“Peanutfish” (“dragonfish”) is a common name for several species of commercially valuable sea cucumbers in the tropical Indo-Pacific. A “peanutfish” population composed of morphologically diverse individuals was discovered in Hatare, Marau, Guadalcanal, Solomon Islands (9.87°S, 160.82°E). To investigate the species composition and genetic structure of the “peanutfish” population, sequences of 464 bp including mitochondrial cytochrome c oxidase subunit I (COI) were analyzed during 2011–2012. Sequences were from 90 individuals, including three morphotypes having different shapes of dorsal papillae, namely (i) sharp, (ii) blunt, and (iii) intermediate and unclassifiable between (i) and (ii). Based on a phylogenetic tree, all individuals of the three morphotypes matched with Stichopus horrens, except for one individual that matched with S. cf. monotuberculatus. There were 32 mutation sites and 29 haplotypes. Haplotype diversity was not different among the three morphotypes. The degree of genetic differentiation (F_ST) between the sharp and blunt types was 0.0174 and not significant (P > 0.05), suggesting that these morphotypes were not genetically differentiated and thus were concluded as the same single species (S. horrens) based on the COI partial sequence, provided that no hybridization occurred with other species. These results indicate that S. horrens has high intraspecies variability in its external morphology. Comparison of data in the present study with sequences deposited in GenBank showed that frequent COI haplotypes of S. horrens were common within the Pacific Ocean, and most occurred sympatrically in Marau. This panmictic occurrence implies that these haplotypes are not geographically isolated from each other.

The dinoflagellate Karenia selliformis forms red tides in coastal waters and damages fishery industries in some countries, although its toxicity remains understudied. In recent years, blooms of K. selliformis have killed a vast amount of marine organisms on the Pacific coast of Hokkaido, Japan, representing the first reports of fishery damage by K. selliformis in this country. It is necessary to understand the lethal density of K. selliformis to marine organisms and environmental conditions that increase its toxicity to establish an alert level for K. selliformis red tides. In the present study, we aimed to identify the level of toxicity to fish of K. selliformis and its environmental modulators by conducting a toxicity assay with medaka fish (Oryzias latipes). First, medaka was exposed to the culture strain of K. selliformis isolated from coastal waters in Hokkaido and a culture strain of a noxious red-tide dinoflagellate Karenia mikimotoi. As a result, the toxicity of K. selliformis was extremely high compared to that of K. mikimotoi. Next, we examined growth and toxicity to O. latipes of K. selliformis cultured under several light intensities. The K. selliformis culture strain could grow at more than 20 µmol photons m⁻² s⁻¹, and its maximum growth rate reached 0.54 day⁻¹ at 400 µmol photons m⁻² s⁻¹. The toxicities of K. selliformis grown under different light intensities positively correlated with its growth rates. Our results revealed that K. selliformis exerts strong toxicity under moderate light intensity, suggesting that its high toxicity to fish requires its active growth.

Acartia (Acanthacartia) tsuensis Ito, 1956, a predominant planktonic calanoid copepod in the brackish-waters of western Japan, was originally described from Japan. This species has been recorded also from other East and Southeast Asian countries. However, a recent genetic study indicated cryptic speciation in specimens so far identified as A. tsuensis from Japan and the Philippines. The present study describes A. tsuensis s. str. based on specimens from Japan, including those collected by the original author of the species, and then the specimens from the Philippines as A. ohnoi n. sp. Previous descriptions of A. tsuensis from Korea and Vietnam somewhat differ from Japanese specimens. The new species is easily distinguishable from the co-occurring related species A. sinjiensis by the short caudal ramus in the female and strong spinules on the urosome in the male. Acartia tsuensis, A. ohnoi n. sp., and A. bilobata can be classified into the A. tsuensis species group because of close morphological similarity. The most distinctive features of the new species, separating it from the other members of the species group, are thick rostral filaments in the female and strong spinules on the urosome of the male. Their maxillule having a unique process on the praecoxal arthrite and their unique male leg 5 suggest a close relationship with a species of a different subgenus, Euacartia.

The two solenid bivalves, Solen gordonis and S. roseomaculatus, are infaunal suspension feeders and commonly cooccur in the same coastal areas of Japan. This study hypothesized that (i) within an area, the two functionally similar and closely related species showed a different microhabitat preference in spatial distribution and (ii) they exhibited a different feeding morphological trait adaptive to a respective preferred environment. Firstly, to reveal the microhabitat preference of the two species, grab sampling of bivalves and sediment samples was made at 49 stations arranged on an approximate 250-m×250-m grid within a heterogeneous, subtidal soft-bottom area (study area: ca. 300 ha) of Sasebo Bay, northwestern Kyushu, Japan. Secondly, to examine the linkage between the spatial distribution patterns and their feeding morphology, the size of gills and labial palps, which are their feeding apparatus, was compared between the two species. The results of the grab sampling showed that the spatial distribution patterns were differed between the species at a local scale, where the main habitat of S. gordonis was located in coarser-grained sediment bottoms than that of S. roseomaculatus. The results of the morphological comparison indicated that the gill fraction of S. gordonis was larger than that of S. roseomaculatus, whereas the palp fraction did not significantly differ between the two species. The interspecific difference in the local distributions might be related in part to the morphological differences in their feeding apparatus. Our findings suggested the importance of feeding morphological traits in understanding the association between the spatial distribution of infaunal suspension-feeding bivalves and sediment grain size composition in coastal soft-bottom habitats.

A new species of photid amphipod, Podoceropsis nanseiae, is described from Kagoshima Bay, Japan. The present new species can be distinguished from all the congeners in having clusters of robust setae on the male gnathopod 2 basis. This species resembles Gammaropsis laevipalmata Ren, 1992, however can be distinguished from G. laevipalmata by the presence of small accessory flagellum in antenna 1, the size of eyes, the shape of palmer margin of male gnathopod 2, and the clusters of robust setae on the male gnathopod 2 basis. The closely related species G. laevipalmata should be transferred to the genus Podoceropsis. Additionally, a partial DNA sequence of the mitochondrial cytochrome c oxidase subunit I (COI) was determined from the type specimens of P. nanseiae sp. nov. for future taxonomic studies.

Unidentified hydromedusae were collected from Nagasaki and Okinawa prefectures in western Japan. In the present study, taxonomic investigations, including morphological observations, of the unidentified specimens were conducted. The specimens collected from Okinawa had the following morphological characteristics: umbrella spherical; without tentacles; umbrella margin with nematocyst ring from which centripetal, flat, nematocyst-containing bands run meridionally on exumbrella from umbrella margin to umbrella apex; manubrium large, cruciform base, the four perradial corners of mouth margin without oral tentacles; gonads perradial on manubrium wall. Based on these findings, we describe it as a new species, Tregouboviopsis gemmula. Additionally, we performed a detailed morphological examination of the species Tregouboviopsis perradialis. To date, this species has only been reported from the central and northern regions of the South China Sea. This study provides additional information on the morphology and ecology of these two species of Ptilocodiidae, which constitute two new records for Japanese waters.