Redigobius bikolanus (Gobiidae), which is distributed widely in the tropical regions of the West Pacific, is categorized as an amphidromous fish. Such fish reproduce in fresh water and newly hatched larvae immediately drift downstream to the estuary and sea where they spend a pelagic stage before returning upstream as juveniles. The morphology of larval R. bikolanus has been reported mainly from Japan, whereas little is known about the early stages of this species on the Asian continent. To understand larval growth of this goby, monthly and bimonthly collections with ichthyoplankton nets were made in the Tien Yen estuary in northern Vietnam, from October 2014 to September 2015. A total of 282 larvae (preflexion to postflexion stages, 2.3-5.4 mm body length) of R. bikolanus were collected from the center of the current in May, but none were captured in the waters near the banks. The larvae were similar in general morphology to those of common gobies, but could be distinguished from other gobiid genera by two very large stellate melanophores on the ventral surface of the trunk and tail. Descriptions of R. bikolanus larvae based on a series of wild specimens are provided. The fishes occurred in low salinities (0.2-2.3 PSU) within 1 m from the surface layer. Together with the wide range of size and developmental stages, this suggests that larvae of R. bikolanus utilize the center of the current of the Tien Yen estuary as their nursery area. This habitat has not been recorded in Japan.
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