Comparative utilization of phytoplankton and vascular plant detritus by the cockle Cerastoderma edule: digestive responses during diet acclimation

Abstract

Contribution to the Theme Section 'Directions in bivalve feeding' ABSTRACT: Physiological components of food absorption (rates of particle clearance and ingestion and absorption efficiencies) were recorded in the cockles Cerastoderma edule fed variable concen- tration diets of phytoplankton (Isochrysis galbana) and detritus from salt-marsh vascular plants (Spartina maritina), after 2 and 12 d of acclimation to these diets. Corresponding digestive processes were also analysed, including determinations of gut passage time of food, gut fullness (GF) and digestive investments, the latter measured in terms of both metabolic faecal losses (MFL) and enzyme activities of the digestive glands. Digestive responses to increasing availability of phytoplankton included faster processing of food particles coupled to improved hydrolytic capacity of digestive glands that resulted in increased gut performance. Compared to phytoplankton, detritus utilization was constrained in the short-term by its refractory behaviour to digestion and the magnitude of meta- bolic faecal losses experienced by cockles fed these food materials. However, increased GF achieved during acclimation to detrital diets compensated for reduced digestibility, allowing for more food to be processed and thus cancelling out the effects of reduced passage time (and the consequent decline in absorption efficiency). Induction of the appropriate set of carbohydrases during the acclimation process resulted in increased amounts of reducing sugars released from natural substrates, with the additional benefit of diminishing the losses of endogenous materials accompanying each cycle of intracellular digestion.