Estimation of Survival Rate at the Settlement of a Bivalve Theoya lubrica

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Survival rate at the settlement of a semelid bivalve Theoya lubrica was estimated from the change in the densities and size compositions of planktonic umboned larvae (shell length : 125-164 μm), metamorphosing larvae (132∼196 μm) and benthic juveniles (more than 164μm). They were simultaneously collected with plankton net, sediment traps set in the strata 0.4, 1 and 2 m above the bottom, and corer-type bottom sampler, in northern Bingo-Nada, the Inland Sea, from June to December. No significant difference was seen in the size composition of animals between the sediment traps and the bottom sampler. The density of metamorphosing larvae (2.9×102∼20.4×102 indiv./m2) was positively correlated to tidal range (210∼416 cm). From mid-June to mid-October, most of the metamorphosing larvae disappeared due to the oxygen-deficient bottom water, while planktonic larvae and benthic juveniles did not decrease markedly. At the settling period excluding the above-mentioned months, survival rate of the umboned larvae was estimated approximately 20 %, whose value was the same rate as that during the metamorphosing stage. This indicates that a large mortality hardly occurred in a transitional stage from planktonic to benthic lives.
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