The Honjo Area of Lake Nakaumi is characterized as a brackish water area semi-enclosed by reclamation dikes. In accordance with the partial removal of these dikes from 2007 to 2009, we studied subsequent differences in environment, and spatial distribution of bivalves between 2006, 2010, and 2014, to determine the influence of the removal. Arcuatula senhousia was the primary dominant species across all years studied, followed by Ruditapes philippinarum and Macoma incongrua. After the partial removal of dikes, the density of A. senhousia decreased. Three areas were designated according to water depth and bivalve distribution. Area I was shallower than approximately 4 m, and bivalves were recognized at almost all stations in this site during each year studied. Areas II and III were deeper than approximately 4 m, and there were no bivalves in 2006 or 2010. In Area II, bivalves, mainly R. philippinarum, were observed in 2014, while none were identified in Area III. Area II was located near the opening slit of Moriyama dike. The inflow of sea water supplied oxygen to the bottom (greater than 4 m) of a limited region (Area II) and possibly enabled bivalves to inhabit this region in 2014.
Chemical defense using secretion of ink containing purple pigments derived from red algae is common in sea hares. Aplysia juliana, however, prefers green algae, and thus, its ink is white, not purple. Since opaline, the sea hares' other defensive secrwetion, is also white, ink and opaline were not distinguished from each other in previous experiments. Thus, deterrence of this white ink alone towards predators has never been tested. In this study, we tested the deterrence of the white ink of A. juliana as well as the extract of their skin, using the Japanese spiny lobster Panulirus japonicus as a model predator. Parallel experiments on Aplysia gigantea, a sea hare with purple ink, were performed to test whether P. japonicus is deterred by purple ink. The skin extract, but not the white ink, of A. juliana was deterrent. In contrast, purple ink, but not the skin extract, of A. gigantea was deterrent. These results show that the skin of A. juliana contains defensive chemicals against P. japonicus, whereas the white ink itself does not.
The Asian mussel, Arcuatula (Musculista) senhousia, forms dense population patches on sandy tidal flats. However, few studies have investigated its secondary production. Therefore, we investigated secondary production of the Asian mussel, as well as the temperature and salinity of water on the Midori river tidal flats, Kyushu, Japan, from April 2012 to July 2013. In this study, we discuss the characteristics of secondary production of A. senhousia and the influence of the mussel on primary producers. The juvenile mussels settled on the sediment in early August 2012 and formed a single cohort. The mean shell length of the cohort was greater than 20 mm over the course of one year. The highest daily secondary production was 2.4 g C m−2 day−1 after the settlement period(from August 31 to October 2, 2012), and accounted for 24% of the total yearly secondary production. Although the yearly secondary production was much higher than that in previous studies, P/Bmax (1.7)was comparable to that in earlier studies; P and Bmax are secondary production and maximum biomass of A. senhousia, respectively. Thus, the high secondary production in this study was sustained by a large mussel biomass. Maximum secondary production of the mussel population reached approximately one-half of the highest primary production in Ariake Bay. Thus, feeding by this bivalve greatly affects both primary producers and circulation of material in their habitats.
Resource partitioning is an important mechanism that facilitates the coexistence of diverse organisms. Hermit crabs depend on the empty shells of gastropods for protection against predation; however, a short supply of this resource results in intense fighting over shells and competition among hermit crab species. To examine whether hermit crab species mitigate shell competition through resource partitioning, we studied the distribution pattern, shell utilization pattern, and frequency of ovigerous Pagurus filholi, Pagurus nigrivittatus, and Clibanarius virescens females, all of which are abundant hermit crab species in the intertidal shore in the Shikoku region of Japan. We found that P. filholi and C. virescens essentially live in sympatry, while P. nigrivittatus prefers a lower intertidal habitat compared to that by the other two species. Additionally, we found a significant difference among the patterns of shell species utilization between P. filholi and C. virescens. Further, while the proportion of ovigerous females reached approximately 30% in C. virescens and P. nigrivittatus, only a few P. filholi females reproduced in our study, suggesting that they breed in distinct temporal windows. Our results demonstrate that these three common hermit crabs spatially and temporally partition their resources, which potentially mitigates interspecific competition.
The pattern of egg production is important for understanding the reproductive biology in various animals. In species that can spawn multiple clutches during a single reproductive season, egg number and/or size in a population often show temporal changes even within a single season. However, there are only a few studies examining the temporal patterns of egg production in decapod crustaceans including hermit crabs. In this study, we investigated whether the clutch size and egg size change during a single breeding season in the hermit crab Pagurus minutus. We collected precopulatory guarding pairs from December 2014 to April 2015 and recorded the clutch and egg size of the newly spawned eggs. Our results demonstrated that both the clutch size and the egg size varied over this period; fewer and larger-sized eggs were laid in December, whereas eggs laid in February were greater in numbers and smaller in size. Given the temporal changes in environmental conditions in the study area, larvae from the two types of eggs experience different conditions. Former larvae are expected to hatch in February and experience a lower water temperature with relatively poor food conditions, whereas the latter are expected to hatch in April, when the feeding conditions are considered better with relatively warmer water temperatures. The pattern of egg production in this species is thought to vary with the environmental conditions at the time of larval hatching.