Zoological Studies

Vol. 40 No. 1, 2001

Induction of Metamorphosis and Substratum Preference in Four Sympatric and Closely Related Species of Sea Urchins (Genus Echinometra) in Okinawa

M. Aminur Rahmani1 and Tsuyoshi Ueharai1,*

1Department of Marine and Environmental Sciences, Graduate School of Engineering and Science, University of the Ryukyus, 1 Senbaru, Nishihara-cho, Okinawa 903-0213, Japan

M. Aminur Rahman and Tsuyoshi Uehara (2001) Metamorphosis and settlement studies were conducted with 20 to 24-d-old laboratory-reared larvae of 4 closely related and genetically divergent sea urchins of the genus Echinometra (E. sp. nov. A, E. mathaei, E. sp. nov. C, and E. oblonga) to assess their preferences for various substrata. All the Echinometra spp. exhibited a similar high rate of metamorphosis in response to encrusting coralline red algae compared to mixed turfs of coralline algae with: regular brown, green, or mixed fleshy algae, suggesting that potent inducing substances may be sufficiently present in red algae. Lack and/or shortage of inducing materials in brown and green algae may account for the very low rate of metamorphosis and survival. Furthermore, aqueous extracts of coralline red algae induced Echinometra spp. larvae to metamorphose, demonstrating that the inducing factor is chemical in nature. These chemicals have been shown by several workers to be proteins which are GABA- mimetic in their interaction with the larval receptors controlling metamorphosis. GABA, which triggers the meta- morphosis of several gastropods and strongylocentrotids, also induced it in Echinometra spp. larvae at concentrations exceeding 10-3 M, indicating that a textural requirement is less likely than chemosensory receptors in inducing metamorphosis. Tests performed by soaking red algal substrata revealed that no inducing substances leak into the surrounding seawater from intact inducing algae. Reduction in the number of live bacteria on the surface of red algae through treatment with antibiotics did not affect the rate of metamorphosis. This is the first attempt to study settlement induction and metamorphosis in 4 closely related but genetically distinct species of the sea urchins of the genus Echinometra. The 4 species did not differ in their rates of metamorphosis on each type of substratum, and each species highly preferred a coralline red algal substratum, which is consistent with their close genetic affinity as well as their sympatric existence in nature.

Key words: Settlement, Echinometra spp., Genetic affinity, Coralline algae, GABA.

*Correspondence: Tel: 81-098-895-8897. Fax: 81-098-895-8897. E-mail: ueharago@sci.u-ryukyu.ac.jp amrahmanbangla@hotmail.com