Mucous-cord secretion for drifting by the clam Meretrix lusoria (Veneridae) under varying light/dark and submergence/exposure conditions

Abstract

Both juveniles and adults of the venerid clam Meretrix lusoria secrete a transparent mucous cord and drift over tidal flats, using the mucous cord as a “sail.” To clarify the ecological roles of mucous-cord secretion, the conditions affecting secretion were examined in the laboratory using clams burrowing in sand. Darkness and sand exposure played important roles in mucous-cord secretion: Under a 12-h : 12-h Light/Dark cycle with continuous sand submergence, 84% of mucous-cord secretions occurred in the dark. Under a 6-h : 6-h Submergence/Exposure cycle in continuous darkness, 79% of mucous-cord secretions occurred within 2 h of the submergence. Since clams secreted mucous cords without being exposed to adverse conditions such as hypoxic waters, it appears that mucous-cord drifting in the field does not necessarily serve as an escape mechanism when the clam encounters unfavorable conditions. On the contrary, it is suggested that drifting is a risk-dispersion (bet-hedging) strategy to sustain the population under the varying and unpredictable conditions of tidal flats.