Effects of food quality and quantity on feeding and absorption in black-ribbed mussels, Septifer virgatus (Wiegmann) (Bivalvia : Mytilidae) dominating wave-exposed habitats in Hong Kong

The black-ribbed mussel Septifer virgatus is an ecologically important species dominating exposed rocky shores in tropical and subtropical areas of Asia. The present study investigates its feeding and digestive responses to variations in food quantity and composition under laboratory conditions. Individuals were exposed to either one of five rations composed of different percentages of silt and the green microalga Dunaliella tertiolecta. Clearance rate (CR: l h^-1) was correlated weakly with both food quantity and quality whereas rejection rate (RR: l h^-1) showed a strong positive correlation with total particulate matter (TPM: mg l^-1), with the lowest threshold of TPM for pseudofaeces production estimated at 10 mg l^-1. Organic fraction in pseudofaeces was also significantly lower than organic content in water (f), evidence of pre-ingestive selection. Absorption efficiency (AE) was a negative exponential function of TPM and a negative quadratic function of organic ingestion rate (OIR). Gut passage time (GPT) showed a negative relationship with food quantity and ingestion rate (IR: mg h^-1). As food concentration increased (TPM), GPT of S. virgatus remained high when OIR was low (-1); further increases in OIR resulted in reductions of both GPT and AE. A prolonged GPT at low OIR resulted in a rapid increase in AR from 0.1 to 0.3 mg h^-1; further increases in OIR, however, did not cause a significant increase in AR. The existence of the regulative mechanism facilitates maximum absorption in wave-exposed habitats characterized by low seston concentrations.