The distribution and population characteristics of the land crab Epigrapsus politus were studied in the Amitori Bay, Iriomote-jima Island, Japan. The crabs were most abundant on the western coast of the bay, particularly at the stations near the tip of the peninsula, where stranded carrion or algae often accumulate. E. politus was found only above the extreme high water of spring tide, where more cobbles and pebbles were piled by the waves. The carapace width (CW) of the individuals ranged from 6.94 to 21.94 mm for males (n＝98) and from 9.83 to 19.98 mm for females (n＝75). Because the population showed a polymodal length frequency or multiple modes, the lifespan was estimated to be at least two years. Adult males showed progressive polymorphism in chelae, and the proportion of heterochelous males and the size difference between major and minor chelae increased with increase in CW. The puberty molt brought about changes in the relative growth rate of the major chela in the males and the abdomen in the females. The size at 50% morphometric maturity was estimated to be about 14 mm CW and corresponded to the CW at about one year of age.

Monthly quadrat sampling of freshwater shrimps infested by the corallanid isopod Tachaea chinensis was conducted in Lake Biwa, central Japan, between April 2014 and September 2015. In total, 3,222 shrimps, including 2,786 Neocaridina spp., 309 Paratya improvisa, and 127 Palaemon paucidens were collected. Individuals of 431 T. chinensis were found from those hosts. The prevalence was 12.2% in Neocaridina spp., 5.5% in P. improvisa, and 24.4% in P. paucidens, respectively. All isopods found on hosts were immature stages and showed an annual emergence pattern; newly hatched juveniles were first sampled in August, increased the body length until May, and were not observed in June–July. To collect T. chinensis apart from the hosts, a cage trap filled with reed roots was put on the study site between June 2014 and September 2015. Isopods were collected in July and September. Specimens trapped in 2015 consisted of adult or premature individuals, and 39 out of 46 adult females collected were ovigerous. During an observation using snorkeling in July 2014, 29 adults were observed under stones and in withered Phragmites australis roots. Thus, T. chinensis reproduces in benthic habitats during summer. Subsequently, juveniles appear and grow via a series of molts on the hosts until the following year.

Calanus helgolandicus is widely distributed across the northeast Atlantic and Mediterranean, and also found in the Black Sea where it is referred to as Calanus euxinus. Previous genetic studies do not include deep-water specimens despite high abundances at bathypelagic and mesopelagic depths. Our objective is to compare the genetic structure of C. heloglandicus from the deep Balearic Sea to that of coastal populations in the Northeastern Atlantic Ocean, the Adriatic Sea, and the Black Sea defined from previous research. We use a portion of the mitochondrial gene cytochrome oxidase I from 41 individuals of C. helgolandicus collected at 2170 m depth in the Balearic Sea to estimate genetic differentiation between geographic regions and elucidate phylogeographic patterns. Results show that populations do not follow an isolation by distance model. Instead, the lowest genetic differentiation is between two distant basins, the deep Balearic Sea and the Black Sea. The results can be explained by the presence of two types of C. helgolandicus, a coastal, shallow water, type and an oceanic, deep water, type that diapauses at great depths. Genetic differentiation between coastal populations is maintained by oceanographic barriers, while differentiation in oceanic populations is lower due to dispersal by deep ocean currents.

The pronghorn spiny lobster, Panulirus penicillatus, is known to have the widest distribution among palinurid lobster species, occurring in tropical and sub-tropical areas of the Indo-Pacific Ocean. In the Pacific Ocean, mitochondrial DNA analyses have revealed that the western–central and eastern populations are genetically isolated. We performed morphological investigations of mid- to late-stage phyllosoma larvae collected in western, central, and eastern areas of the Pacific Ocean. The larvae from the western area had a significantly longer cephalic shield, shorter abdomen, and longer eyestalk than those from the eastern area. Additionally, for larger larvae, the widest part of the cephalic shield in larvae from the western area was located closer to the middle of the median line of the cephalic shield than in larvae from the eastern area. The ratio of width to length of cephalic shield and the ratio of cephalic shield width to thorax width are key traits for distinguishing between the larvae from the western and eastern areas. The morphology of larvae from the central area tended to be similar to that of larvae from the western area.

Two specimens of a freshwater fish parasite Argulus japonicus Thiele, 1900 were found to be mixed in a sample of over 600 specimens of a brackish-water fish parasite Caligus orientalis Gusev, 1951 in a glass vial kept at Hiroshima University. These specimens were all collected in September 1973 from the body surface of a moribund Japanese white crucian carp, Carassius cuvieri Temminck & Schlegel, 1846, in drought-induced brackish waters of Lake Shinji, Shimane Prefecture, Japan. This collection indicates that A. japonicus can survive in brackish waters under unusual climate conditions.

Mysid shrimps of the genus Heteromysis surprisingly appeared in coral reef exhibition tanks and connected filtration tanks of the public aquarium center Haus des Meeres, Vienna, Austria. This material is first described here as Heteromysis (Olivemysis) domusmaris sp. nov. based on a specific set of five flagellate spines (modified setae) on the antennular trunk in both sexes. Besides features typical of the subgenus Olivemysis, the new species is also characterized by features of the antennae, eyes, rostrum, third thoracic endopod, by modifications of male pleopods 3, 4, and by spine patterns on the endopods of uropods and telson. The most similar species are known from (sub) tropical waters of the SW-Pacific, to a lesser extent also from the Indian Ocean and Atlantic. Short notes are given on color, foregut, larval morphology, and swimming behavior. Locomotion and respiratory movements are documented by video clips in the supplement.

The seaward migration and larval release of Coenobita brevimanus Dana, 1852 on a sandy beach of Iriomote Island, Japan, was studied between May and November 2009 and between May and December 2010. Seaward migration and larval release were mainly observed for several days before and after new moons during the period from June to November, and the migration was mainly focused on 0.82 days after the new moon. Around full moons, some crabs were also found, with the highest numbers at 1.34 days after spring tides (semi-lunar rhythms). Almost half of the crabs that appeared on the beach entered seawater, and almost half of the immersed crabs released zoeas or exhibited swinging behavior. Appearance of crabs on the beach and entering seawater occurred only after sunset between 19:00 to 23:10. The peak time of the appearance on the beach was within a certain range each month. Crabs entered seawater at a mean of 2 h after the nighttime high tide. Seaward migration of the crabs showed clear seasonality, and many crabs migrated and entered seawater during the period from July to September, the months with higher temperature (＞25°C). This species shows a phenology and rhythm of spawning common to coenobitids on the Ryukyu Islands.

A second species of the ilyograpsine crab, Mirograpsus Komai & Fujita, 2018 (Macrophthalmidae) is described from Vanuatu. It is compared to the type and only species, M. asper Komai & Fujita, 2018, from Ryukyus in Japan, differing in the carapace shape, armature of the chelipeds and ambulatory legs, and structure of the male pleon.

Responses of larval individuals of the fish-parasitic gnathiid isopod Elaphognathia cornigera under presence or absence of adult male were examined by a laboratory rearing experiment. Female larvae molted into adults after 29.5±4.8 days (mean±s.d.) under male absence since the final parasitism against 17.4±3.4 days under male presence. Ovulation in females was never observed under male absence during the experimental period while it occurred within a day after maturation of female larvae under male presence. The delayed molting and inhibition of ovulation under male absence are considered to provide additional time for finding a mate when the availability of males is low and to decrease reproductive failure of females. In contrast to females, solitary reared male larvae took less time to mature than those reared with a male. The delayed molting of male larvae under male presence was suggested to contribute to reduce potential attacks from adult males when male-male competition exists. Precopulatory mate guarding behavior by adult males was observed more frequently for female larvae than for male larvae. This indicates that adult males of the gnathiid can discriminate the sex of larvae as well as larvae recognize the presence of males.

Population structure, growth and reproductive activities were investigated over one year for the boring isopod Sphaeroma sieboldii Dollfus, 1889 for a population burrowing in sand-stone. The occurrence of heterosexual pairs peaked in spring, followed by the peak occurrence of ovigerous females and females cohabiting with early juveniles in the summer. This suggests the reproductive season is from spring to summer. For adult males that had penes, three reproductive stages were recognized: large-sized males with mature appendices masculinae, intermediate-sized males with immature appendices masculinae, and small-sized male without appendices masculinae. Most of the males pairing with females were large-sized males with mature appendices masculinae, while most of the pairing females were smaller than the partners, carrying small, non-functional oostegites. Males with mature appendices masculinae were frequent from winter to summer, covering the reproductive season. Size-assortative pairing was found in the peak season of heterosexual coupling. Sex ratios were female-biased in all months. Observation of the interior reproductive organs confirmed that adult females had ovaries, while most males had both ovaries and male reproductive tracts. Some males with mature appendices masculinae were found to have degenerated ovaries. These findings suggest that protogynous sex change occurs in S. sieboldii.

The blue crab, Callinectes sapidus Rathbun, 1896, is native to the Atlantic coasts of the Americas and globally one of the most highly invasive marine species. In the present study, the species’ diet and the foraging behaviour was studied in the Thermaikos Gulf and Papapouli Lagoon for the first time. Surveys were undertaken using fyke nets, shore surveys, scuba and snorkelling. Additional data were compiled from systematic interviews with mussel farmers, shellfish traders and fishermen. In both Thermaikos Gulf and Papapouli Lagoon C. sapidus was found to prey on a wide variety of species including economically important molluscs, fishes, and crustaceans, indicating a substantial potential impact on fisheries and aquaculture in the region. Observation showed that over 6 (2009–2014) years, the blue crabs became dominant in Papapouli Lagoon at the expense of the native commercially fished crab Carcinus aestuarii Nardo, 1847 according to fisheries data. Potential management implications are discussed.

A new species of the alpheid shrimp genus Potamalpheops Powell, 1979 is described from an anchialine cave on Nggela Pile Island, Solomon Islands. Potamalpheops nazgul sp. nov. belongs to the P. monodi (Sollaud, 1932) species group, defined by the presence of two pairs of cuspidate setae on the distal margin of the telson and feebly or non-enlarged chelipeds. The new species is morphologically closer to P. pininsulae Bruce & Iliffe, 1992 and it is characterised among others by the presence of a well-developed antero-mesial tubercle on the eyestalk, a short rostrum with a dentate ventral carina, a toothed distal margin of first antennular segment and a non-strongly inflated carapace. The description of this new species raises the number of known species of Potamalpheops to fifteen.

Egg loss from ovigerous females has hampered larval culture experiments for life history studies of the coconut crab Birgus latro. We conducted two preliminary experiments to develop a method to artificially incubate and hatch embryos separated from the mother: 1) manipulation of incubation duration in a pseudo-terrestrial environment and 2) manipulation of incubation temperature. In experiment 1, we incubated embryos on medical gauze moistened with seawater for 7 or 17 days at 27–28°C before incubation by immersion in seawater (27–28°C), or we continuously incubated them in seawater only. In experiment 2, we similarly incubated embryos on medical gauze until 1–2 days before hatching at 21–22°C, 24.5–25.5°C, or 27–28°C before incubation in seawater (27–28°C). Successful hatching occurred, but embryos did not hatch synchronously, and hatching continued for approximately a week. Incubating embryos in seawater continuously led to the highest hatching rates of morphologically normal zoeae; however, hatching rates of normal zoeae did not exceed 50%. Increased incubation temperature reduced the incubation duration until hatching. Zoeae could metamorphose into megalopae, but survival rates were generally low. Further studies are required to improve the hatching rate of viable larvae under artificial conditions.