Animal shape has become sophisticated through evolution or phenotypic plasticity to adapt and survive changing environmental conditions. Sexual differences, which are sex-specific characteristics, form an important research topic that has also long fascinated biologists. Morphometrics is a traditional approach that enables the quantitative description and interpretation of variation in shape based on morphological features. In this study, we attempted to apply two morphometrics approaches (landmark-based or measurement-based) to identify sexual dimorphic traits in the carapace of the horsehair crab, Erimacrus isenbeckii (Brandt, 1848). Both approaches revealed that its carapace showed apparent sexual dimorphism, enabling the discrimination of the male and female carapace with more than 90 % accuracy. Moreover, the landmark-based approach found that carapace shape showed fluctuating asymmetry as well as directional asymmetry in both males and females. Fluctuating asymmetry consists of small random deviations from perfect bilateral symmetry and may be tightly linked to environmental changes. Our data suggest that responsiveness to environmental stressors during carapace formation might be different in female and male horsehair crabs. This study indicates that landmark-based geometric and traditional morphometrics can serve as a strong tool to distinguish female from male horsehair crabs using carapace shape. This is the first report to detail the use of morphological features to show sexual differences in horsehair crab. Morphometric parameters obtained from this study will be a useful reference dataset that can be applied for geographic comparisons of carapace shape of horsehair crab.

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Kunimasu, Oncorhynchus kawamurae, spawning is poorly understood. At the only identified spawning site in Lake Saiko, Japan, we observed a spawning aggregation on a gravel area at ~30 m depth. Spring water is crucial for salmonid spawning; however, spring water discharge is limited to a few positions in Lake Saiko. The magnitude of spring water discharge can be extrapolated from the difference between the underground and lake-bottom-water temperatures. Here, to assess the effects of spring water discharge, eyed eggs were buried at two positions with different spring water discharge magnitudes. Because O. kawamurae eyed eggs were not available, those of the closely related Himemasu, Oncorhynchus nerka, were used. We found that eggs buried in gravel with little spring water discharge did not survive at all, whereas eggs buried in gravel with high spring water discharge showed high survival rates. Given these results, maintenance and management of spring water is essential for O. kawamurae conservation.

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Kunimasu, Oncorhynchus kawamurae, spawning and its inhibiting factors were investigated in Lake Saiko, central Japan, by direct observation while scuba diving and recording images using underwater time-lapse cameras. The cameras were installed on three stations in gravel areas with high or low spring water discharge at a depth of c.a. 30 m from November 2016 to February 2017. Kunimasu were observed to visit these areas, and digging behavior was observed. It was inferred that the number of fish visiting the gravel areas was related to the size of the gravel areas rather than the magnitude of spring water discharge. The underwater cameras confirmed the presence of freshwater eels, which were actively digging gravel and eating Kunimasu eggs. Two eels captured were identified to be European eel, Anguilla anguilla, by mitochondrial DNA sequence analysis. Since we found only a few Kunimasu eggs on the spawning ground, the egg-eaters appear to have had a substantial negative impact on the reproduction of Kunimasu. These results indicate that the maintenance of gravel areas and spring water, as well as the expulsion of the European eel, are vital for the conservation of the Kunimasu population in Lake Saiko.

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Nymphonella tapetis is a sea spider showing a larval endoparasitism on Manila clam (Asari) and some other bivalves. A large-scaled outbreak of the species occurred in Tokyo Bay in 2007 and caused serious damage to the Asari fishery. The species has been recorded afterward in several places in Japan and seemed to be expanding its distribution. In the present study, we conducted a population genetic analysis of N. tapetis using mtDNA COI sequences from the populations of five localities in Japan: Tokyo Bay (Chiba Prefecture), Mikawa Bay (Aichi Pref.), Matsukawaura (Fukushima Pref.), Tateyama (Chiba Pref.), and Misaki (Kanagawa Pref.). Phylogenetic tree from a partial COI sequence (562 bp) robustly supported a monophyly of N. tapetis of these localities. Parsimony haplotype network analysis showed the two genetic lineages among these populations: the Tokyo Bay (Tokyo Bay, Mikawa Bay and Matsukawaura) and the Tateyama (Tateyama and Misaki) lineages. The lower average nucleotide sequence divergence of the Tokyo Bay lineage suggests its younger history than another lineage. As the localities of the Tokyo Bay lineage are remotely separated each other and the N. tapetis larvae lack the planktonic stages, they are considered sharing a common source such as a recent commercial circulation of Manila clam. Although Tateyama and Misaki are geographically much closer to Tokyo Bay compared to Mikawa Bay and Matsukawaura, the migration between the Tateyama + Misaki population and the Tokyo Bay population seems to be an unlikely event.

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Genetic diversity of common freshwater shrimp Palaemon paucidens in the Kanto region (Tokyo, Kanagawa and Chiba) of Japan, was investigated using mitochondrial 16S rDNA sequence analysis. All 98 individuals collected at 31 localities were determined to be type A, which included 52 individuals collected at 18 localities in Inner Tokyo Bay Basin. A total of 12 haplotypes were detected, which were phylogenetically classified into 3 groups (designated by A-I, A-II and A-III). Number of individuals comprising each group was 44 for A-I, 38 for A-II and 16 for A-III. A-II was determined to be an indigenous group distributing central to northern Japan. A part of A-I group may be domestic alien probably originated from Lake Biwa. A-III was determined to be invasive group probably originated from commercially imported individuals from Korea and/or China. Although the human-mediated contact between indigenous and alien groups must threat the genetic integrity of indigenous group, nothing is known about hybridization and genetic introgression among these groups.

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Temperature is an important environmental factor affecting the larval survival, development and duration of decapod crustaceans. We investigated the effect of temperature on larval performance in six terrestrial hermit crab species (genera Birgus and Coenobita): B. latro, C. brevimanus, C. cavipes, C. purpureus, C. rugosus, and C. violascens. Larvae were cultured at six temperature levels between ~19 and 34 °C. Larvae could not survive to the megalopal stage at temperatures below 20 °C. Megalopae appeared at temperatures above ~22–23 °C, and survival rates to the megalopal stage were high at ~25–31 °C and tended to decline over ~32 °C. Intra- and interspecific variations were found in the developmental pathways to the megalopal stage: 4 zoeal stages in B. latro, 4–8 zoeal stages in C. cavipes and 4 or 5 zoeal stages in the other Coenobita species. The proportions of larvae that completed the shorter developmental pathways tended to increase with increasing temperature in each species. Total zoeal duration tended to decrease with increasing temperature in all species, and C. cavipes, with prolonged developmental pathways, exhibited the longest developmental periods to the megalopal stage. The lower threshold temperature for larval development was estimated to be 16.4–19.3 °C based on the heat summation theory equation. Thus, larvae of coenobitid crabs are not able to develop and survive at low-temperature conditions below 20 °C, exhibiting typical thermal adaptation of subtropical and tropical species.

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A total of 73 small juveniles (46.0-115.2 mm folk length) of the genus Beryx were caught by a large mid-water trawl performed in the West Mariana Ridge, western North Pacific in 2013. Mitochondrial DNA sequence (COI gene) analysis performed on 66 individuals revealed that all individuals were Beryx mollis. Oval posterior nostril, a characteristic of B. mollis, was confirmed to be valid in these small juveniles. These juveniles appeared to be captured in epipelagic zone. This is the first report that many small juveniles of B. mollis were caught.

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Genetic diversity and population structure of the mudskipper, Periophthalmus modestus, were investigated using mitochondrial control region (D-loop) sequence analysis. Sixty-eight haplotypes were detected in 196 individuals collected at 19 localities [Seto Inland Sea (n=11), Pacific coast (n=4: Mie Prefecture, Ehime Prefecture, Kochi Prefecture and Chiba Prefecture), Japan Sea coast (n=1), Ariake Sea (n=1) and Okinawa Prefecture (n=2: Haneji flats and Sashiki flats)], and an average haplotype diversity of h=0.923 was obtained. Analysis of molecular variance (AMOVA) indicated that Edo R., Ibi R., Souzu R., Arita R., Kashima R., Haneji F., and Sashiki F. populations were isolated from Seto Inland Sea populations. No significant genetic differentiation was observed among Seto Inland Sea populations. These results suggest that the regional populations in Seto Inland Sea have a metapopulation structure.

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