A new species of the caridean genus Alvinocaris, A. marimonte sp. nov., is described and illustrated on the basis of specimens collected from hydrothermal vents at three locations in Izu-Bonin and Mariana Arcs of the north-western Pacific Ocean, Myojin Knoll (1224 m depth), NW Eifuku (1574–1582 m depth) and NW Rota seamounts (525 m depth). It is the 17th species assigned to Alvinocaris, and differs from all other congeneric species in several morphological characteristics derived from the rostrum length and armature, the carapace armature, the shape of the posterior margin of the telson and the armature of the third and fourth pereopods. Results of molecular phylogenetic analyses based on partial sequences of the mitochondrial COI and nuclear ITS-I genes compared between specimens of A. marimonte sp. nov. and other known species of Alvinocaris supported our recognition of the new species, although available data are still limited. Alvinocaris marimonte sp. nov. is represented by two morphotypes, distinguished by the armature of the posterior margin of the telson (spines vs plumose setae), which were not detected as separate species by the molecular analysis. Our discovery of the intraspecific variation in the armature of the posterior margin of the telson led us to reassess diagnostic characters used to distinguish species within the genus Alvinocaris.

The morphology of the zoeal stages IX and XI of the shrimp Thalassocaris lucida is described and illustrated in detail from plankton specimens identified by barcoding of the mitochondrial 16S ribosomal RNA gene (sequence similarities 99.4–99.6%). The present study confirms the larval morphology of Thalassocaris, that shows distinct features within Pandaloidea: (1) carapace broad and dorsoventrally flattened, (2) coxal endite of maxilla with only one lobe, (3) basis of third maxilliped with a distal globose lobe. On the other hand, the funnel-shaped eyes, and the development with long series of larval stages of Thalassocaris indicate affinities with some genera of Pandalidae which corroborates the results of recent phylogenetic analyses in abandoning the family status of Thalassocarididae.

Three specimens of the panopeid crab Acantholobulus pacificus are reported from the subtidal zone of the Port of Yokohama, Tokyo Bay, Japan. The discovery of the species in the bay is the first record of the species from the western North Pacific, as well as from Japan. Its presence there is treated as an introduced species. A description of the Japanese specimens is provided.

Three specimens of the crangonid sand shrimp species Crangon uritai are reported from the muddy intertidal zone of Newport in Port Phillip Bay, Victoria. The discovery of the species in the bay is the first record of the genus Crangon from Australian waters and the first report of the East Asian coastal species Crangon uritai from the southern hemisphere. Its status as an introduced species is suggested and the likely vector for introduction is discussed. A key to the identification of crangonid shrimp species from Port Phillip Bay is included.

The parapagurid hermit crab, Paragiopagurus ventilatus, is reported from hydrothermal vents on the Nikko Seamount, Mariana Trough, at a depth of 410 m. This is the first record of this species outside Taiwan, and represents a major range extension. The animals were found using empty tubes of the siboglinid worm Lamellibrachia satsuma for housing, representing the first example of this carcinoecium for a hermit crab. However, it was observed that some of them changed their housing to gastropod shells during rearing in aquaria. Originally, this species was reported to use gastropod shells for housing. It is considered that the use of siboglinid tubes is due to necessity rather than preference, because empty gastropod shells are very scarce in the collection site.

A new species of the palaemonid shrimp genus Periclimenes, P. cannaphilus, is described from upper bathyal hydrothermal vents of the Bonin-Mariana Arc in the north-western Pacific at depths of 392–456 m. A symbiotic relationship between the new shrimp species and a siboglinid tube worm Lamellibrachia satsuma is suggested by their simultaenuous collection and further observations in situ. Similarities in the morphology and symbiotic association suggest that the new species is closely related to P. thermohydrophilus, also associated with L. satsuma in shallow hydrothermal vent fields in Kagoshima Bay, southern Japan, but differences in the rostral shape, the position of the epigastric tooth on the carapace, and the development of the hepatic tooth on the carapace morphologically differentiate the two species. Phylogenetic analysis based on sequences of the mitochondrial DNA COI gene supports the recognition of two clades corresponding to these two taxa.

Nautilocaris saintlaurentae gen. nov., sp. nov., is described and illustrated from hydrothermal vents in the North Fiji Basin and Lau Basin in the south-western Pacific. This new taxon is assigned to the Alvinocarididae. It has a well-developed, dorsally dentate rostrum, carapace with pterygostomian spine present, eye broadly fused mesially, epipod–setobranch complex present above pereopods, and appendix internae on second to fourth pleopods greatly reduced. This combination of characters places it in an intermediate position between a group of species in the genus Alvinocaris with well developed, dorsally dentate rostrum, carapace with pterygostomian spine present, eyes narrowly fused mesially, epipod–setobranch complex absent and well-developed appendices internae on second to fourth pleopods, and Mirocaris fortunata with rostrum unarmed, carapace with rounded pterygostomian angle, eye broadly fused mesially, epipod–setobranch complex present above pereopods and appendices internae on second to fourth pleopods greatly reduced. The habitat of the new species is briefly described.