Comparative studies of the genus Echinometra from Okinawa and Mauritius

Okinawan sea urchins, the genus Echinometra, are four independent species. But which species are the same species as E. mathaei and E. oblonga as described by Blainville 1825 is still open to question. To answer this question, a field survey of genus Echinometra was made in Mauritius (the type locality of E. mathaei) according to the characteristics used to classify Okinawan Echinometra: appearance, pore pairs, spicules in gonads and tubefeet, sperm shape, and distribution on a reef. The results of cross-fertilization between Echinometra from Mauritius and Okinawa are also reported. Mauritian Echinometra are classified into three groups which resemble Okinawan Echinometra sp. B, D, and violet spine color Echinometra. The latter has almost the same characteristics as Okinawan Echinometra sp. B but with violet spines, a spine color not found in Okinawa. In cross-fertilization experiments, the sperm of Mauritian Echinometra sp. B-like and violet Echinometra fertilized Okinawan Echinometra sp. B with almost 100% success. However, fertilization was unsuccessful with other Okinawan Echinometra species. Therefore, it could be said that Okinawan Echinometra sp. B is the same as Mauritian Echinometra sp. B-like, and the counterparts of Okinawan Echinometra sp. A and C are not distributed in Mauritius. The descriptions of E. mathaei most match Mauritian Echinometra sp. B-like and the type locality of E. mathaei is Mauritius. Thus, it is probable that Mauritian Echinometra sp. B-like is E. mathaei. Therefore, Okinawan Echinometra sp. B, which most resembles Mauritian Echinometra sp. B-like, would be also E. mathaei Echinometra sp. A and C would be considered to be new species. Echinometra sp. D is thought to be the same species as Mauritian black Echinometra. Whether Okinawan Echinometra sp. D is the same as E, oblonga remains as a problem for future research. Because the type locality of E. oblonga is not known and, it is suggested that the E. oblonga described in the Indo-West Pacific is a complex species.