Abstract: Selective behaviors in salinity variations of Corbicula japonica larvae were experimentally studied with vertical salinity gradients in glass cylinders. Experiments were conducted in dark and illuminated conditions at 7 stages(16, 25, 47, 68, 141, 213 and 309 hours after fertilization). Larvae were active in selecting specific ranges of salinity that changed along developmental stages. Early straight-hinge larvae(25 and 47 hours after fertilization)selected high salinities(15.21-19.21psu on average)while late straight-hinge larvae(68 hours after fertilization)selected moderate salinities(13.45-14.29psu). Postlarvae(141 and 213 hours after fertilization)selected low salinities(6.44-11.38psu). When downward illuminations were given from the water surface, larval aggregations were formed in higher layers than in dark conditions. At 309 hours after fertilization, postlarvae did not form clear aggregation, and some individuals were found settling on the bottom.

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Abstract: The purpose of this study was to investigate the effect of a change in food availability on the growth and reproduction of the hermaphrodite sea hare Aplysia juliana. All groups of A. juliana were fed algae for five days(conditioning period), and then we stopped feeding half of each group for the subsequent five days(experimental period)while the other half of each group were fed algae. There was a negative relationship between the initial body weight of A. juliana and the increment rate in body weight under the fed condition, and positive relation ships were found between the weight of A. juliana and fecundity: i. e., the number of egg masses and the weight of them. When we stopped feeding, small individuals had more copulations than their counterparts which were under the food-rich condition although there was no increase in the number of copulations with large individuals. However, both the number of egg masses and the egg mass weights decreased when we stopped feeding. This implies that the degree to which small individuals divide energy into reproductive activity as males, though not as females, might increase when food availability decreases.

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Abstract: We observed that a small number of embryos in brood tubes of the lecithotrophic capitellid, Capitellasp. I, remained and metamorphosed into crawl-away juveniles in the parental brood tube after the parent and other larvae had left. In order to investigate whether this behavior might have a genetic basis, we rearedthe planktonic and non-planktonic individuals separately in population cages for three consecutive generations. In each generation, three food levels were used, producing mature worms of different sizes. Food supply appeared to have no direct effect on the number of non-planktonic larvae per first brood produced. Brood size ranged from 3 to 371 embryos per brood tube over the different food treatments, and was closely correlated with the size of the female parent. We found 2 to 8 non-planktonic individuals per brood irrespective of brood size. High proportions of non-planktonic larvae per brood(>20%)were only found in the smallest broods(<50 embryos per brood). In all broods with >100 embryos, >95% of the individuals were planktonic. Since the proportion of non-planktonic individuals did not increase with short-term selection for this trait over two generations, we concluded that it does not have a genetic basis. Nevertheless, the small number of non-planktonic individuals in this lecithotrophic species may be important over relatively small scales(on the order of 100m)in the recovery of relict populations decimated by the hypoxic conditions which often develop in organically enriched sediments.

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Abstract: Fish farming using net pens has become very widespread in the coastal areas of various countries in the world since the1970s. Now, it is one of the most important ways to obtain food resources from the coastal seas. This cultivated fishery, however, tends to cause eutrophication of the water and organic enrichment of the bottom sediment around fish farms, a problem that has yet to be addressed satisfactorily. In this study, we focus on the organic enrichment of the sediment below the fish farm, and develop techniques to treat the organically enriched sediment with biological activities such as feeding and reworking the sediment of a small deposit feeding polychaete, embers of the macrobenthic anim als in the organically enriched areas throughout the world. We created a mass-culture of this polychaete, and spread it on the organically enriched sediment below a net pen in a fish farm, in Amakusa, Kyushu, western Japan, in December 2003. Rapid population growth was observed after spreading the colonies, and it reached extremely high densities of approximately 130, 000indi./m² in the sediment within three months. As the Capitella population increased, the organic matter content and AVS of the surface layers of the sediment decreased markedly. Thus, spreading the artificially cultured colonies of Capitella is an effective technique to treat the organically enriched sediment and to prevent further progress of the organic enrichment of the sediment below the fish farm.

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