon et al., 1980; Dauer et al., 1981). Several works have since shown strong relationships between suspension feeding activity and hydrodynamic characteristics of the bottom layer (Dauer et al., 1981; Frechette et al., 1989; Eckman and Duggins, 1993). According to Bock and Miller (1997), the concentration of particulate organic matter in the water column is the most important factor inducing change in feeding mode from deposit to suspension feeding. The particles in suspension offer a nutritional value 15 to 40 times higher than those found in the sediment layer (Bock and Miller, 1995) which would favour the choice of suspension feeding over deposit feeding when hydrodynamic conditions and suspension of particles allowed.
The annelid tubeworm Lanice conchilega was considered as a deposit feeder for a long time, but is in fact capable of modifying its feeding strategy according to its situation by changing from deposit to suspension feeding (Buhr, 1976; Fauchald and Jumars, 1979). Buhr and Winter (1977)suggested that a density-dependant process played a role in inducing the transition. At low densities (several dozen individuals per square metre), L. conchilega would be preferentially deposit feeding while at high densities (several thousand individuals per square meter), competition at the sediment surface would force animals to adopt a suspension feeding mode.
Since 1985, an intertidal population of L. conchilega has colonised the eastern side of the Bay of Veys (in the west of the Seine bay, English Channel, France). The species has proliferated in the area and densities can exceed 7000 ind m−2 (Ropert, 1996). A feature of this population is that it is strictly geographically limited to a shellfish production area of around 200 ha. Although the increase in the Lanice population has not had a major impact on the quality of the shellfish produced, it does pose the question of whether competition for food resources is occurring with the cultivated Pacific cupped oyster Crassostrea gigas.
The feeding behaviour of C. gigas has been studied by several groups (e.g., Gerdes, 1983; Barillé et al., 1993Barillé et al., 1994; Raillard et al., 1993) while only Buhr (1976)and Buhr and Winter (1977)quantified and analysed feeding behaviour in L. conchilega. The objective of the present study was to examine diets and physiological energetics of L. conchilega and C. gigas in order to investigate any potential ecological relationship between the species. In vitro experiments were conducted in parallel on oysters and annelids in order to: (1) determine the spectrum of particle size retained by L. conchilega and C. gigas; (2) evaluate filtration efficiency in the two species; (3) make a comparison of growth potential between the two species based on respiration measurements, food retention, biodeposit production and estimation of the assimilation rate; (4) evaluate competition between the two species by extrapolation to their biomass in the natural environment.
Section snippets
Sampling and conditioning of L. conchilega
Individuals were sampled from the central area of the intertidal population located in the mesolittoral zone. The animals were sampled using a TASM corer (Souza Reis et al., 1982; Sylvand, 1995) of 0.02 m2 area to a depth of 30 cm. The samples were carefully washed in seawater on a 1-mm mesh and brought to the laboratory. On arrival at the laboratory, the sand tubes were separated from the animals which were then deposited on a clean sediment surface of 250–500 μm sized particles. After 48 h,
Retention efficiency
Retention efficiency of different sized particles was calculated as the difference between the fraction consumed by the animal and the result from the control chamber. For each recording, the retention efficiency was calculated for each of 9 size classes following the formula (IFREMER, 1987):Where Ri is retention efficiency for particle size class i, [v]con is the particulate volume measured at the output of the control chamber and [v]meas is the particulate volume
Retention spectra and filtration efficiency
One hundred and twelve measurements were made in total over the three diets. Because differences were found in the particulate charge, two series of measurements were recorded with natural seawater at different flow rates (351 ml h−1 and 740 ml h−1). At the end of the experiments, only 48 measurements indicated animal filtration activity (significance in a Mann–Whitney test) and were retained for analysis.
When the flow rate was over 500 ml h−1, particle retention by L. conchilega was
Discussion
Whatever the type of diet supplied in our experiments, L. conchilega demonstrates the ability to collect particles in suspension. Retention takes place from particle sizes around 4 μm and upwards. Negative retention values obtained below 4 μm do not necessarily demonstrate output of particles by the animals. Similar phenomena were observed by Vahl (1972)on Mytilus edulis for particles below 2 μm. Vahl (1972)suggested that this apparent emission of particles was due to particle aggregation or
Acknowledgements
The authors wish to thank to Dr. H. McCombie for useful comments, and the translation of this paper. Special thanks are extended to P. Geairon for his technical assistance. This work was supported by funds from the Regional Council of Basse-Normandie, `Agence de L'Eau', and the Normandy Shellfish Farmers Association of Normandie (SRC).
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