Monactinus biwaensis (Negoro) Matsuoka et Kayama was described as a new species in 1954 as Pediastrum biwae Negoro from Lake Biwa in Central Japan. This species is characterized by projections from adjacent marginal cells that either face each other or diverge. However, there was also the view that P. biwae was not an independent species but rather a variety of P. simplex as P. simplex var. biwaense. Subsequently, no other water systems yielded Pediastrum species with this morphology, and P. biwae was long considered endemic to the Lake Biwa–Yodo River system. Measurement of the side ratio of marginal cells in Monactinus simplex collected from Lake Biwa and molecular phylogenetic analysis revealed that M. biwaensis and M. simplex are clearly distinguishable. This result strongly suggests that two morphologically similar species, M. simplex and M. biwaensis occur in Lake Biwa at present. Fossils of M. biwaensis have been found in the Ayama Formation of the Late Pliocene Kobiwako Group and in the Ma0 Member of the Early Pleistocene Osaka Group, leading to the assumption that this species was endemic to Lake Biwa–Yodo River System. However, M. biwaensis was also found in the Middle Pleistocene Karato Formation on Himeshima Island of Oita Prefecture and in the Sahama Mud Member along the coast of Lake Hamana, which are distant from the Lake Biwa–Yodo River System. In addition to this, M. biwaensis has also been observed in surface sediments from Harima Nada and Isa Bay, both located outside the Lake Biwa–Yodo River System. Furthermore, M. biwaensis occurs in other countries outside Japan, primarily in the Southeast Asian region. Taken together, these findings clearly indicate that M. biwaensis is not endemic to the Lake Biwa–Yodo River System.

The ocean covers 70% of the Earth’s surface, and understanding environmental changes in the ocean is directly related to understanding the global environment. Environmental DNA (eDNA) metabarcoding has become a widely used method for monitoring marine biodiversity, complementing traditional visual surveys; each approach has its own strengths and limitations. However, benthic organisms have rarely been targeted, as many eDNA studies—especially in Japan—have focused on fish. In this study, we sampled 5 L of seawater each from the bottom and surface waters at 17 m and 34 m of Ise Bay, central Japan (in total 20 L), and carried out metabarcoding targeting the mitochondrial 16S rRNA gene (brittle star-specific primer set “16SOph1”) for the water samples. Brittle stars, with about 2,100 known species found from shallow coasts to 8,000 m depths worldwide, are highly diverse and abundant benthic animals. Our results showed that the metabarcoding detected five species of brittle stars, compared to two species by direct collection, and that eDNA obtained from surface water contained DNA of species inhabiting the sea bottom, which indicate that (1) metabarcoding of brittle stars can be performed in tens of meters deep, and (2) eDNA of benthic species can be detected even in surface waters. Further research is needed to confirm the extent of vertical eDNA transport.

The periodicity of vulvar opercular decalcification, mating, and oviposition was examined in two sentinel crabs, Macrophthalmus japonicus and M. abbreviatus. For both species, the percentage of decalcified females varied with the semi-monthly tidal cycle of peaks around spring tides. Most females with decalcified opercula oviposited eggs within two days of sampling, and their opercula were recalcified within one day of oviposition in M. japonicus and within half a day in M. abbreviatus. Both surface copulation and underground copulation have been observed in M. japonicus. However, only surface copulation has been observed in M. abbreviatus. Nevertheless, copulations were rarely observed in either species. The ovigerous females of both species fed actively on the surface and spawned the next brood shortly after hatching. These results suggest that opercular decalcification occurs after ovarian maturation and that, likely, the eggs are fertilized by stored sperm in many females. Although the influence of opercular calcification on mating systems remains unclear, short decalcification periods may enable females to avoid multiple matings. Fertilization using stored sperm allows successive oviposition and reduces the risk of predation.

In March 2023 and April 2025, large numbers of Calanus jashnovi were collected in the Kii Channel of the eastern Seto Inland Sea. To distinguish C. jashnovi copepodids from co-occurring closely related Calanus sinicus, which is the dominant large-sized copepod in the Seto Inland Sea, their prosome lengths from the copepodid stage CIV onward were measured. The length distributions of CIV and CV each showed clear bimodal patterns, of which the large and small modes were assumed to be C. jashnovi and C. sinicus, respectively. Adult C. jashnovi were far less abundant than CIV and CV. The appearance of C. jashnovi in the Seto Inland Sea is thought to result from ontogenetic seasonal vertical migration to the surface layer, combined with the occasional intrusion of warm Kuroshio surface water into the Kii Channel. The mean prosome length of C. jashnovi CV in April 2025 was much larger than in March 2023, which is considered to be due to the difference in food abundance and/or quality in the surface water where they grew up from CI to CV. In the Kii Channel, C. jashnovi numerically accounted for 70% and 44% of the Calanus population in March 2023 and April 2025, respectively, which would have contributed as a food source for planktivorous fish.

Antarctodius echinicola, a new species of the odiid amphipod associated with a sea urchin Clypeaster japonicus, is described from Ehime Prefecture in Japan. This species can be distinguished from its congeners by the non-keeled pereonites, the pleonite 3 without posterolateral projection, and the antenna 1 flagellum. Diagnoses of the family Odiidae and the genus Antarctodius are emended and a key to the species of the genus is provided.

We described here a new species of Amphictene (Annelida, Polychaeta, Pectinariidae) from the sandy shore of Kouri-jima, Nakijin, Okinawa, Japan. Amphictene nakijin sp. nov. is characterized by the following characters: an anterior dorsal lobe on segment 2; ventro-median lobes on segments 3–5; anterior conical or rounded lappets on the ventro-lateral lobe of segment 2; lateral ear-like lobes on segment 4; slender dorso-lateral pads on segments 3 and 4; an inflated ventro-lateral lobe on segment 6; 17 segments with notopodial chaetae and 13 segments bearing uncini on neuropodia; neuropodial uncini with 2–3 rows and 10–13 columns of teeth, with both distal and basal teeth small; 9–10 pairs of weakly curved scaphal hooks; and the scaphe that is deeply furrowed posteriorly and with a short anal flap. We also obtained a mitochondrial 16S rRNA gene sequence of this new species to serve as a reference for future studies.

Predator–prey interactions can be divided into pre- and post-capture phases, with the latter involving the processing of prey exhibiting secondary defenses such as chemical toxins, adhesive secretions, or rigid shells. Generalist predators often adjust their handling strategies according to these defenses. Among starfish that perform external digestion, prey-handling behavior has been well documented for bivalve predation. However, little is known about their responses to prey with other defensive traits. The blue bat star, Patiria pectinifera, everts its stomach to digest prey externally, but no study has yet compared its handling behavior among different prey types. Here, we investigated how P. pectinifera handles three prey items with distinct secondary defenses: the limpet Lottia peitaihoensis, the bivalve Mytilisepta virgata, and the crab Hemigrapsus sanguineus. Field surveys revealed frequent predation on shelled invertebrates, and laboratory observations showed that repeated stomach eversion occurred at different frequencies among prey species. The starfish opened bivalves, inverted limpets after detachment, and immobilized crabs through strong adhesion to the substratum. In addition, crabs were observed stealing conspecific carcasses from feeding starfish, indicating kleptoparasitism during digestion. Patiria pectinifera exhibited prey-specific, flexible handling behavior and repeated digestion, suggesting an adaptive mechanism for efficient external digestion. These findings highlight behavioral plasticity in starfish and expand our understanding of predator–prey relationships in marine benthos.