Sexual differences in morphology and behavior are common. Although males typically have larger weapons and are more aggressive because of sexual selection, such aggressive traits can benefit both sexes in year-round non-sexual resource competition. These benefits might affect female trait expression and cause sexual differences in seasonal trait changes, with male traits peaking in the reproductive seasons and female traits remaining stable or even diminishing. Here, sexual differences in major cheliped size and use in the hermit crab Pagurus filholi were examined across reproductive and non-reproductive seasons. Individuals were collected from April to July 2018 (reproductive season), and from October 2018 to January 2019 (non-reproductive) on an intertidal rocky shore in southern Hokkaido, Japan (41°44′N, 140°36′E). The female major cheliped was smaller in the reproductive season than in the non-reproductive season, with the smallest in ovigerous females; their cheliped use did not show seasonal change. In contrast, males possessed a larger major cheliped, and used it more, in the reproductive season than in the non-reproductive season. Whereas females mainly used their cheliped defensively and more often against female opponents, males used it in both defensive and offensive behavior regardless of the opponent’s sex. These results suggest that the function of aggressive traits in each sex can be shaped by different selective pressures, such as reproductive investment in females versus mating success in males.

Two new deep-sea myzostomid species, Pulvinomyzostomum unicorne sp. nov. and Eenymeenymyzostoma abyssale sp. nov., are described from the Northwest Pacific off Japan. Pulvinomyzostomum unicorne sp. nov. was collected from the crinoid host Pentametrocrinus varians at a depth of 1506 m near Shima Spur, Japan, NW Pacific Ocean, characterized by its distinctive scalloped body margin with 20 equal-sized conical cirri, lack of dorsal ridges or color patterns, a smooth proboscis, and a single cirrus extending from the base of the parapodia. Eenymeenymyzostoma abyssale sp. nov., the deepest record for the genus, was found at 511 m depth on the black coral Leiopathes cf. annosa at the Nishi-Shichito Ridge, Japan, NW Pacific Ocean. This species exhibits an oval, thick body without marginal cirri, transparent and orange coloration internally when alive, and uniquely lacks parapodial cirri. Molecular phylogenetic analyses based on COI, 16S, and 18S sequences strongly supported the placement of these new species within their respective genera.

Two gnathiid species were collected from around Kagawa Prefecture, Seto Inland Sea, western Japan. One species was described as Caecognathia tubicola sp. nov. with detailed morphological examinations of male and female adults and larvae. A total of 130 individuals of C. tubicola were obtained from empty tubes of the polychaetous annelid Spirobranchus akitsushima in the intertidal zone. This species probably completes its life cycle around the aggregation of empty calcareous tubes of S. akitsushima on bedrock, as larvae at all developmental stages were found. Some adult females have significantly narrow thoraxes, a morphology that may be adapted to the elongate tubes. Small numbers of Elaphognathia cornigera (Nunomura, 1992) were also found in empty S. akitsushima tubes.

This study examined the reproductive strategies of four Pleuromamma species (P. abdominalis, P. gracilis, P. xiphias and P. indica) in Sagami Bay, Japan. Key reproductive traits, including egg production rates (EPR), and the relationship between body size and egg size, were analyzed to uncover interspecific variability and adaptive strategies. The present result was considered the first species-level report on the egg production rates (EPR) of the genus. Average EPRs were almost the same among the species, i.e., around 20 eggs female⁻¹ d⁻¹. Comparisons of the female body, egg and clutch carbon contents suggest variations in reproductive patterns across species. Among the four species, P. gracilis, the smallest and distributed near the surface of the food-rich environment, produced both the highest egg carbon (1.88% of body carbon) and clutch carbon (43%). Its high egg carbon content corresponded to those of egg-brooding species. Our result implies that the small body size of P. gracilis reduces predation pressure, allowing it to perform diel vertical migration (DVM) over shorter distances near the surface and to invest energy obtained from feeding into its reproduction.

The boring sea urchin of the genus Echinostrephus (Echinometridae, Camarodonta, Echinoidea, Echinodermata) is composed of two species, namely, Echinostrephus aciculatus A. Agassiz, 1863, and Echinostrephus molaris (de Blainville, 1825). In Japan, E. aciculatus is distributed from Tateyama, Chiba Prefecture, to the Osumi Islands, Kagoshima Prefecture, whereas E. molaris is distributed from the Tokara Islands, Kagoshima Prefecture, and southwards. They can be distinguished by the color of spines and by the number of pore pairs in each row on ambulacral plates. Echinostrephus aciculatus has four pore pairs in each row on the aboral side, whereas E. molaris has only three pore pairs in each row across all the ambulacral plates. However, these spines are fragile, and they often break, making them an unreliable identification trait. Pore pairs also often show individual variation. Thus, re-examining these species based on molecular data is necessary. In this study, Echinostrephus spp. were collected from tropical waters (Okinawa and Ogasawara Islands) and subtropical waters (Shirahama, Wakayama Prefecture) within the Kuroshio Current basin, and molecular and morphological comparisons were conducted. Molecular phylogenetic analysis based on mitochondrial COI sequences showed that E. aciculatus and E. molaris formed a distinct clade, and E. molaris collected from Japan showed no regional variations, making Shirahama a northernmost record. Spine color and the number of pore pairs, especially at the first to third ambulacral plates from the aboral side, were confirmed to be valid taxonomic traits that clearly distinguish each species clade.

The genus Peronella Gray, 1855 is a widespread group of sand dollars native to the Indo-Pacific region. However, compared to adjacent areas, their presence in Taiwan is extremely limited, with only two recorded species, including one only represented by a single denuded test. Here, we report new Peronella specimens from northern Taiwan. Based on petal outline, lateral outline, apical system, periproct position, and oral plating pattern, these specimens are identified as Peronella pellucida Döderlein, 1885. Their morphological variations during growth as well as individual differences are described. We found notable intraspecific variations in the gonopore size and periproct, suggesting that some previous diagnostic features for this genus may not be reliable. In contrast, hydropore distribution and periproct position emerge as the most reliable diagnosis. Additioanlly, except for the juvenile and deformed specimens, all other specimens exhibit four gonopores, consistent with the previous key diagnosis of Peronella. Our neighbor-joining tree based on mitochondrial COI sequences of four available Peronella species reveals strong monophyletic supports for each species and that P. pellucida is phylogenetically closer to P. rubra Döderlein, 1885. Our morphological measurements, combined with molecular evidence, provide valuable data for a better understanding of this underexplored group.

The calanoid copepod Calanus sinicus is the biomass-dominant zooplankton in temperate coastal waters of the Northwest Pacific from spring to early summer, and is an important prey for planktivorous fish such as sand lance. To estimate population biomass of postnaupliar C. sinicus, the temperature and body length regression equation was obtained for each instar stage based on prosome lengths of more than 10 individuals in five months at different temperatures (January, March, May, June, and July) between 2018 and 2021. Using the present equations and the known prosome-length and carbon-weight regression equation of C. sinicus, its seasonal population biomass could be estimated from temperature and stage-specific numerical abundance more accurately and easily than before.

The freshwater microalga Haematococcus lacustris, the most prolific natural producer of astaxanthin, has potential as a live microalgal diet for aquaculture. However, its low salinity tolerance restricts its use in marine aquaculture. To apply H. lacustris as live microalgal diet in marine aquaculture, this study examined the synergistic effect of nitrogen starvation and high light through four-shift salinity acclimation (FS) on the survival and astaxanthin accumulation in seawater. The living cell densities of cultures subjected to the FS were comparable between sufficient nitrogen and low light (LL + N; the control) and nitrogen starvation and high light (HL−N), reaching 0.50 × 10⁵ and 0.58 × 10⁵ cells mL⁻¹, respectively. Rapid formation of red cells with stable survival was observed when cells were cultured in HL−N. HL−N achieved an astaxanthin yield of 5.50 mg L⁻¹, a 7.9-fold increase over the control (0.70 mg L⁻¹). Salinity acclimation maintained the survival, while nitrogen starvation and high light enhanced astaxanthin accumulation. The synergistic effect of salinity acclimation, nitrogen starvation and high light promoted the rapid formation of red cells, which enhanced tolerance to salinity stress (such as seawater salinity) and boosted the astaxanthin accumulation in H. lacustris.