Abstract
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.
Highlights
Mudskippers are amphibious gobies belonging to the subfamily Oxudercinae[12]
We report on the basic ecology of P. septemradiatus inhabiting from brackish to freshwater reaches of the Mekong River, discuss how studying this species will benefit the understanding of vertebrate transition to land from waters of different salinities, and propose a possible scenario for the expansion of the distribution range of this species into freshwater reaches of the Mekong River
We found no individual of P. septemradiatus in the reaches further upstream from these spots to the border with Cambodia
Summary
Mudskippers are amphibious gobies belonging to the subfamily Oxudercinae[12]. These fishes usually inhabit intertidal mudflats of tropical and subtropical coasts and estuaries, and show various degrees of behavioural, morphological and physiological adaptations to terrestrial environment. Mudskippers emerge from water for various activities such as feeding and defending territories, while during high tide some species remain out of water but others retreat into burrows[13]. We report on the basic ecology of P. septemradiatus inhabiting from brackish to freshwater reaches of the Mekong River, discuss how studying this species will benefit the understanding of vertebrate transition to land from waters of different salinities, and propose a possible scenario for the expansion of the distribution range of this species into freshwater reaches of the Mekong River
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