In this review, studies on the growth of the Japanese sea cucumber Apostichopus japonicus are summarized. All the recent studies referred have been using the new method of estimation of body size of A. japonicus from only one simultaneous measurement of body length and breadth taken from living animals. There were differences in individual growth rates (uneven growth) of A. japonicus seedlings in a tank, which was mainly due to the unevenness in the amount of food for each individual in the tank. However, such uneven growth may not occur in nature. In their natural habitat, juveniles grew up well and were uniform in their size within the same year-group, revealed by the cohort study of juvenile sea cucumber from an intertidal habitat in the Hirao Bay, Yamaguchi. The artificially-reared seedlings released in an artificial habitat also grew up well and uniform in their size within the same year-group, revealed in the Funka Bay, Hokkaido. In the nursery habitats of juvenile sea cucumber, the origin of the natural food resources was also investigated based onδ13C andδ15N plots of juveniles’ tissue samples, and their potential food sources. The recent studies on the inventory surveys and taxonomy of the Japanese holothurians were also summarized.
Since the discovery of deep-sea hydrothermal vent communities in 1977, researchers have made efforts to understand the mechanisms underlying the biodiversity of these communities. “Biodiversity” is hierarchical in nature, ranging from genetic to ecosystem levels and local to regional scales, and encompassing genetic, species, ecosystem, and landscape diversity. A previous paper suggested that interactions between local and regional ecological processes in hydrothermal vent ecosystems could be understood through a metacommunity framework. As a first step toward understanding the complex interactions in these ecosystems, summarizing evolutionary-ecology studies conducted at different hierarchical levels for each vent taxon could be useful. Barnacles are suitable model species for studying evolutionary ecology at various spatiotemporal scales because of the good availability of fossil records, and because of their wide distribution and high potential for laboratory/field-based experiments using both larvae and adults. In this paper, studies on hydrothermal vent barnacles at each hierarchical level of biodiversity are reviewed.
To understand the relationship between the environmental characteristics and migration habits of the Japanese turban snail Turbo cornutus, we performed tagging and release of turban snails on four fishery-grounds on the coast of Niigata, Japan. We released a total of 4,657 tagged individuals and were able to recapture 168 individuals. The analysis of the recapture data using LASSO–COX proportional hazard model showed that an increase in areas of macroalgal-bed and rocky-bottom suppressed the dispersal of the turban snails from the fishery-grounds. The partial regression coefficient of the area of rocky-bottom was larger than that of the macroalgal-bed, suggesting that an increase in the area of rocky-bottom was more effective in suppressing the dispersal rate of the turban snails from fishery-grounds compared with that of the macroalgal-bed. Conversely, an increase in the area of sandy-bottom on the fishery-ground caused an increase in the dispersal rate of the turban snails, even if other environmental characteristics were fixed. The results provide useful information for seed release and habitat construction for the Japanese turban snails.
Portunid crabs are strong predators in shore ecosystems, and often prey on hermit crabs. In a previous study, we observed the loss of larger individuals of the hermit crab Diogenes nitidimanus during the mid-reproductive season. Therefore, we experimentally investigated predation intensity of the portunid crabs Charybdis japonica and Portunus pelagicus on D. nitidimanus collected from the tidal flat of the Waka River to elucidate the effects of predator body size, prey shell type and shell size, and season. Each predator was given six prey items with a combination of two shell types (Umbonium and Batillaria) and three shell size classes. We replaced eaten prey items daily with identical ones in terms of shell size and type, and recorded the number of prey items eaten over 10 days per predator. However, we were unable to analyze the effect of season for P. pelagicus because of the species’ limited periods of collection. C. japonica preferred prey with small, Umbonium-type shells. The total number of prey items eaten increased in summer, but not with body size. By contrast, P. pelagicus preferred prey with Batillaria-type shells, and predator body size tended to be a more important factor in the context of medium- and small-sized shells. Batillaria-type shells are likely more resistant to crushing pressure than Umbonium-type shells. More smaller prey were eaten during the experiment, which does not explain the loss of larger individuals at the study site.
The Ashizaki tidal flat in inner Mutsu Bay, northern Japan, represents the northern limit of the range of the endangered mud snail Batillaria multiformis in Japan. In this study, the population structure and tidal zonation of sympatric B. multiformis and B. attramentaria were examined at stations on the Ashizaki tidal flat from 2010 to 2015. A cohort of smaller individuals (shell length [SL]: 10.0–17.1 mm) of B. multiformis was formed in 2010; however, few cohorts formed during the following years, leading to a decrease in the population density. Batillaria multiformis recruited in the summer and fall, with SL growth of 5–6 and 10–17 mm in June–July and September, respectively, of the second year, and of 20 mm in May–July of the third year. By contrast, B. attramentaria recruited every year (SL growth: 5–6 and 10–17 mm in May and summer, respectively, of the second year, and 15–20 mm in the third summer) and maintained a high population density throughout the study period. The two species recruited in low- to mid-intertidal zones in most cases. Subsequently, a proportion of B. attramentaria moved to the high-intertidal zone, while B. multiformis remained in the low-intertidal zone. Our data suggest that the populations of the two batillariid species studied herein exhibited slower growth rates and attained larger adult sizes compared with other populations in Japan. Less frequent recruitment of B. multiformis in our study site implies that this population of B. multiformis may be vulnerable.
In order to evaluate the effect of 35.5 km2 of land reclamation in Isahaya Bay in the inner part of the Ariake Sea, Japan, we analyzed the interannual variation in the bottom sediments and community structures of macrobenthic fauna across the whole area of the Ariake Sea. Sediment samples were collected from 82 fixed subtidal stations consisting of four parts (inner, central-north, central-south, and mouth parts) in June 1997 (two months after the complete closure of the inner Isahaya Bay by the 7-km dike), June 2002 (two months after the temporary recovery of limited tidal currents within the inner Isahaya Bay by the opening of the water gates in the dike), June 2007, and June 2015. The median particle diameter of the bottom sediments decreased at many stations around the central to mouth parts of the Ariake Sea from 2002 to 2007, and it increased from 2007 to 2015. A total of 29,885 macrobenthic invertebrates (mean density 7,289 ind./m2) were collected from the 82 fixed subtidal stations in June 1997, and the most dominant taxa were polychaetes, gammaridean amphipods, ophiuroids, and bivalves. In June 2002, 62,314 individuals (mean density 15,199 ind./m2) were collected from the same stations. However, the number and density of the macrobenthos declined to 11,992 (2,925 ind./m2) in June 2007 and 17,669 (4,310 ind./m2) in June 2015. A similar pattern of the variation (i.e., an increase from 1997 to 2002, and a decrease from 2002 to 2007 and 2015) was commonly observed in the densities of each dominant taxon, and for all macrobenthos in each of the four parts of the Ariake Sea, although the variation was more remarkable in the inner and central-north parts than in the others. Our results suggest that the macrobenthic community has declined from 2002 to 2007 and 2015 not only around Isahaya Bay (in the inner part of the Ariake Sea), but across the whole area.