Habitat use by the gnathiid isopod Elaphognathia discolor living in terebellid polychaete tubes

The pattern of habitat utilization by Elaphognathia 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 gracilibranchis was collected in each sample, while the other species were obtained only intermittently. Both larval and adult E. discolor inhabited the tubes of N. gracilibranchis throughout the year, with their prevalence in samples ranging from 57.1 to 80.0%. A large range of larval stages was found within N. gracilibranchis tubes, including the first larval stage and premature females, suggesting that this gnathiid species may use N. gracilibranchis tubes 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 of E. discolor can be classified as semelparous harem polygynandry.

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