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Several studies indicate that burrows play an essential role in burrowing fish, especially mudskippers, but little is known on burrow morphology and utilization of Periophthalmodon septemradiatus. This study was therefore conducted at five sampling sites from the estuary to upper reaches of Hau River to contribute to the knowledge of the behavior of Pn. septemradiatus, via in situ and ex situ observations of the utilization of burrow structures. Burrows made by polyester resin were used to determine their morphology. The male mudskippers Pn. septemradiatus were observed to excavate burrows using their mouths during the low tide. The burrows' shapes were found to be J, U and W, with 1-3 openings and one bulbous chamber. The chamber was used as egg container where the mudskipper deposits their fertilized eggs. W-shaped burrows were found to be interconnected chambers where the mudskipper used it to change direction of movements inside the burrow. For the J-and U-shaped burrows, the commonality at all five sites was identified. The W-shaped burrows were common and were only found at two sites (Long Duc and An Lac Tay). For the burrows' structure, the openings to the surface were oval, circular and bulbous in shape. Yet, there were variations between sites, and the burrows' size was similar during the dry and wet seasons. At different sites, the depth of the burrows seasonally changed. The burrows used by mudskipper provide shelter, spawning sites, and access to feeding grounds for other mudskipper species. 'Fish Use for conducting Research' singed by Vice Head of Department of Research Affairs on 8 th November 2018 for the study from April 2017 to April 2019. Burrow structureThe burrow structures of Pn. septemradiatus were studied in situ, by creating casts of burrows at field sites when the fish relocated out of burrows, to avoid polyester resin's effect on fish. The constructed burrows by Pn. septemradiatus were

There has been a long-standing controversy about whether vertebrates emerged in the Paleozoic from marine or freshwater environments. Several hypotheses have proposed coastal, estuarine and riparian areas as sites of the transition. Here, we report the ecology of an amphibious fish Periophthalmodon septemradiatus, which we presume is in the process of niche expansion into terrestrial habitats from estuarine to freshwater environments along the Mekong River, Vietnam. Adult fish are highly terrestrial and have not been observed to venture into water during our survey. Courtship behaviour was observed, and fertilised eggs were recovered from burrows in both brackish and freshwater environments. The smallest fish collected at 12, 96, and 148 km from the river mouth were juveniles shortly after starting an amphibious life. These findings suggest reproduction in both brackish and freshwater environments. In contrast, otolith Sr:Ca ratio indicates larval hatching only in brackish water. Analysis of a 940-base pair (bp) segment of the mitochondrial cytochrome c oxidase subunit II and a 934-bp segment of the mitochondrial D-loop demonstrated no genetic segregation between populations. The fish may provide a unique opportunity to study how ambient salinity affects the biology and ecology of a living vertebrate during transition from water to land.

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