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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
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