Body size structure, abundance and gastropod shell use by the local population of the land hermit crab Coenobita brevimanus were investigated in the abandoned village of Amitori on Iriomote Island, Okinawa, Japan. Amitori was inhabited by a maximum of 200 people between the 17th century and 1971 and is now used as a research center of Tokai University. Results of mark-recapture experiments showed higher recapture rates than previous studies and an estimated population size of > 1000 crabs. The hermit crabs in Amitori, which use shells of edible marine gastropods such as Turbo (Marmarostoma) argyrostomus likely discarded by the former residents, grow to be larger than crabs living on an adjacent uninhabited island and an inhabited village without shell middens. The proportion of abraded and broken shells used by the crabs in Amitori is higher than that of crabs in another village on an adjacent island where edible gastropod shells continue to be discarded by residents. The crabs in Amitori use either small shells with little damage, or relatively large shells with more damage. Since the growth and abundance of C. brevimanus individuals depend on the continued supply of new marine gastropod shells, the proportion of large crabs in the uninhabited village of Amitori will likely decrease in the future.

To assess macrozoobenthic diversity and habitat conditions following the 2011 tsunamis, we conducted a series of field surveys in the Samegawa and Momiya River estuaries (Fukushima and Ibaraki Prefectures, respectively). We compared measured parameters with existing published datasets for 10 sites along the northeastern Honshu coast. Faunal diversity was higher at the Samegawa site (140 taxa in total, of which 31 were endangered and 51 were endemic; the faunal list included stenohaline marine taxa), likely because of the high habitat diversity at this location and seawater discharge from the thermal power plant. Cluster analysis differentiated distinct faunal community groupings associated with two habitat types: (i) marine-dominated sites, including the Samegawa Lagoon, Mangoku-ura, and Matsushima Bay and (ii) sites with riverine influence, including the mouths of the Samegawa and Momiya Rivers and brackish lagoons along Sendai Bay. The population size of the dominant mud snail Batillaria attramentaria in the Samegawa Lagoon declined steeply after the tsunamis but gradually recovered within five years. Microsatellite DNA analysis showed that the genetic diversity of this population did not significantly change following the tsunamis. After 2016, ongoing restoration work caused drastic habitat degradation at the Samegawa site, resulting in mass mortalities of polyhaline and stenohaline marine taxa and overall reductions in faunal diversity.

Between 2011 and 2017, we studied the effects of habitat characteristics on the abundance of the Japanese abalone (Haliotis discus discus) on the coast of Sado Island, Niigata, Japan. Subtidal areas (depth, 1–7 m) along the island were evaluated using quadrat surveys performed by scuba divers. The habitat characteristics (water depth, percent macroalgal cover, number of macroalgal species, and percent boulder cover) were recorded within each quadrat, along with the number and size of the abalones. Nonlinear models of the habitat characteristic data revealed differences in the habitat characteristics that affected the abundance of large (≥90 mm) and small (<90 mm) abalones. The large abalones were more prevalent between 3 and 5 m, and their abundance increased as the macroalgal cover decreased. Alternatively, most small abalones inhabited areas covered up to 40% by macroalgae. These results suggest that Japanese abalones ontogenetically shift their habitat requirement and thereby contribute to the restoration of their habitats in coastal areas.

Between 2013 and 2017, we collected 30 sea urchins that belonged to the genus Echinometra from Banshozaki Cape, Shirahama, Wakayama, Japan. These specimens included 6 greenish individuals that resemble Echinometra. sp. C (reported by Arakaki et al., 1998), a species distributed in only Okinawa and southwards. Molecular analysis based on partial sequences of the mitochondrial cytochrome oxidase subunit I gene (1,080 bp) suggested that the 30 sea urchins from Shirahama were Echinometra mathaei, Echinometra oblonga, Echinometra sp. A. and Echinometra sp. C. Five of the 6 greenish specimens were genetically identical to Echinometra sp. C from Okinawa. Triradiate spicules present in the gonads, a diagnostic character of Echinometra sp. C., further confirmed the taxonomy of the 5 individuals. The remaining green specimen was identified as Echinometra mathaei on the basis of both molecular analysis and spicule morphology. To the best of our knowledge, this is the first record of Echinometra sp. C on the main island of Japan (Honshu). Here, we have discussed the reasons for the emergence of tropical sea urchins around Banshozaki Cape, Shirahama.