Ecophysiological Study on the survival strategy of mudskippers in subtropical mangrove swamps of Oceania

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The Australian mudskipper, Periophthalmus_minutus, inhabits high intertidal mudflat areas that experience seawater inundation only during the high water spring tides. Our study site remained emerged usually for 7-12 days during neap tides in one tidal cycle, throughout a year. As a result, burrow water salinity increased from 33 ± 5 psu in March (end of the rainy season) to 84 ± 7 after an extended period of emersion in August (dry season). P.minutus remained inside its burrow during these long emersion periods. Experimental exposure to either freshwater (FW) or 200% SW did not affect the chloride cell density, of P.minutus. However, apical pits of the chloride cells were rapidly closed in response to FW, but not to 200% Sw, exposure. Chloride cell size significantly increased after 9-14 day exposure to 200% SW. Plasma sodium concentration did not change by the exposure to 200% SW but decreased significantly after 5 days in FW. No mortality occurred during exposure to 200% SW, whereas 50% of FW-exposed fish died within 4 days. These results demonstrate that the exceptionally high salinity tolerance of P.minutus is made possible by the high density and adaptive hypertrophy of the opercular chloride cells. This allows P.minutus to effectively regulate ion concentrations of the body fluids during the extended burrow confinement during which concentrated seawater is the only source of water supply to the fish.The gonads of P.minutus were immature throughout the dry season, but rapidly grew with time during the rainy season, attaining the largest size in January to February. Subsequently, the gonadal size decreased at end of the rainy season (March). Spawning occurred in neap tides in February. These results demonstrated that gonadal maturation is triggered by the advent of the rainy season, and that spawning takes place in a semi-lunar cycle.