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Tylorrhynchus osawai, nereidid polychaete that is considered to support estuarine biodiversity, undergoes a unique epitokous metamorphosis for reproductive swarming. The reproductive swarming of this species has been observed in Japan for more than 100 years; however, the benthic phase of this species has not been well researched. In this study, we aimed to elucidate the morphological allometry and life history, specifically the reproductive swarming, of this species. To accomplish this, surveys were conducted to collect atokes and epitokes from a tidal river in Kyushu, Japan. A total of 1670 T. osawai specimens were collected and preserved in 80% ethanol. The body widths of these specimens were measured, while several intact specimens underwent additional measurements of body length and the number of chaetigers. The morphological allometry of this species differed between atokous and epitokous forms, indicating that its heteronereis form might be suitable for migration from a tidal river to the ocean. The survey results pertaining to the benthic phase of this species indicated that its lifespan was approximately one year or longer. In addition, the settlement, growth, and maturity of this species exhibited a degree of plasticity, potentially influenced by habitat landscapes and sediment conditions. Based on the surveys conducted on reproductive swarming, climate change could have caused the peak of reproductive swarming to shift slightly later than that of several historical observations. Our findings contribute to elucidating the ecological significance of epitokous metamorphosis in nereidid polychaetes and to conserving T. osawai populations. [ABSTRACT FROM AUTHOR]

Knowledge of fish movement ecology contributes to the improvement of fishery resource management actions. We investigated the horizontal movement of the black sea bream Acanthopagrus schlegelii around the natural coast of Nishinoumishima Island in Hiroshima Bay, using active acoustic tracking. Six adult fish equipped with depth transmitters were tracked for a total of 54 days in the bay. Five tagged fish were fully tracked for 9–10 consecutive days after release. Tagged fish were found in the intertidal zone and adjacent subtidal zones, which were shallower than 12 m, and within 170 m off the coastline. The mean daily movement distance and 95% kernel utilized density of tagged fish were 191 ± 272 m and 60,207 ± 99,437 m2, respectively. The swimming depth of the tagged fish was 1–4 m above the seafloor. We opine that the littoral area (< 12 m in depth), which includes the intertidal zone and adjacent subtidal zone, is an important habitat for black sea bream. [ABSTRACT FROM AUTHOR]

Genetic homogeneity among populations of marine organisms with high dispersal potential is maintained by high gene flow whereas genetic differentiation is often formed under the presence of dispersal barriers. One of the possible dispersal barriers for intertidal species in Japan is the Sea of Japan because several tidal-flat species are absent in the region due to the small tidal range. In this study, we aimed to validate this possibility by examining the population genetic structure of the intertidal crab Macrophthalmus japonicus, which possesses a long planktonic larval period indicating high dispersal potential but is absent on the Sea of Japan side in Honshu Island. In total, 83 haplotypes of the mitochondrial cytochrome c oxidase subunit I were recovered from 154 specimens collected from 10 Japanese sampling sites and three GenBank sequences from China and Korea. All populations showed high genetic diversity except for the northernmost population at Asadokoro site in Mutsu Bay. The FST values among populations were low, even between populations with great distances such as the Pacific side of northeastern Honshu and Kyushu Islands (ca. 1500 km). On the other hand, the FST values between Asadokoro site and the other Japan sites were high. The isolation of the northernmost population despite the high dispersal potential of M. japonicus is probably because of the absence of neighboring populations that can supply larvae to the northernmost population and the invasion of larvae from the Pacific side is blocked by ocean currents. [ABSTRACT FROM AUTHOR]

The Japanese pygmy octopus, Octopus parvus, is a small octopus that commonly occurs along the coast of southern Japan; however, the ecological information is limited. This study investigated the annual seasonal occurrence of O. parvus in the intertidal zone of Fukue Island, Nagasaki, Japan, and examined its growth, maturation, and feeding habits. We found 140 individuals in the intertidal zone from August to January, and zero from March to July. During the emergence period, they appear to move from the high to low intertidal zone, and subsequently migrate to the subtidal zone. During low tide at night in the intertidal zone, we observed that the octopuses actively captured and fed on prey, such as fish, crabs, hermit crabs, shrimp, gastropods, and bivalves. The body size of collected individuals increased significantly from summer to winter. Both males and females were found to be characterized by similar body sizes and growth, although gonadal development occurred earlier in males. However, no mating or brooding behaviors were observed during the course of the study. Our observations thus indicate that O. parvus uses intertidal zones as feeding grounds for rapid growth and maturation, and may thereafter move to the subtidal zone for spawning. [ABSTRACT FROM AUTHOR]

Macrophthalmus banzai is an intertidal crab species of Macrophthalmidae inhabiting muddy tidal flats in the northwestern Pacific. A previous study on the population genetic structure of Japanese M . banzai based on the mitochondrial COI gene sequences revealed the presence of two genetically distinguished groups, i.e., the northern group (Honshu, Shikoku, and Kyushu Islands) and the southern group (the Ryukyu Islands). In the present study, we newly determined the COI gene sequences of M . banzai collected from Taiwan and conducted population genetic analyses of these sequences together with Japanese sequences obtained from GenBank to reveal the genetic relationship of this species between Japan and Taiwan. The SAMOVA and pairwise ΦST analysis showed that the Taiwan population is more closely related to the northern group than the southern group. This indicates that the populations of M . banzai are not genetically differentiated by latitude but probably by the pathway of the Kuroshio Current, resulting in the isolation of the population in the Ryukyu Islands. Such a pattern is consistent with the population genetic structure of the fiddler crab Tubuca arcuata shown by a previous study, whereas the pattern differs from those of other intertidal invertebrates. The difference in the larval durations may have influenced the difference in population genetic structures among species. The present study provides a further case of the genetic structure of intertidal species that are not simply regulated by geographic distances.

The recruitment patterns of black-spot tuskfish Choerodon schoenleinii were compared with those of orange-dotted tuskfish (OT) C. ancholago. Individuals at the settlement and juvenile stages were collected using a push net in the intertidal zone and a seine net in seagrass beds. Relative values of standard length to total length (TL) for the two Choerodon species indicated an inflection point at approximately 10-20 mm TL, suggesting that both species complete metamorphosis at a similar size. However, their temporal and spatial occurrence patterns were distinct. Newly settled individuals (postflexion stage) of black-spot tuskfish were collected mainly within foliage of Sargassum plants of the intertidal zone around March-April, while those of OT occurred in both the intertidal zone and seagrass beds mainly in May and June. In seagrass beds, the abundances of both species increased from May or June; however, the TL of black-spot tuskfish was bigger than that of OT. Therefore, black-spot tuskfish progressively migrate to seagrass beds after settling and completing metamorphosis in the intertidal zone, indicating that they utilize two different habitats as nursery grounds. Early spawning and migration from the intertidal zone by this species contributed to the occurrence of relatively large individuals in seagrass beds. [ABSTRACT FROM AUTHOR]

Generally, the gene flow of marine organisms is well maintained, but some local populations of coastal species are genetically differentiated even on a small scale (genetic patchiness). Small-scale isolation can be crucial for understanding genetic diversity within a species. The present study examined the population genetic structure of the sand bubbler crab Scopimera ryukyuensis, which is endemic to the Ryukyu Islands in the northwestern Pacific. A total of 52 haplotypes of mitochondrial cytochrome c oxidase subunit I were recovered from 197 specimens collected from four islands. The haplotype and nucleotide diversities were relatively high in the central Ryukyus (Amami-Oshima and Okinawa Islands) with some exceptions but were low at the southern edge of the geographical distribution of the species, i.e., the southern Ryukyus (Ishigaki and Iriomote Islands). Pairwise F ST analysis suggested that the gene flow of S. ryukyuensis was largely restricted. The local populations of the species are differentiated among islands, except for stations on Ishigaki Island and a station on Iriomote Island. Moreover, a clear intra-island population genetic structure was observed within Amami-Oshima and Iriomote Islands, e.g., only 20 km between stations. Small-scale isolation among local populations may be a common tendency for coastal species in the Ryukyu Islands, considering the results of previous studies on corals.