Around the Akashi Strait, about half of the catch of the sand lance in Harima-Nada and Osaka Bay is caught in winter to early spring. The annual catch has continued to decrease after the 1990s, and it is suggested that the decline in catch was caused by copepod shortage as food. In the present study, frequent samplings of copepods were conducted from December to early April around the Akashi Strait for three years to evaluate copepod abundance and biomass. Around the Akashi Strait, the dominant species by abundance were Paracalanus spp., Oithona spp. and Microsetella norvegica, but by biomass, the most dominant was Calanus sinicus. Cluster analysis showed that copepod adult communities, based on biomass, were classified into five groups, and the representative species were Oithona similis, Subeucalanus crassus, M. norvegica, Euchaeta plana and C. sinicus, respectively. Especially from February to early April, most of the copepod communities were classified into cluster group 5, characterized by a high biomass of C. sinicus. This period coincides with the active feeding on copepods by sand lance. These results suggest that C. sinicus is the most important prey item for sand lance around the Akashi Strait. In addition, the temporal recruitment of pelagic species such as S. crassus and E. plana contributed to an increase in copepod biomass. There is concern that environmental shifts will lead to declines in C. sinicus, resulting in a decline in the total copepod biomass in this area.

The close relationship between hermit crabs’ fitness and the size and species of gastropod shells they occupy is largely based on studies of young and adult crabs. When hermit crabs settle as megalopae, they too require a shell, but there are few studies of shell selection and use by this early life stage. We examined shell selection by megalopae and early juveniles of the hermit crab Pagurus filholi for three gastropod shell species, Nassarius fraterculus, Batillaria cumingii, and Homalopoma sangarense, all common in the study area and used by young and adult P. filholi. The early crab life stages did not prefer any shell species. Shell species had no significant effect on the survival of megalopae or early juveniles that were exposed to high water temperatures, nor on crab growth rates or vulnerability to predators. Under desiccating conditions, megalopae and early juveniles had significantly lower survival in Homalopoma shells than in Batillaria shells. These results suggest that variation in fitness across shell species has not selected for shell species preferences by early life stages of P. filholi perhaps explaining why they occupy shell species more equitably compared to later life stages for which fitness varies with the species of shell they occupy.

We describe a novel foraging technique of the blue bat star Patiria pectinifera that we call “standing-on-tiptoe” behavior. We examined whether the starfish uses this technique to catch shore crab Gaetice depressus in laboratory experiments. Patiria pectinifera in an aquarium with live crabs showed standing-on-tiptoe behavior at a significantly higher frequency (20 out of 47 cases) than those with dead crabs (one out of 23 cases) or no crabs (one out of 23 cases). Nine out of the twenty starfish showing this behavior succeeded in catching and consuming the shore crabs. This behavior could be a tactic for foraging for agile prey, such as shore crabs.

In the autumn of 2021, a large-scale harmful algal bloom (HAB) emerged along the southeast coast of Hokkaido, Japan, and this was predominantly composed of the dinoflagellate Karenia selliformis. The emergence of K. selliformis-dominated HABs was the first observation in Japan; therefore, no previous reports exist on the impact of HABs on zooplankton in the region. This study investigated the effects of K. selliformis on the survival of copepods—a critical component of the zooplankton community. The results indicated that the survival rates of all six copepod species examined were markedly reduced, particularly at elevated K. selliformis concentrations or during extended exposure. The copepod survival rate decline occurred even in the absence of direct contact or ingestion of K. selliformis, thus implying that K. selliformis may excrete harmful substances extracellularly. Feeding experiments revealed that Neocalanus plumchrus consumes K. selliformis cells; however, the ingestion rate diminished with increased concentrations of K. selliformis cells. The results suggest that larger copepods with smaller surface-to-volume ratios may possess greater resilience to the harmful substances compared to smaller species. Further, the HAB in southeast Hokkaido likely exerted deleterious effects on lower trophic levels within the marine ecosystem by disrupting copepod survival and feeding activity.