Two individuals of “monster larvae” belonging to the genus Cerataspis (Decapoda: Penaeoidea: Aristeidae) were collected in the western North Pacific in 2016. The smaller one (carapace length 7.3 mm) was collected by daytime plankton net operation towed at a shallow layer (25 m to the surface), and the larger one (carapace length 11.5 mm) was collected by a nighttime plankton net operation towed from 208 m to the surface. These larvae have a pair of large spines on the lateral surface of the carapace, and in this regard they agree with C. petiti Guérin-Méneville, 1844 reported from the Atlantic. However, the mitochondrial 16S rDNA sequences of these two individuals matched with those of C. monstrosus Gray, 1828, the senior synonym of the widely used name Plesiopenaeus armatus (Spence Bate, 1881), suggesting that the presence and absence of the spine might be intra-specific morphological plasticity or variations among different developmental stages.

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We investigated genetic population structure and tetrodotoxin (TTX) content of yellowfin toxic goby Yongeichthys criniger in the Japanese coastal area. Two divergent mitochondrial lineages (lineages A and B) were found, and the frequencies were clearly different among Okinawa-Iriomote Island, Amami Oshima Island and northward of Yakushima Island. Genetic heterogeneity among three groups were supported by principal coordinate analysis based on the pairwise FST values. TTX contents were measured in goby samples from Kochi Prefecture (Kashiratsudoi River) and Yakushima Island (Isso Port and Anbo River). TTX contents of skin and muscle tissues of Kashiratsudoi River and Isso Port samples were relatively high, while TTX was not detected in most individuals collected in the Anbo River. Therefore, it is suggested that the TTX contents of this species considerably vary even among geographically close areas and/or seasonally.

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Genetically distinct three types (designated as A, B and C) in the common freshwater shrimp Palaemon paucidens in Japan have been determined using allozyme, 18S rDNA and 16S rDNA analyses. Based on the morphological and genetic analyses, Katogi et al. (2019) proposed type B population distributing in the northern Japan to be a new species Palaemon septemtrionalis. In the present study, phylogenetic analysis in amphidromous type B was performed using 16S rDNA sequences of 185 individuals collected throughout the distributional range of this species in Japan. Two highly divergent clades (designated as B-I and B-II) were observed in the phylogenetic tree, in which the mean nucleotide sequence divergence (K2P) between the clades was 3.9 %. B-II clade was further subdivided into subclades B-IIa and B-IIb, in which the mean K2P between the subclades was 1 %. Geographic distributions of these clades were clearly separated. B-I corresponding to P. septemtrionalis was distributed in the northern Japan (north of Miyagi Prefecture to Hokkaido) and rivers on the Sea of Japan side. B-IIa and B-IIb were observed in rivers on the Pacific Ocean side. B-IIa was distributed from the northern Japan (north of Miyagi Prefecture) to central Japan (Kanagawa Prefecture) and B-IIb from central Japan (Shizuoka Prefecture) to southern Japan (Kyushu). Northern area of Miyagi Prefecture was determined to be the boundary zone between B-I and B-IIa, where B-I and B-IIa individuals were observed to coexist in a river of the Oshika Peninsula. Sagami Bay at central Japan was considered to be the boundary between B-IIa and B-IIb. In the appendage morphology, longer propodus than carpus in the second pereiopod, that was reported as one of specific characteristics of P. septemtrionalis, but we found this to be size dependent characteristic and not to be applicable for species identification.

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We compared the feeding patterns of marbled flounder Pseudopleuronectes yokohamae in Tokyo Bay between 1980s (high abundance) and 2000s (low abundance). The stomach contents weight was significantly lower in 2000s compared to that in 1980s. The dietary composition changed substantially between 1980s and 2000s; main constituents of the stomach contents were annelids, molluscs and echinoderms during 1980s. In 2000s, however, they fed predominantly on annelids. Observed changes in the stomach contents may be associated with changes in abundance and species composition of macrobenthic community, and could have affected changes in the growth and reproductive patterns between 1980s and 2000s.

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The inachid genus Platymaia includes 4 species in Japanese coasts, but their larval development has been poorly known because of their deep-sea habitat. In the genus, the first zoeal stage was described in P. alcocki in Japan, however, some morphological characters, from the recent viewpoint, are insufficiently described. We re-describe the first zoea of P. alcocki in detail from laboratory-hatched materials, and compared with previous description of P. wyvillethomsoni. The zoeas of Platymaia species are very similar to each other, it can be distinguished by lengths of the rostral and dorsal carapace spines. Additionally, among the first zoeas of 6 inachid genera, Platymaia is closely related to the genus Cyrtomaia especially in the endopodal setations of the maxillule and the second maxilliped.

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Mid- to late stage phyllosoma larvae, assigned to 7th and 8th stages, collected south of the Ogasawara Islands, Japan, were determined to be Panulirus brunneiflagellum Sekiguchi & George, 2005 by DNA barcoding, and morphological description was given for the first time. Phylogenetic analysis based on the mtDNA COI sequences indicated that the most closely related species to P. brunneiflagellum was P. marginatus, and hence P. brunneiflagellum was indicated to belong to ‘Panulirus longipes group’. The larval morphology also indicated P. brunneiflagellum to be a member of the same group, in which the ratio of carapace shield width to thoracic width was larger than that of P. japonicus but similar to those of the other species of ‘Panulirus longipes group’ previously reported.

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Two Japanese codling Physiculus japonicus Hilgendorf, 1879 and P. maximowiczi (Herzenstein, 1896) have been suspected to be conspecific. Many Japanese literatures have described phenotypic differences between these two morphs; P. japonicus possessing larger eye (eye diameter/snout length larger than 2/3) and pale brown body is distributed deep seafloor (150‒650m) in south of Tokyo Bay, while P. maximowiczi possessing smaller eye (eye diameter/snout length less than 2/3) and dark brown body is distributed shallow seafloor (≤ several tens of meters) in south of Hokkaido. We collected 44 Physiculus individuals (13 from shallow area and 31 from deep area) from northern to central Japan and performed nucleotide sequence analysis on the mitochondrial COI, 16S rRNA and Dloop regions. Body color largely varied, but dark brown individuals were abundant in the shallow group and pale brown individuals were abundant in the deep group. Ratio of eye diameter to snout length also largely varied, but the average was larger in the deep group than in the shallow group. In contrast, genetic difference among individuals and between shallow and deep groups was very small, and phylogenetic analysis indicated all individuals analyzed to be conspecific. These indicate that P. maximowiczi is a junior synonym of P. japonicus and phenotypic differences are intraspecific variation probably in response to the environmental diversity.

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Abundance of the invasive and predatory moonsnail Laguncula pulchella was investigated at 10 tidal flats along the coast of Hiroshima, western Japan. They were found from two tidal flats where this species has already been known to occur. The size and density of egg collars of L. pulchella became smaller from 2015 to 2017 but thereafter recovered in 2018. This tendency was common between the two sites, indicating that the population and body size of adult L. pulchella are highly variable among years.

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Recent molecular analyses revealed Palaemon paucidens De Haan, 1844 to be species complex, since three genetically distinct types (designated by types A, B and C) were detected. A mitochondrial 16S rDNA sequence of the lectotype specimen collected by P. F. von Siebold nearly two centuries ago in Japan and held in Naturalis Biodiversity Center (formerly Rijksmuseum van Natuurlijke Historie), Leiden, The Netherlands, was successfully determined to be haplotype JA1 of type A. JA1 is the most predominant haplotype mainly observed from central to southern part of Japan and would be the most accessible type to Von Siebold whose activity range was limited around Nagasaki.

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Four specimens of the freshwater palaemonid prawn, three Macrobrachium australe and one M. lar, were collected in the Koonoura River, Nagasaki, Japan. The former is the first record in Nagasaki Prefecture. Occurrence of these species was thought to be due to larval dispersal by branched flow from the Kuroshio Current. However, these two species are unlikely to overwinter, because the samples are only subadults and the water temperature of this river drops below 10 °C in winter.

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