Feeding interactions between planktonic copepods and red-tide flagellates from Japanese coastal waters

Abstract

Feeding interactions between inshore marine copepods Pseudodiapto~nus marinus and Acartia ornonl, and 15 red-tide flagellates were studied by examining egesbon rate, mortality and egg production rate of the copepods offered a suspension of each phytoplankton species. Among several species of poor quality as food, Olisthodiscus luteus (Raphidophyceae) was nearly completely rejected by P. mannus, and Gymnodinium nagasakiense (Dinophyceae), Heterosigma akashiwo (Raphidophyceae, N a g a s a k ~ University strain), Chattonella marina (Raphidophyceae) and Fibrocapsa japonica (Raphidophyceae) were almost entirely rejected by A. omorii. Since these flagellates are of preferred cell size and have no hard undigestible cell walls, the rejective feeding by copepods was suspected to b e chemically mediated. Effects of chemical stimuli from 0 . luteus (to P. marinus) and G. nagasakiense (to A. omorii) were examined in detail by conducting comparative feeding experiments in a suspension of Heterocapsa triquetra (Dinophyceae), a normally edible species. Addition of filtrate from the cellhomogenate of exponentially growing 0 . luteus or G. nagasakiense to the suspension reduced copepod filtering rate on H. triquetra, indicating deterrent chemical compounds are intracellularly present. These compounds were ephemeral, being deactivated within 12 h at 20C. Chemically-mediated rejection by copepods is a n important factor in the development of monospecific red tides.