ABSTRACT
Echinometra mathaei is widely distributed in the Indo-Pacific warm temperate to tropical regions, and one of the commonest species in shallow water habitats. It has been well known that this species shows a great color variation, and Okinawan population has been grouped into 4 types, A, B, C and D. Field observations on ecological distribution and habitat preference revealed that these 4 types occur in different habitats showing partial overlapping. Type A prefers more or less protected habitats, occurring predominantly in constantly submerged ones such as tide pools, moats and shallow reef slopes, spreading over various bottom substrates from rocky to shingle/rubble bottoms. Usually it does not use burrows excavated on the reef rocks. Types B and C occur mostly on intertidal rocky substrates in the excavated burrows. Although Type D has a great overlapping with Types B and C, it prefers reef margin or higher places in the intertidal zone on the shore exposed to wave action and occupies deep burrows. Ecological implication of the burrow occupation is discussed. Observed habitat segregation and great variation in morphological as well as ecological traits of these sympatric populations have stimulated studies of the Okinawan populations from various view points.
