Activity of the shallow-water hydrothermal vent crab Xenograpsus testudinatus is affected by tides, water temperature, and light intensity

Abstract

The activity pattern of the shallow-water hydrothermal vent crab Xenograpsus testudinatus in the vent and peripheral regions of Kueishan Island, Taiwan, was observed using an underwater time-lapse camera with infrared lighting during 7 consecutive 2-3 d periods. Hourly tide levels, in situ water temperature, pH, and light intensity were recorded to examine any effects of these environmental factors on crab activity. Time series analysis using spectral density plots indicated that crab activities were not rhythmic. Nevertheless, cross-correlation analysis and Pearson's correlation showed that crab density in the vent region was negatively correlated with tide but positively correlated with water temperature and light intensity. Stepwise multiple regression analysis showed that water temperature, tide levels, and light intensity are significant predictors of crab density. There were no significant differences in crab density among specific tide periods, and thus, a previous hypothesis that vent crabs emerge in large swarms during slack waters was rejected. The vent region is a refuge from predators, with crabs more active during daytime low tides and during periods with higher water temperatures. Crab density in the peripheral region is positively correlated with tide but negatively correlated with pH, indicating that crabs there are more active at high tides while foraging and during periods with lower pH, probably when there are fewer predators. Stepwise multiple regression revealed pH and tides were significant predictors for crab density in the peripheral region.