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.
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.
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.
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.
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.
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.