| Summary: | 碩士 === 國立中山大學 === 海洋生物研究所 === 94 === Marine and aquatic species relying on external fertilization must have special adaptation against sperm-dilution in the water. In addition mating competition may be another important source of limitation to an individual’s reproductive success. If the two above mechanisms are in direct conflict, real adaptation would reflect whichever is more critical to one’s fitness. In using the same logic, the unidirectional induction of spawning, i.e., females inducing males, but not males inducing females, has been reported as an evidence for the relative importance of natural selection, that is, sperm-dilution, in limiting the fitness of brittlestars in shallow waters. If the above deduction is correct, one would predict more adaptations to offset the adverse effect of sperm-dilution in various aspects of spawning-related characteristics.
Two species of brittlestars. Ophiocoma dentata, living in subtidal zone, and O. scolopendrina, living in intertidal zone, were studied at Kenting, southern Taiwan.
Experimental approaches were adopted to figure out if these two species have any adaptation in space and in time that can perceivably counteract the effect of sperm dilution. Spatially, the male O. dentata has the ability to search for females, and the males living with females have higher rates of responding to female spawn than unpaired males; the males do not actively search for eggs released in the water. In O. scolopendrina, neither male nor female had the ability to distinguish the sex of other individuals, they do not form pairs in nature. Males, however, have strong ability to search released eggs and then approach and spawn near the eggs. Temporally, the males of the O. scolopendrina, are sensitive to tidal rhythms in terms of inducibility by female spawns, they only spawn at the time of low tide. The possible effect of water level and photophase were both ruled out. In contrast, no such tidal rhythms of male inducibility was found in the subtidal O. dentata. Sexual selection needs not be invoked in any of the above behaviors, whereas natural selection against sperm dilution is an acceptable explanation.
Last, the male O. scolopendrina displays interference behavior by blocking other males from approaching eggs are in the water. The blocked individuals are less likely to spawn.
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