Predator-prey systems of drifting seaweed communities off the Tohoku coast, northern Japan, as determined by feeding habit analysis of phytal animals

We collected phytal animals (fauna associated with drifting seaweeds) off the Tohoku coast in northern Japan in May and June 1994 to examine their feeding habits. Phytal animals were assigned to the following three feeding habit groups: (i) carnivores preying on crustaceans: a gammarid (Stenothoe sp.) and a decapod megalops; (ii) omnivores feeding on crustaceans and pinnate diatoms: a cirripede (Lepas anserifera), three gammarids (Calliopius sp., Jassa slatteryi and J. marmorata), an isopod (Idotea metallica), a decapod crustacean (Planes cyaneus), a decapod megalops and two caprellids (Caprella penantis and Caprella mutica); and (iii) herbivores grazing on pinnate diatoms, fucoids and seaweeds other than fucoids: three gammarids (Allorchestes angusta, Amphithoe lacertosa and Perampithoe lindberi), an isopod (Cymodoce japonica) and a polychaete (Platynereis bicanaliculata). Pinnate diatoms in the gut contents were composed mainly of Licmophora spp., Navicula spp., Nitzschia spp. and Cocconeis spp. and seaweeds other than fucoids (i.e. Ectocarpus siliculosus and Sphacelaria furcigera). The latter two algae seemed to be epiphytes on drifting seaweeds. These results suggest that phytal animals play an important role in drifting seaweed communities through trophic linkage with two primary producers (i.e. drifting seaweeds and the epiphytes growing on them).