Habitat use by the gnathiid isopodElaphognathia discolorliving in terebellid polychaete tubes

Abstract

The pattern of habitat utilization byElaphognathia discolor, a gnathiid isopod inhabiting terebellid polychaete tubes, was examined in bimonthly samples taken between June 2003 and April 2004 from Shizugawa Bay, north-eastern Japan. Nine species of terebellid polychaetes were obtained, and gnathiids were found in tubes of four terebellid species. Of the terebellids,Nicolea gracilibranchiswas collected in each sample, while the other species were obtained only intermittently. Both larval and adultE. discolorinhabited the tubes ofN. gracilibranchisthroughout the year, with their prevalence in samples ranging from 57.1 to 80.0%. A large range of larval stages was found withinN. gracilibranchistubes, including the first larval stage and premature females, suggesting that this gnathiid species may useN. gracilibranchistubes for resting and moulting throughout the larval period. The distribution of adult gnathiids within tubes significantly differed from random. Adult males rarely coexisted with other males in tubes, while several larvae and females often lived together. Groups of female gnathiids in polychaete tubes were usually attended by a single male, resembling the ‘harems’ observed in other gnathiids and suggesting that the mating system ofE. discolorcan be classified as semelparous harem polygynandry.