The raninid genus Cyrtorhina Monod, 1956, is revised and restricted to Cyrtorhina granulosa Monod, 1956, from West Africa. A new genus, Flaberhina, is established for C. balabacensis Serène, 1971 from the Philippines and New Caledonia. Despite their superficial similarities, the two genera can easily be separated by the structures of the carapace, eye, antenna, pterygostome, thoracic sternum, pleon and penis. The taxonomy of the subfamily Cyrtorhininae Guinot & Ng, 2020, which contains both fossil and extant taxa, is also reviewed.

We conducted laboratory experiments to evaluate the dietary effects of phytoplankton and zooplankton on larval survival, duration and growth of four amphidromous atyid shrimps, Caridina leucosticta, C. multidentata, C. serratirostris, and C. typus. Larvae were reared by being fed commercially preserved or cultured phytoplankton Tetraselmis sp. and cultured zooplankton rotifers. Larvae survived to juveniles by being fed cultured phytoplankton and rotifers; however, C. serratirostris larvae died due to being trapped to the surface tension of the water during moulting. Rotifers significantly improved larval survival and development, particularly in C. multidentata and C. typus. C. leucosticta larvae moulted to juveniles with high survival rates by being fed only cultured phytoplankton, suggesting that they may develop in eutrophic brackish environments with substantial phytoplankton biomass. C. multidentata larvae had prolonged larval durations under limited nutritional conditions, suggesting their ability to broadly disperse in the sea. C. typus had a longer larval duration than C. leucosticta. Thus, Caridina species might exhibit different larval dispersal strategies in the sea. Larval duration varied, but body size did not vary among the species under the different feeding conditions, indicating that larvae moulted to juveniles after reaching the threshold size of each species needed to migrate to their adult freshwater habitat.

An ovigerous female of the cymothoid isopod Nerocila trichiura (Miers, 1877) was collected from the isthmus of Cheilopogon doederleinii (Steindachner, 1887) (Beloniformes: Exocoetidae), in Kowaura Bay, Mie Prefecture, central Japan. This represents the third record of N. trichiura from the western Pacific Ocean, and C. doederleinii is a new host of the isopod. A skin wound caused by the isopod was found at attachment site.

Two species of bopyroid isopods are first reported on crabs collected from rocky beaches of Yoron Island, middle part of Ryukyu Islands, Japan. A bopyrid, Kepon grapsi (Nobili, 1905) nov. comb., is recorded based on both sexes obtained from the branchial chambers of Grapsus tenuicrustatus (Herbst) (Grapsidae); the genus Lobocepon Nobili, 1905 is synonymized with Kepon Duvernoy, 1840. The finding of K. grapsi nov. comb. in the present study represents the first record from Japan. An entoniscid, Allocancrion yunnu gen. et sp. nov., is described on the basis of the adult females, males, and epircaridium larvae from visceral cavities of Plagusia squamosa (Herbst) (Plagusiidae). Allocancrion gen. nov. is similar to Cancrion but it is differentiated from it by the male lacking pereopod 6 and the epicaridium larva with a very short, unmodified, pereopod 6.

Pleonobopyrus kumanonadensis gen. et sp. nov., a parasite of the crangonid shrimp, Prionocrangon dofleini Balss, 1913, is described on the basis of a female specimen collected from Mie Prefecture, off Owase, Pacific coast of central Japan. The specimen was attached to the ventral surface of the abdomen of the host shrimp. The species of the bopyrid subfamily Hemiarthrinae Markham, 1972, are known to occur on the abdomens of caridean shrimps, but morphological characteristics of the new species did not match those of Hemiarthrinae nor any other subfamilies. The attachment posture is backwards in contrast to that found in species of Hemiarthrinae. This new genus and species is the first abdominal bopyrid found on species of Prionocrangon.

The Neptune lobster, Metanephrops neptunus (Bruce, 1965), is reported from 300–600 m depths off the Pratas Islands southwest of Taiwan in the South China Sea. Seven males and three ovigerous females were captured in July 2019. Key morphological features and internal structures are described, with particular attention to the chelae, mandibles, gills, cephalothoracic and abdominal sculpturing, and gastric mills. The red-and-white coloration is described and illustrated from fresh specimens. The geographic range, depth of occurrence, stomach contents, and fecundity of the species are also noted.

The alpheid shrimp Salmoneus tafaongae Banner & Banner, 1966 was described based on a fragmentary holotype from Samoa, which was subsequently lost in the devastating fire of the Hawaii Marine Laboratory in 1961. The species is here fully redescribed based on new material from French Polynesia, Australia (Great Barrier Reef) and Solomon Islands, and compared to closely related species.

Mating pairs of the potamid crab Geothelphusa dehaani were observed in a mountain stream of Fukuoka Prefecture, Japan, during the survey of over ca. 3-year period. Five mating pairs in copulatory posture were found in the afternoon in October and November. Mating pairs remained immobile for at least several minutes. No guarding behavior was observed after the release of copulation. They were all found on the bottom substrate under the water in pools. Females were either in the soft-shell or hard-shell condition, and even included one juvenile among the adults.

The aim of this study was to examine patterns of abundance, density, size, and shell use in Coenobita brevimanus and C. cavipes, occurring on Sakiyama Village, Iriomotejima Island, southwestern Japan. Crabs were captured and their body size and the type of adopted gastropod shell were recorded. Relative abundances were similar among species (67 and 64 for C. brevimanus and C. cavipes, respectively) and density of C. brevimanus was as high as 0.23 m⁻². Overall, C. brevimanus had larger body sizes (palm width 5.9–32.7 mm) than C. cavipes (4.2–14.2 mm). Patterns of gastropod shell use were also significantly different between species. While C. brevimanus used were the marine snails Turbo (Marmarostoma) argyrostomus and Mancinella armigera (each species comprised about 35% of all occupied shell), the most adopted gastropod shells by C. cavipes were the land snails Satsuma caliginosa caliginosa (38%) and Acusta despecta sieboldiana (19%). Interestingly, almost all (>90%) shells occupied by C. brevimanus were abraded, whereas for C. cavipes 56% of adopted shells were non-abraded. Generally speaking, C. brevimanus in Sakiyama, where the supply of artificial shells is relatively more limited than in more populated areas near to the village, had smaller body sizes, and occupied older shells.

The fiddler crab, Austruca lactea inhabits tidal flat areas and is widely distributed across Japan, the Korean Peninsula, China, Taiwan and northern Vietnam. Fiddler crab habitat is being lost rapidly due to human impacts and this species is currently listed as endangered in Japan. We studied the population genetic structure of A. lactea using mitochondrial DNA control region markers to define management units. A. lactea individuals were sampled from Osaka, Hiroshima and Kumamoto in Japan, and from Taichung and the Penghu Islands in Taiwan. All local samples exhibited high levels of genetic diversity, and no sampled populations showed evidence for a significant decline in effective population size that can result from population bottleneck effects. Pairwise F_ST estimates distinguished three discrete A. lactea populations corresponding with, the Seto Inland Sea (Osaka and Hiroshima), Kyushu (Kumamoto) and Taiwan. The three populations showed clear differences in historical population expansion times and their population dynamics after expansion. Results of the study indicate that A. lactea dispersal is limited geographically and that high levels of genetic diversity are maintained both within and among populations.

The body color of the soldier crab, Mictyris guinotae, varies from a pale blue to a dark ink-blue. We describe the composition of black and blue body color forms in animals obtained during a year-long collection effort. Body color showed clear seasonal changes; almost all crabs were black between April and September (non-breeding season) and blue between late November and February (breeding season). Larger black crabs, >7 mm in carapace length, first showed blue coloration in October, suggesting that color change is associated with sexual maturity. Some newly molted black individuals with white exuviae were found inside the burrow in March. The timing of the color change from blue to black was later in females because ovigerous females are unable to molt. Black body color during summer months may mitigate the harmful effects of ultraviolet radiation and provide visual crypsis from predators during activity on the surface of sand flats.

We investigated the reproductive traits and population structure of a Petrolisthes japonicus population through monthly sampling for one year on an intertidal cobble and boulder shore on the Boso Peninsula, Japan. The main reproductive season lasted for four months, from June to September. The sex ratios were approximately 0.5. Newly recruited juveniles were found mainly from August to October, and three age groups were detected using body size histograms. The growth curves of both sexes were significantly different, and males grew larger than females. Our analyses suggest that females and males in the 0+age group begin to breed in the year after hatching and that a relatively large proportion of 0+aged individuals die during the end of or after the reproductive season. A relatively small proportion of 0+aged individuals survive even after the reproductive season and breed in the next year as the 1+age group; then, individuals that reach the 2+age group likely die by the end of summer. Consequently, the longevity of P. japonicus is estimated to be 1–2 years.

Metapenaeopsis provocatoria owstoni (Decapoda: Penaeidae) is distributed in the waters of Japan, Korea and Taiwan. There is, however, no study available on the population biology of this species to design its sustainable management and conservation protocols. We, therefore, studied for the first time its oocyte development, ovarian maturation, size at sexual maturity and spawning season using samples collected from Kagoshima Bay, southern Japan. Histological observation of ovaries confirmed seven oocyte developmental stages. This species exhibited asynchronous ovarian development, which indicates multiple spawning in a single reproductive season. Ovarian maturity stages were classified into three categories based on the developmental stage of the most advanced oocytes in the ovary: immature, maturing and mature. Females containing oocytes that either had evidence of germinal vesicle breakdown or migrating nucleus were defined as mature. The gonadosomatic index exhibited a positive but gradual relationship with ovarian maturation progression. In contrast, macroscopic staging showed a sharper relationship with histological stages of ovarian maturity, which exposed its potential as a maturity index for M. provocatoria owstoni. The size at sexual maturity was estimated to be 12.8 mm CL, and the spawning season appeared to last from April to December, with May to June being the main part.

A hemiarthrine bopyrid, Quadripediphryxus mayuzumii gen. et sp. nov. (Isopoda: Epicaridea: Bopyridae), is described based on a pair of specimens infesting the ventral surface of the abdomen of a snapping shrimp, Synalpheus streptodactylus Coutière, 1905 (Decapoda: Caridea: Alpheidae). The shrimp was collected from ornamental coral rock of an unidentified species of Poritidae (Cnidaria: Anthozoa) from Ehime Prefecture, Japan. The female of the new genus is similar to those in the genera Hemiphryxus Bruce, 1978, Eriphrixus Markham, 1990, and Micropodiphryxus Boyko, 2012, in having seven and four pereopods on the concave and convex sides of the body, respectively. However, female of the new genus can be distinguished from those in the other genera by the presence of a tiny triangular pleotelson bearing uropods. This finding represents the 12th species of hemiarthrine bopyrids found infesting the abdomen of caridean shrimps recorded from Japan.

A deep-sea isopod, Janirella (Janirella) longicauda sp. nov. (Asellota: Janirellidae), was found off the coast of Iwate Prefecture, west of the Japan Trench, at a depth of 3,370–3,547 m by trawling. Janirella longicauda sp. nov. can be distinguished from other species of Janirella Bonnier, 1896 by their long posterior pleotelson, the size differences between the posterior and anterior lateral processes in the pleotelson, the long and tapered rostrum and lateral processes in the pereonites, and the absence of dorsal spines on the pereonites and pleotelson. As a result of the description of this species, the number of species of Janirella (Janirella) known from NW Pacific becomes 12 species.

A new species of cavernicolous sesarmid crab is described from Vanuatu. Karstarma umbra sp. nov. belongs to a group of three species that has the chitinous distal part of the male first gonopod slender and elongate. The new species can easily be distinguished from K. balicum (from Bali, Indonesia) and K. loyalty (from Loyalty Islands) by its distinctly longer ambulatory legs and its relatively more slender male first gonopod. Karstarma umbra is morphologically closest to K. waigeo (from Indonesian Papua) but has proportionately longer legs and the outer margin of the male first gonopod is distinctly concave. The taxonomy of the genus is discussed and a revised key to the 18 species is provided.

A new species of Bathyceradocus, one of the maerid amphipods, is described based on the specimens collected from sunken wood on the bottom of the Kumano-nada, central Japan. Bathyceradocus japonicus sp. nov. can be distinguished from its congeners by the number of the accessory spines on the pleonite 1, the number of articles on the accessory flagellum, and the strongly oblique palm of the gnathopod 2 with the dentate margin. A brief note on the behavior of the new species and a key to species of the genus are provided.

The cymothoid isopod Ceratothoa oxyrrhynchaena Koelbel, 1879 was collected from the buccal cavity of yellowback sea-bream, Dentex hypselosomus Bleeker, 1854, in the southern Sea of Japan. There was a linear positive relationship between isopod body length (female, 22.6–35.0 mm; male, 9.6–17.0 mm) and fish standard length (115–249 mm). Body lengths of paired female and male isopods were also correlated with each other. These results indicate that isopods parasitized smaller fish than those examined and grew steadily as their hosts grew. This paper also suggests that the isopod can stay over three years in the buccal cavity of fish.

A new species of Hexapinus (Hexapodidae) is described from Hatoma Island, Ryukyu Islands, Japan. Hexapinus patuma, new species, is morphologically close to H. latus Rahayu & Ng, 2014, but can be distinguished by the structures of the carapace and third maxilliped, as well as the degree of setation of ambulatory legs.

An ovigerous female of Nerocila phaiopleura Bleeker, 1857 was collected from the body surface of a Japanese sardine, Sardinops melanostictus (Temminck & Schlegel, 1846) (Clupeidae), caught in Hitachi Port, Ibaraki Prefecture, central Japan. This collection extends the northern distribution limit of N. phaiopleura from Kujukuri, Chiba Prefecture, to Hitachi Port, Ibaraki Prefecture. In Japan, N. phaiopleura has so far been reported from coastal waters of the western North Pacific ranging from southern Kyushu to central Honshu, the East China Sea off Kyushu, and the Seto Inland Sea. While much remains unknown on its distribution in the southern Sea of Japan, N. phaiopleura is found from waters affected by a warm current, the Kuroshio, and its branch, the Tsushima Current. The species has not been found from subtropical waters of the Ryukyu Islands to date. Due to low water temperatures, the species is not likely to occur in subarctic waters affected by a cold current, the Oyashio, and the Kuroshio-Oyashio transition waters off northeastern Japan.

A rare xanthid crab species Cyrtocarcinus truncatus (Rathbun, 1906) is recorded from Ouvéa, Loyalty Islands, New Caledonia, and Ishigaki Island, Ryukyu Islands, Japan. This represents the first record of the species from outside Hawaii. Morphological features and its variations are described in detail.

The caligid copepod Gloiopotes huttoni (Thomson, 1890) was collected from the body surface of a black marlin, Istiompax indica (Cuvier, 1832), in the western North Pacific Ocean off Yonaguni-jima Island, the Ryukyu Islands, southern Japan. This represents the first record of G. huttoni from I. indica in Japanese waters. This copepod is known to use almost exclusively billfishes as its hosts, but due to the past confused billfish taxonomy, the billfish hosts have been recorded using various scientific names, most of which have recently been relegated to junior synonyms. Thus, based on the current taxonomy of billfishes and the literature published between 1890 and 2019, this paper also reviews the hosts and distribution of G. huttoni. Five species of billfishes in two families [black marlin; striped marlin, Kajikia audax (Philippi, 1887); blue marlin, Makaira nigricans Lacépède, 1802; sailfish, Istiophorus platypterus (Shaw, 1792) in the Istiophoridae; and swordfish, Xiphias gladius Linnaeus, 1758 in the Xiphiidae] serve as hosts for G. huttoni in the Pacific and Indian oceans, but four non-billfish species [bigeye tuna, Thunnus obesus (Lowe, 1839); yellowfin tuna, Thunnus albacares (Bonnaterre, 1788); wahoo, Acanthocybium solandri (Cuvier, 1832); and blue shark, Prionace glauca (Linnaeus, 1758)] are regarded as accidental hosts.

An ovigerous female of Lernaeenicus ramosus Kirtisinghe, 1956 was collected from an areolate grouper, Epinephelus areolatus (Forsskål, 1775), in coastal waters of Suruga Bay (western North Pacific Ocean), Shizuoka Prefecture, central Japan. This represents a new host and the easternmost distribution records for L. ramosus. Based on the literature published between 1956 and 2019, this paper tabulates the known hosts and distribution records of the species in the Indo-West Pacific region. The recent collections of L. ramosus from Japan further support a previous suggestion that the geographical distribution of the species in Japanese waters is affected by two warm currents, the Kuroshio, and its branch, the Tsushima Current.

First stage larva of the deep-sea giant shrimp, Sclerocrangon rex is described and illustrated under laboratory conditions. The larvae hatched in the most advanced condition, such as in having stalked eyes, among the congeneric species with abbreviated development. The newly hatched first larvae were staying together by clinging maternal pleopods until the second stage.

Argulus coregoni Thorell, 1864 is briefly described based on specimens from the body surface of amago salmon, Oncorhynchus masou ishikawae Jordan & McGregor, 1925, cultured at a trout farm in Tokushima Prefecture, Shikoku, western Japan. This represents the first confirmed record of A. coregoni from farmed salmonids in Shikoku. A list of occurrence records of A. coregoni from fishes reared in Japan is also provided based on the literature published between 1950 and 2019. The species has so far been reported from three species and two subspecies of salmonids and one species of plecoglossid reared at 14 sites in 10 prefectures (Tochigi, Tokyo, Nagano, Ishikawa, Gifu, Aichi, Wakayama, Hyogo, Tokushima, and Yamaguchi). Most of these sites were governmental or research facilities, and it is evident that little attention has been paid to A. coregoni at commercial tout farms. The species is considered to occur at more trout farms in Japan.