The first record larvae of the genus Peltogaster (Rhizocephala: Peltogastridae) with eyes and unilobed male aesthetasc.

The complete larval development of Peltogaster lineata Shiino, 1943 (Rhizocephala: Peltogastridae), including five nauplii and one cypris stage, is described and illustrated using SEM. The development took 3.5-4 days at a water temperature of 22-23 °C. Peltogaster lineata has the peltogastrid type of development. Nauplii possess a large and clearly reticulated flotation collar, six pairs of dorsal shield setae with the U-shaped second pair, long segmented frontolateral horns each with two subterminal setae, and a long seta at the antennal basis. The attachment disc in female cyprids has a flap-like extension at the posterior margin. Cyprids of both sexes possess two sensory setae at the attachment disc. The large male aesthetasc is unilobed, the female subterminal aesthetasc terminates into two thin long filamentous processes. Larvae of P. lineata have distinctly visible nauplius eyes.

Ecology determines appearance: a new taxonomic solution for the soft bottom dwelling spider crab Pisoides bidentatus (A. Milne-Edwards, 1873) from the Sea of Japan, with remarks on Pisoides ortmanni (Balss, 1924) and the northwestern Pacific Pugettia Dana, 1851 (Majoidea: Epialtidae).

The northwestern Pacific soft bottom dwelling spider crab, Pisoides bidentatus (A. Milne-Edwards, 1873) (Brachyura: Majoidea: Epialtidae), is re-described using an integrative approach. As a result, it is transferred to the genus Scyra Dana, 1851 (Epialtidae) based on morphological and molecular data, as well as already described larval morphology, which certainly better reflects the phylogenetic relationships of the species. Japanese Pisoides ortmanni (Balss, 1924), for which morphology and ecology are very similar to P. bidentatus as well as Pugettia quadridens (De Haan, 1839) and the closely related Pugettia ferox Ohtsuchi & Kawamura, 2019, are also transferred to Scyra. The "reduced" morphology of both species is very different from the related northwestern Pacific Scyra spp. and Pugettia spp., which are mostly decorating algae-dwellers. We associate the relatively long ambulatory legs, a smooth tear-drop shaped carapace with a reduced dorsal armature and slightly pronounced pseudorostral spines, characteristic of P. bidentatus and P. ortmanni with their open soft (mainly sand) bottom habitats, where it is difficult to find a foothold or shelter, but it is necessary to resist constant currents. Such morphology is also characteristic of spider crabs of the genera Libinia Leach, 1815 and Doclea Leach, 1815 (Epialtidae: Pisinae), which are also living in similar habitats/conditions and represent a convergent morphological appearance.

Larvae of two parasitic barnacles, Parasacculina pilosella (Van Kampen et Boschma, 1925) (Rhizocephala: Polyascidae) and Sacculina pugettiae Shiino, 1943 (Rhizocephala: Sacculinidae) studied by scanning electron microscopy.

The complete larval development of Parasacculina pilosella (Van Kampen et Boschma, 1925) and Sacculina pugettiae Shiino, 1943 including five naupliar stages and one cypris stage is described and illustrated using SEM. P. pilosella and S. pugettiae have a sacculinid type of development. Nauplii possess a naupliar eye, short frontolateral horns with terminal processes, and a ventral process between the furcal rami. Larvae lack a flotation collar, seta 6 on the antennule and a seta on the antennal basis. Cyprids have a nearly straight LO2. Breakage zone and a spinous process are present only in male larvae. Nauplii of the two species differ by the morphology of the furca: in P. pilosella, the furcal rami are longer and not drowned into cuticular sockets. Naupliar antenna of S. pugettiae has a lateral seta on the endopod which is lacking in P. pilosella. Dorsal head shield setae 1 and 2a are present in S. pugettiae nauplii and not found in P. pilosella larvae. In P. pilosella, all dorsal setae have subterminal pores, whereas in S. pugettiae, pores of the setae 2 are shifted proximally. It is possible that the presence/absence of setae 1 and 2a is the distinctive feature of nauplii of the families Sacculinidae and Polyascidae.

Porcellanid crab Pachycheles stevensii parasitized by rhizocephalans Lernaeodiscus spp.: population biology in the northwestern Sea of Japan.

We examined the population and reproductive characteristics of the anomuran crab Pachycheles stevensii Stimpson, 1858 (Decapoda: Porcellanidae) heavily infested by 2 rhizocephalans of the genus Lernaeodiscus in the northern part of the species range (Peter the Great Bay, Sea of Japan) in 2020-2021. The prevalence of infestation reached 32.3%, and L. rybakovi proved to be much more numerous than L. kasyanovi. The sex structure of the P. stevensii population was represented by males (38.8%), females (50.6%), and modified specimens (10.6%), which were extremely feminized males. The latter had a broadened abdomen, smaller chelae, shortened gonopod, and 3 additional pairs of female-type pleopods. Thus, the initial sex ratio in the P. stevensii population approximated the expected 1:1, as in most porcellanid crabs. No significant modifications of the secondary sex characters of the female hosts were observed. The size structures of males and females did not significantly differ. P. stevensii produced 1 clutch yr-1. Oviposition and incubation lasted from summer to early fall; recruitment was noted in the fall. There was a correlation between the reproductive cycles of the host and parasites: rhizocephalans with mature externae were found from June-October. A large number of ovigerous female P. stevensii simultaneously carried parasitic externae of L. rybakovi; fecundity was considerably higher in non-infested females.

Sympatric Two-species Infestation by Rhizocephalan Barnacle Parasites in the Spider Crab Pugettia aff. ferox Ohtsuchi & Kawamura, 2019 from Peter the Great Bay (Northwestern Sea of Japan).

Sympatric two-species infestation by rhizocephalan parasites in the spider crab Pugettia aff. ferox Ohtsuchi & Kawamura, 2019 (Brachyura: Epialtidae) was investigated in the Vostok Bay (Peter the Great Bay, northwestern Sea of Japan). Morphological and molecular analyses showed that this crab was infested simultaneously by Sacculina pugettiae Shiino, 1943 and Parasacculina pilosella (Van Kampen et Boschma, 1925) (Cirripedia: Rhizocephala). Sacculina pugettiae was found in the northwestern Sea of Japan for the first time. The two rhizocephalan species are clearly distinguishable by the morphology of their external cuticles, the shape and position of their receptacles, and the structure of their colleteric glands. Retinacula are present in the mature externae of both species. Molecular analysis showed that these rhizocephalans are unrelated, although both species parasitize Pugettia aff. ferox and are sympatric. Sacculina pilosella should be placed in the genus Parasacculina Høeg & Glenner, 2019, belonging to the family Polyascidae Høeg & Glenner, 2019. The intensity of infestation reached two externae in P. pilosella and three externa in S. pugettiae per host. A simultaneous settlement of two rhizocephalans on the same crab specimen was shown for the first time. The intensity of the two-species multiple infestations reached four externae per host. Externae with developing embryos occurred from June to September in P. pilosella and July to September in S. pugettiae, at water temperatures of 15-24°C, indicating that the reproductive periods of these species are confined to the summer months in the investigated locality.

The parasitic barnacle Peltogaster reticulata Shiino, 1943 (Rhizocephala,Peltogastridae) from Russian waters of the Sea of Japan: morphological description, molecular identification and complete larval development.

A rhizocephalan parasite of the hermit crab Pagurus minutus Hess, 1865 (Decapoda, Paguridae) is described from Russian waters using morphological and molecular methods. Partial sequences of the mitochondrial cytochrome c oxidase subunit I gene (COI) were used as a genetic marker. We refer this species to Peltogaster reticulata Shiino, 1943 (Rhizocephala, Peltogastridae) based on its morphological description, since genetic data for the species from its type locality are lacking. The species can be distinguished from its known congeners by the reticular pattern of the external cuticle. Peltogaster reticulata is the sister taxon to P. postica Yoshida Osawa, 2011 (in Yoshida et al. 2011), inhabiting the same host in southwestern Japan. The complete larval development of Peltogaster reticulata, including five naupliar and one cypris stage, is described and illustrated using SEM.

First stage larva of the mud shrimp Nihonotrypaea makarovi Marin, 2013 (Decapoda: Axiidea: Callianassidae) obtained in the laboratory.

First stage larva of a new mud shrimp Nihonotrypaea makarovi Marin, 2013 (Decapoda: Axiidea: Callianassidae) obtained from the ovigerous female is described and illustrated for the first time. The first zoea of N. makarovi is well distinguished from the first larvae of N. japonica and N. petalura, sympatric callianassid species inhabiting Russian waters of the Sea of Japan, only by the greater size and by the presence of two terminal setae on the antennal endopod.

The number of zoeal stages in larval development of Nihonotrypaea petalura (Stimpson, 1860) (Decapoda: Axiidea: Callianassidae) from Russian waters of the Sea of Japan.

Zoeal stages of the mud shrimp Nihonotrypaea petalura (Stimpson, 1860) (Decapoda: Axiiidea: Callianassidae) inhabiting Peter the Great Bay (Sea of Japan) is described and illustrated from the larvae reared in the laboratory individually. The development included seven zoeal stages before molting to the megalopa. At 22-23°C the first megalopa was attained 26 days after hatching. A range of the number of zoeal stages in the family Callianassidae is analyzed. The possibility of variation of the number of zoeal stages of burrowing shrimps in different populations of the same species and/or under different laboratory conditions is discussed.