Male-biased sex ratios of Tripneustes gratilla around Taiwan

Abstract In a preliminary study of the sea urchin, Tripneustes gratilla at Hobihu, south Taiwan, biased sex ratios were found in 2010. Three hypotheses were proposed to explain the mechanism and tested in a wider region around Taiwan. The Sampling Bias Hypothesis was not supported since external morphology, including “masking” percentages, i.e., percentages of debris cover on top of urchin tests, and the height:diameter ratios of urchin tests were not different between the sexes. The Innate Hypothesis was not supported since an island-wide investigation revealed unbiased sex ratios in six out of seven surveys carried out in March/April when the urchins started to appear on the surface of the substrate, but the sex ratios were male-biased in all 11 surveys, later in the year. Moreover, artificially propagated sea urchins raised in seawater ponds had unbiased sex ratios, whereas the wild population in the same region and season had significantly male-biased sex ratios. With the progress of season, the sex-ratios became more and more biased with increasing urchin sizes, in two field cases. These phenomena falsified the Innate Hypothesis. The Differential Mortality Hypothesis was the only one that could not be falsified. Females obviously suffer higher mortality rates than males in the field, presumably due to predators. It is suggested that females attracted predators during repeated spawning when they released eggs. The additional mortality rates of females than males were estimated to be no less than 20% per month based on the change of sex ratios through seasons.