Sequential hermaphroditism (sex change) has been reported in many decapod crustaceans, but it has not been fully reviewed to date. In this review, I have integrated and sorted descriptions regarding hermaphroditic decapod crustaceans. Eighty-five species have been recognized as hermaphrodites and 57 of those species are protandrous (male first) hermaphrodites including protandrous sex changer and protandrous-simultaneous hermaphrodite. The sexual maturation pattern of the remaining 28 species is still unclear, although simultaneous hermaphroditism has generally been speculated from sexual characteristics in their morphology or reproductive organs. Moreover, I have summarized the emerging knowledge of the adaptive significance of their sex change. After briefly reviewing theoretical works, I draw attention to recent empirical works exploring the proximal factors causing life history variation. I discuss life history variation within a population and among populations is caused by growth rate variation, mating system and diet.

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Sagitta crassa is the only autochthonous chaetognath in Tokyo Bay, central Japan. We analyzed the gut contents of S. crassa at a fixed station in May and September 1997. The species fed on prey organisms whose body widths were in a range between 1.4-6.0% of the length of S. crassa. Prey with body widths less than 2% of the length of S. crassa were minor components of the gut contents. Although the cyclopoid copepod Oithona davisae was abundant in the water column, the prey selectivity of S. crassa on O. davisae was low. Our results suggest that S. crassa consumes catchable prey organisms that are larger and easier to detect. As planktonic species diversity in the Tokyo Bay ecosystem is low, it has been assumed that a simple trophic pathway from the most dominant primary consumer O. davisae to the secondary consumer S. crassa exists. However, such a simple assumption is inadequate to understand the prey-predator relationships and food-web dynamics in Tokyo Bay.

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In this study, we identified 9 polymorphic compound microsatellite markers in the toxic dinoflagellate Alexandrium catenella, which was isolated from Thau lagoon (France, Mediterranean Sea), using the compound microsatellite marker technique. These new microsatellites were characterized by screening DNA templates from 43 A. catenella clonal strains, which were collected from a seawater sample from Inokushi Bay (Oita Prefecture, Japan). These loci provide one class of highly variable genetic marker: the number of alleles ranged from 3 to 8, and the estimate of gene diversity varied between 0.285 and 0.762. We also analyzed the same 43 DNA samples using microsatellite markers previously identified for A. catenella, comparing the PCR amplification success, the number of alleles and gene diversity. These three parameters were not significantly different, showing that the compound microsatellite markers have the same potential to reveal A. catenella genetic structure. This simple and efficient method reduces the costs for developing SSR markers and for analyzing the genetic structure of populations, therefore, suggesting the effectiveness of applying this method to other species.

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A 17-month field survey and laboratory experiments were conducted to investigate the life cycle, seasonal population fluctuations, and salinity tolerance in the poecilostomatoid copepod Hemicyclops gomsoensis associated with the burrows of the mud shrimp Upogebia major and the ocypodid crab Macrophthalmus japonicus in the mud-flats of the Tama-River estuary, central Japan. On the basis of sample collections in the water column and from the burrows, it was revealed that H. gomsoensis is planktonic during the naupliar stages and settles on the bottom during the first copepodid stage to inhabit the burrows of U. major and, to a lesser extent, those of M. japonicus. While females carrying egg-sacs were present throughout the year, the copepods' reproduction took place mainly during early summer to autumn with a successive decrease from autumn to winter. Occasionally the copepod populations in the burrows suffered from severe flushes of river water that led to salinity decreases in the burrow water to fatal levels, but usually the salinity in the burrow was within optimal levels and permitted recovery and maintenance of the populations.

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This paper studies heteropods in terms of species composition, distribution and abundance, and analyzes the dominant species' contribution to the total heteropod abundance in the East China Sea by step-wise regression and other ecological indices based on data collected from four oceanographic cruises in the East China Sea (23°30'-33°N, 118°30'-128°E) during 1997 to 2000. A comparison was made between results from the present study and that of the year 1959. Eleven species were found in the above mentioned four oceanographic cruises during the research - 9 in autumn, 7 in summer, 2 in spring and only 1 in winter. Four species, namely Atlanta rosea, A. peroni, A. lesueuri and Protalanta souleyeti, are defined as ‘common species’ based on the fact that they were found all over the East China Sea. Others are thereby considered ‘rare species’, namely A. inclinata, Firoloida desmaresti, Atlanta sp., Oxygyrus keraudreni, A. turriculata, A. helcinoides and A. depressa. The analysis of regression contribution shows that of all heteropod species, A. rosea is the most important one, whereas the importance of A. peroni and A. lesueuri is revealed in autumn. By comparing data from 1959 with that from the present sampling period, an increase in the number of heteropod species could be observed. Taking into consideration the close reliance of heteropods on warm currents, they might serve as an effective indicator of warm current intrusion into the East China Sea.

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Behavior of the common lancelet Branchiostoma japonicum in sediment and effects of their turbation on sediment were observed in a tank to discover their ecological significance in a sandbank with a dense population of the animals. Direct observation using a video camera revealed that a lancelet frequently moves in the sediment only a short distance (a few centimeters at most) over the course of a day and rarely swims out from the sediment, and only for a few seconds. Sand ridges 4 cm high formed in the tank with lancelets at a density of 560 animals m^-2 became almost level during an incubation period of 10 days, in contrast to a tank without lancelets, where there was no change. Dissolved O₂ concentration in the interstitial water of the sediment with lancelets was 2-5 times higher than that without lancelets at the end of incubation for 7 or 12 days. These results indicate that bioturbation by a dense population of lancelets has significant effects on the physical and chemical conditions of the sediment.

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