The symbiotic scale worm polychaete Arctonoe vittata mainly exploits the limpet Niveotectura pallida and the sea star Asterias amurensis as hosts in southern Hokkaido, Japan. Its size distribution was different between the two host species, in which smaller individuals were often observed on the sea star, while larger ones were in the mantle cavity of the limpet. The host exploitation pattern of the scale worm was examined by several experiments. Scale worms infesting the sea star and the limpet showed significantly higher survival rates than those detached from the hosts under a risk of crab predation. Larger scale worms showed significantly higher survival rates in the limpet than in the sea star. In competition trials for the limpet host, relatively larger scale worms won and smaller ones sometimes died probably due to severe struggle. These results suggest that the scale worm switches its host species from the sea star to the limpet as it increases in body size, because smaller individuals improve their survival rate in the sea star, while larger ones are able to guard their host limpet from conspecifics and improve their chances of survival. The achievement of optimal host exploitation in Arctonoe vittata therefore seems to be closely correlated to ontogenic growth.

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A new species of sea anemone from the Sea of Japan is described as Cribrinopsis japonica sp. nov. In Toyama Bay, pinkish-colored anemones have been frequently caught by the local fishermen but never identified. In this study, specimens were collected from the Sea of Japan (36°58'5N, 137°22'7E) between 2010 and 2012. These anemones are identified as a new species of Cribrinopsis differing from known species in the verrucae, cnidae, and the arrangements of tentacles and mesenteries. The species may be confused with C. fernaldi Siebert and Spaulding, 1976, which is reportedly present in the Sea of Japan, but decamerous C. japonica sp. nov. is distinctly different from the hexamerous C. fernaldi.

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Use by a symbiotic crab Sestrostoma toriumii of burrows of a mud shrimp Upogebia yokoyai was investigated in laboratory experiments. Crabs used shrimp burrows in the presence of the host when the mud surface was covered with water. Crabs entered shrimp burrows soon after introduction to the tank (taking a mean of 656 s to enter), where they spent 61–81% of their total time in the experimental tanks. Half the crabs left the burrow at least once on the first day, whereas only a few crabs left on the second day. Although the crabs were expelled by the host shrimps in some cases, it is not known whether exiting is always due to host aggressive behavior. Two individuals that did not enter a shrimp burrow buried themselves in the sediment most of the time. Burying in the surface sediment may also be an adaptive behavior of S. toriumii when the crab has left the burrow for some reason. By living in the shrimp burrow, S. toriumii would benefit through predator avoidance.

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Population structure, growth, reproductive activities and spatial distribution in a population of the neritid gastropod Clithon retropictus are described, and the effects of habitat stones on distribution and individual activities are investigated. Monthly changes in coupling behavior and abundance of egg capsules indicated that the main reproductive season is June. The occurrence of small juveniles indicated that recruitment occurred mainly between August and September in the lower intertidal region of the adult distribution range. Recruits grew to a large size (ca. 9 mm SL), reaching maturity in June of the second year (ca. 1 year old). Longevity was estimated to be at least 3.3 years. More than 90% of snails occurred on stones throughout the year, predominantly underneath stones on exposed substratum rather than those on submerged substratum. Snail abundance was significantly influenced by stone coverage, water condition and time of year (month). Stone manipulation revealed a positive relationship between snail abundance and stone abundance. Mating behavior occurred mainly on the upper or lateral surface of stones. The occupancy periods of individual snails on stones were positively correlated with stone size in autumn, but not in the other three seasons. Distances moved by individual snails per day were significantly shorter in winter than in the other seasons.

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Nipponomysis surugensis (Murano, 1977) (Crustacea, Mysida, Mysidae) was first reported on the basis of specimens collected from Suruga Bay, central Japan. No further collections have been made to date since the original record. Specimens recently found in the Seto Inland Sea, western Japan, were referred to N. surugensis. Our study revealed that N. surugensis is very remarkable among the members of Nipponomysis Takahashi & Murano, 1986 by having four-segmented carpo-propodi on the third to eighth thoracopodal endopods, as opposed to three segments in the remaining species. This rare species is redescribed based on newly-obtained specimens. An updated key to the species of Nipponomysis is provided.

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