Nursery ground and morphological characters of the symbiotic goby fish, <i>Eutaeniichthys gilli</i>, in Tosa Bay, southwestern Japan

The goby fish Eutaeniichthys gilli, distributed in eastern Asia, utilizes shrimp burrows as shelters and spawning nests in tidal flats. Despite previous research, comprehensive understanding of the distribution and morphological development of larvae and juveniles remains limited. This study aimed to characterize the morphological features of E. gilli larvae and juveniles and to investigate how these traits change throughout ontogeny, thereby offering insights into larval dispersal within inner bays and the mechanisms facilitating successful settlement. To clarify these ontogenetic changes of larval distribution, monthly sampling and morphological observations were conducted at the Susaki Inlet of Tosa Bay, southwestern Japan, in 2015–2016. Sampling was carried out monthly in the inner, middle, and outer areas of the inlet using larva and beam trawl nets. Eutaeniichthys gilli specimens were only found in June 2016 and only from the inner inlet. A total of 69 specimens were collected, and morphological measurements were performed. Morphological data were then compared with those of reared individuals previously reported in the literature. While the overall morphology was largely consistent with earlier descriptions, differences were evident in pigmentation patterns. A comparison of their eye diameter-to-body length ratio, which is particularly relevant to larval development, revealed a marked decrease during ontogeny. Because no larvae or juveniles were collected from the middle and outer inlets, most E. gilli individuals were assumed to be vertically dispersed after hatching, remaining in the inner part of the inlet.