FEEDING PHYSIOLOGY OF CERASTODERMA-EDULE IN RESPONSE TO A WIDE-RANGE OF SESTON CONCENTRATIONS

The suspension-feeding activity of Cerastoderma edule (L.) was investi gated in response to a wide range of experimental seston concentration s which approximated the changes in seston quality and quantity found in the natural environment over tidal cycles of sediment resuspension. The different seston concentrations were produced by adding increasin g quantities of resuspended fine sediment to a relatively low and cons tant algal concentration (0.59 mg dry wt l(-1)). The total seston conc entrations (total particulate matter) varied over a wide range from 1. 6 to 570 mg l(-1), and the organic fraction (particulate organic matte r) between 0.62 and 79.8 mg l(-1). The organic content of the diets de creased with the increasing seston concentration, from 38.8% at the lo west concentration to 14% at the highest concentration. A significant negative relationship was found between clearance rate of C, edule and seston concentration, with very low values at 570 mg l(-1) of total s eston. Filtration rate increased with seston concentration from a mini mum value of 2.67 mg h(-1) al the lowest concentration to a maximum of 84.0 mg h(-1) at a concentration of 300 mg l(-1). Above 300 mg l(-1) there was an abrupt decline in filtration rate. Pseudofaeces productio n started at a concentration of 4.8 mg l(-1), showing its highest valu es at around 300 mg l(-1). The organic content of pseudofaeces was sig nificantly lower than the organic content of the seston, suggesting th at C, edule is able to preferentially select organic particles for ing estion instead of inorganic particles. Selection efficiency was mainta ined at a high and constant level at seston concentrations between 4.8 and 97 mg l(-1), declining at the highest seston concentrations with the lowest organic content. Thus the physiological sorting and selecti on mechanism appears to be very effective at seston concentrations bel ow 100 mg l(-1), this ability declining abruptly at higher concentrati ons. Ingestion rate was constant in the lower range of seston concentr ation (1.6 to 34 mg l(-1)), increased at concentrations around 250 mg l(-1), and declined abruptly at the higher food rations. Absorption ef ficiency appeared relatively independent of seston concentrations over a large range, but it was reduced al concentrations above 250 mg l(-1 ). We found a very effective digestive mechanism to regulate absorptio n rate at seston concentrations between 1.6 and 250 mg l(-1). The resu lts suggest, that C. edule can compensate efficiently for a decrease i n seston quality over a wide range of seston concentration (1.6 to 300 mg l(-1)) by maintaining an effective preingestive mechanism of selec tion for organic particulate matter, as well as increasing filtration and rejection rates. As a consequence, this species is well adapted to Living in turbid environments, such as intertidal mudflats, which are characterised by marked fluctuations in seston quality and quantity, caused by resuspension of fine sediments during periods of high curren t velocities on the flood or ebb tides.