Size distribution and growth patterns of gobies in habitats associated with a natural estuary of inner Tokyo Bay, central Japan

Size distribution and growth patterns of Gymnogobius breunigii, G. macrognathos and G. uchidai in habitats associated with the natural estuary of Obitsu-gawa River of inner Tokyo Bay, central Japan were studied from specimens collected from May to December 2005 and March to April 2006 and specimens from the shallow tidal creek and soft sediment pools from July 2009 to June 2010. The gobies showed preference for a particular estuarine habitat at a given stage of ontogenetic development and habitat shift corresponded with change in body shape implying the level of competence exhibited by these species during ontogeny. The occurrence of larva and the early J1 in the lower estuary around May was timed with spring bloom that was attributed to increase in the secondary production of estuaries in the bay particularly the zooplankton, which was the main prey item of gobies during the pelagic phase. The movement in the tidal creek from late J1 to J2 was related with shift to benthic lifestyle with preference for benthic prey but it could also be attributed to refuge from predators as well as temperature-mediated growth and morphogenesis in the shallow habitat. Relevant changes in body shape was a prelude to benthic life particularly the narrowing of the head, reduction in the angle of the mouth on the ventral surface bringing the mouth closer to the substrate, deformation at the anterior trunk region that resulted to a tumescent appearance on the dorsal region of the trunk as well as a relatively elongated and ventrally displaced tail. All these characters were attained at J3. The gobies gained competence when they reached J3 having the ability to explore “high-risked” habitats such as the middle estuary and a specialized habitat like the soft sediment pools. The J3 of G. breunigii on the other hand corresponded with upstream movement to preferred adult habitats. Changes during the adult stage was primarily associated with the elaboration of the morphological form inherent at J3. The study proposed that the estuarine habitats associated with the lower region of Obitsu-gawa River are important for gobies during the transition from pelagic to benthic life that would likely increase their survival and subsequent recruitment into the estuary.