Sex allocation in scleractinian corals
Kazuhiko Sakai1), Kenichi Shinjo2), and Chio Hayashi3)
1) Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, Sesoko, Motobu, Okinawa 905-0227, Japan, 2) Department of Biology, University of the Ryukyus, Nishihara, Okinawa 903-0213, 3) Department of Chemistry, Biology and Marine Science, University of the Ryukyus, Nishihara, Okinawa 903-0213,
We studied sex allocation of two hermaphroditic scleractinian coral species (Goniastrea aspera and Favites chinensis) on a fringing reef of Sesoko Island, Okinawa, by collecting colonies of various sizes just before the spawning. The two species belong to the same family (Faviidae), having the same growth form (massive), and having a center of distribution at the low intertidal. We measured volume of testes and ovaries, and examined the theoretical prediction that testes to ovaries (T/O) ratio is decreasing function over colony size under the assumption that fertilization occurs locally and fitness through sperm is in the diminishing return.
We compared sex allocation of G. aspera and F. chinensis at the low intertidal. G. aspera had higher T/O ratio than F. chinensis. We detected a negative correlation between colony size and T/O ratio in F. chinensis, but no significant correlation was detected in G. aspera. G. aspera showed high cleverage rate in self-fertilized eggs six hours after the spawning, but F. chinensis showed no self-fertilization in six hours. The theory predicts that higher self-fertilization results in lower T/O ratio. Hence, self-fertilization appears to be rare in the field, and it cannot explain the difference in sex allocation between the two species. Population density was much higher in G. aspera than in F. chinensis. If the population density is responsible for the difference in sex allocation between the two species, sex allocation pattern should be reversal, because sperm limitation should be severer in species with low population density. G. aspera brooded planulae after the spawning, but F. chinensis did not. Planula brooding seems to result in higher T/O ratio in G. aspera than in F. chinensis, and the non-significant correlation between colony size and T/O ratio in G. aspera.
We then compared sex allocation in G. aspera between the low intertidal and the reef edge on the same reef. G. aspera had higher T/O ratio at the low intertidal than at the reef edge. Further, T/O ratio was negatively correlated with colony size at the reef edge, but not at the lower intertidal. Population density was much higher at the low intertidal than at the reef edge, but the density cannot explain the observed difference in sex allocation with the same reason mentioned above. G. aspera brooded more planulae at the low intertidal than at the reef edge. Thus, planula brooding also seems to explain the intraspecific variation in sex allocation between the low intertidal and the reef edge in G. aspera.
