Seasonal fluctuations of the sponge-dwelling gnathiid isopod Elaphognathia aff. cornigera

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While records of seasonal abundance patterns are available for gnathiids (Crustacea, Isopoda, Gnathiidae) in temperate zones and cold water populations, none exist for tropical and subtropical populations. We found the gnathiid isopod Elaphognathia aff. cornigera inhabiting sponges on a subtropical tidal flat in the Haneji Island Sea, Ryukyu Archipelago, southwestern Japan. To examine seasonal abundance patterns of this species, we conducted monthly sampling from December 2004 to November 2006 of the demosponge Haliclona sp. in which the gnathiids dwelled. All of the adult males, adult females in various reproductive phases, and larvae of Elaphognathia aff. cornigera were found throughout most of the year, suggesting that the species reproduces year-round. However, gnathiid density showed a remarkable seasonal pattern, with the maximum abundance occurring in spring and the minimum abundance in summer. These results were quite different from temperate species, in which female development and reproduction are restricted to warmer months and the highest densities are often recorded in the summer. Warm subtropical winters would likely not disturb gnathiid growth and reproduction, but extreme environmental conditions, including summer high water temperatures and typhoons, may affect gnathiid density.