A variety of symbiotic relationships can be found in marine ecosystems. Among these, inquilinism, which utilizes structures such as bores created by rock-boring organisms and bodies of sessile organisms, is predominant. This study describes the diversity of symbionts in rocky pits created by rock-boring sea urchins. In addition, the morphology and ecology of the unique symbiont Broderipia iridescens (Trochidae, Gastropoda, Mollusca), which exclusively lives in pits of sea urchins, flattening of coiled shells, and host urchin tracking behavior will be presented. We will also introduce the ecology of Rugilepas pearsei (Thoracica, Cirripedia, Arthropoda), a symbiont of the toxic sea urchin, by modifying its body surface, as revealed by a study of the sea urchin symbiosis biota in subtropical coastal areas. This series of studies has deepened our knowledge of the symbiosis associated with sea urchins in the waters around Japan, both as inquilinism and body-surface symbiosis.

The objective of this study was to determine the characteristics of the composition and spatiotemporal distribution of operational taxonomic units (OTUs) of nematodes in two tidal flats, namely, Nakatsu in Oita and Umino-Koen in Kanagawa, which differed in the density of Manila clams (Ruditapes philippinarum) and the other macrobenthos. There was no difference in the median particle size of the sediment between the two tidal flats, but the sorting coefficient was smaller in Umino-Koen, and the amount of organic matter was larger in the Nakatsu tidal flat. The density of nematodes was higher, and the number of OTUs was lower in Umino-Koen, which has a higher clam production than the Nakatsu tidal flat. A total of 219 OTUs were confirmed in both tidal flats, but 16 common OTUs were confirmed even by one individual on both tidal flats. In Nakatsu tidal flats, the OTU presumed to be Daptonema sp., which is common during all seasons at the two stations, was a dominant species that always inhabited extensively. It has been reported that the genus Daptonema is a non-selective deposit feeder (1B feeding type) nematode. In the Umino-Koen, where macrobenthos density is high, the presumed OTU Oncholaimus sp. was the dominant species that stably inhabited every year. Oncholaimus nematodes may thrive in tidal flats with a large amount of benthos because they are omnivore predators (2B feeding type). The characteristics of the composition and spatiotemporal distribution of nematode OTUs in the two tidal flats were different.

This study examines the effects of removing long-spined sea urchin species Diadema setosum on algal coverage, biomass, and sessile animal biomass in experimental plots that simulate barren grounds. Algal coverage and biomass increased in removal plots, and no algae were observed in non-removal plots where D. setosum was maintained at high density. A positive correlation was found between the algal coverage and biomass in the removal plots. Further, sessile animal biomass in the removal plots was higher than that in the non-removal plots. Our results suggest that the feeding pressure of D. setosum affects not only algal coverage and biomass but also sessile animal biomass.

The life history of Palaemon paucidens, a common freshwater shrimp inhabits many freshwater areas in Japan, remains unclear. In the present study, we surveyed a single population in a stream in Kesennuma, Miyagi Prefecture, Japan to elucidate the life history of this species. The spatio-temporal distribution pattern of the surveyed population was investigated in samples from seven stations established along the upstream and downstream sections of the stream. Size composition, state of sexual maturation, and appearance of hatched larvae were observed in samples collected from the mouth of the stream. Molecular species identification was used to confirm that the samples were P. paucidens. The results indicated that the total population consisted of three year-classes, and that many of the matured individuals (one or two years old) of both sexes seasonally migrated to the mouth of the stream. However, because some individuals remained in the freshwater area upstream, the migration of this species at our study site was partial. Larvae appeared at the same time as adults migrated downstream, and larvae hatched only at night. Based on age estimations in the field and the spawning intervals of females in a laboratory, it was peculated that two-year-old individuals would die after reproduction while one-year-olds would migrate upstream again. As the seasonal migration of both males and females corresponded to reproduction, we conclude that P. paucidens at our study site show diadromous migration and that their main migration pattern is catadromy.

The macrozoobenthic community structure was examined at 21 sites along the Pacific coast of Japan, from Chiba to Aomori Prefectures after the 2011 tsunamis, using government survey datasets. The taxonomic diversity was low (20–42 taxa) in river-dominated sites on the Sanriku Coast, including Unosumaigawa and Orikasagawa in Iwate Prefecture. Endangered species were scarce at six sites along the Sanriku Coast (2–5 taxa) compared with the other 15 sites, including Aomori Prefecture (7–21 taxa). Cluster analyses based on the qualitative survey data classified the macrozoobenthic community types into four groups: (1) Low saline estuaries, (2) Sanriku Coast, (3) High saline inlets, and (4) Moderate saline estuaries. Qualitative analyses revealed that the community structure in Iwate Prefecture was typically characterized by marine andepifaunal organisms, such as limpets, gastropods, and barnacles, due to the steep, marine-affected, and substrate-rich river mouth habitats. Contrastingly, the quantitative survey data showed that the community structure at three sites in Iwate Prefecture was characterized by the highest diversity and density. Based on this survey, we listed 31 taxa that were absent from Iwate Prefecture, including 18 endangered species. This clearly showed that the macrozoobenthic community structure in Iwate Prefecture differed from those at other sites on the northeastern coast of Japan. Such discontinuity may partly be caused by the effects of cold Oyashio Currents in the Sanriku Region.

The tidal flats at Otomo-ura (Iwate Prefecture), which had been converted to farmland through a reclamation project from 1959 to 1969, have been partially restored after the tsunami caused by the Great East Japan Earthquake in 2011. In the course of monitoring the benthic fauna of the tidal flats for 11 years after the tsunami, we found some adult sentinel crabs of the genus Macrophthalmus at a muddy area for the first time on July 28, 2022. During subsequent observations of the mud flat, we noted that the adult male crabs exhibited waving display of vertical non-forward-pointing type. Allocleaning, performed by both sexes, was also observed. Morphological characters include: the carpus and propodus of the ambulatory leg 3 of the adult male are not associated with tuft of setae on their ventral surfaces, and the subdistal tooth of leg 3 is not distinct. Based on these behavioral and morphological characteristics, the macrophthalmid crab was identified as Macrophthalmus japonicus. This is the first record of this species along the coast of Iwate Prefecture. In addition, five of six females captured on July 28 and August 14, 2022, were ovigerous (range in carapace width: 19.1–25.0 mm), which indicates that the crabs are reproducing on the tidal flat.