This study aims to test a hypothesis that the hardiness to lower temperatures of oceanic sea-skaters (Halobates germanus and H. micans) inhabiting the highest annual precipitation areas around the equator in the Pacific Ocean is greater than that of sea-skaters inhabiting nearby areas where there is less precipitation. This hypothesis is related to the sudden decrease in air temperature by about 5 ºC near the sea surface when it is rainy. Samples were taken from two sampling areas in the tropical Pacific Ocean (high annual precipitation area: 8ºN-5ºS, 136ºE-156ºE; low annual precipitation area: 12ºN, 135ºE). Semi-cool-coma temperature (SCCT) and cool-coma temperature (CCT) were recorded. The gap temperature for cool-coma (GTCC) was calculated as the difference between the surface sea water temperature at sampling sites and the CCT. The CCT (mean ± SD [n] = 12.8 ± 3.3 ºC ) and SCCT (17.1 ± 1.8 ºC ) of the specimens collected from the high precipitation area around the equator were significantly lower (CCT: p < 0.001, SCCT: p = 0.001) than the CCT (mean ± SD [n] = 17.9 ± 1.8 ºC ) and SCCT (18.0 ± 1.8 ºC ) found in specimens collected outside the high precipitation area. The GTCC (16.8 ± 3.4 ºC ) shown by the specimens collected from the high precipitation area was significantly higher (p < 0.001) than that of those collected from an area outside the high precipitation area (10.9 ± 1.9 ºC ). These results support our hypothesis.
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