Carybdea prototypus has potent venom and is responsible for serious envenomation incidents in the coastal regions of Japan. However, the natural habitat of the benthic polyp, which is a source of medusae, has not yet been identified. Previous studies on cubozoans, including polyp collection records and environmental DNA analyses, have suggested that C. prototypus polyps may inhabit low-salinity environments. However, the role of salinity in regulating the life-history processes of this species remains unclear. Therefore, in this study, we examined the effects of salinity on key developmental processes in C. prototypus, including planula settlement, polyp growth, asexual reproduction, and medusae metamorphosis. The planulae and polyps were reared under controlled salinity conditions ranging from 15 to 33. Planulae were successfully attached across all salinity treatments, although survival was reduced at 15. Metamorphosis into creeping polyps proceeds faster under moderately low-salinity conditions. After attachment, the time required to transform into settled polyps decreased with decreasing salinity. Polyp proliferation was active at low salinity, whereas polyp size increased with increasing salinity. In contrast, metamorphosis into medusae occurred only under high-salinity conditions. At salinity of 15, polyps degenerate into tissue masses. These findings indicate a clear salinity-dependent shift in life history strategy, in which low salinity promotes clonal expansion through asexual reproduction. Meanwhile, higher salinity facilitates somatic growth and metamorphosis into medusae. This study provides experimental evidence that salinity regulates key life-history transitions in C. prototypus and suggests that this polyp may inhabit low-salinity environments such as estuaries and seagrass beds of Zostera marina. Fluctuations in salinity likely are instrumental in shaping the distribution and population dynamics of this species in coastal environments.

Guanine-type choroidal tapetum comprises a palisade of cells containing stacks of guanine crystals within the choroid, reflecting light from behind the photoreceptors and resulting in increased sensitivity. Although many cartilaginous fishes possess a guanine-type choroidal tapetum, such structures have at no time been investigated in Hexanchiformes. We investigated a quantitative chemical analysis of choroidal tapeta in two deep-sea hexanchiform sharks, the frilled shark Chlamydoselachus anguineus and sharpnose sevengill shark Heptranchias perlo. Both sharks exhibited eyeshine and contained silver choroidal tapeta in the whole area. The locus tapetalis, with guanine values exceeding 1.27 mg/cm2 in C. anguineus and 0.49 mg/cm2 in H. perlo, was developed in the temporal fundus in both sharks, the results suggesting that such facilitates naso-lateral detection with higher photosensitivity for predation in the deep-sea environment. The possible ecological advantages of the tapetum in hexanchiform sharks and their evolutionary relationship among other cartilaginous fishes are discussed.

Ocean acidification, caused by increased atmospheric CO2, threatens marine organisms that depend on calcium-based structures such as jellyfish statoliths. This study investigated the effects of low pH on the morphology and statolith formation of ephyrae in Aurelia coerulea, comparing two developmental pathways to form ephyra: polyp-strobilation and planula-strobilation. Under the pH 6.8 condition, polyps failed to produce viable ephyrae, whereas planula-strobilation succeeded in releasing ephyrae with normal morphology, though statoliths were absent. Under the pH 7.8 condition, both strobilation types produced normal-shaped ephyrae with reduced statolith size but increased statolith number compared with the control (pH 8.1), suggesting a compensatory response to acidification. Statolith morphology differed between pathways: planula-strobilated ephyrae had needle-shaped statoliths with high aspect ratios, indicating a rapid, early-stage crystallization process. Despite their minimal body size and statolith development, planula-strobilated ephyrae maintained the functional mass of statoliths necessary for survival. This rapid, morphologically minimized development suggests that planula-strobilation is an adaptive reproductive strategy in response to environmental stress. Our findings suggest that A. coerulea possesses a flexible life history strategy that may facilitate its resilience to ongoing ocean acidification scenarios.

Phylllorhiza punctata von Lendenfeld, 1884 originates from Australia and Coral Triangle, but its occurrence has been reported in the Atlantic Ocean and Mediterranean Sea, so it is necessary to better understand its geographic range. Two specimens were collected in Nagasaki and Kagoshima Prefectures, Japan, and morphological and molecular studies suggest that they are P. punctata. . These are the first records of the species in Japan, and it is possible that individuals have been transported to Japan by the Kuroshio Current.

In the present study we reviewed the life cycles and reproduction strategies of the order Rhizostomeae. We found 28 species with described life cycles representing ∼30% of the valid species. The metagenetic life cycle of most scyphozoans, which includes the benthic asexually-reproducing polyp and the pelagic sexually-reproducing medusa, is exhibited by all rhizostome species. Rhizostomeae are dioecious with only two exceptions described as hermaphroditic. Sexual dimorphism can be found in species with special external structures utilised for brooding but others show no sexual dimorphism despite the colour of mature gonads. Six asexual reproduction modes have been described for the production of new polyps but rhizostome polyps propagate through a main mode that differs among taxa. Species belonging to Dactyliophorae produce new polyps by podocysts whereas the Kolpophorae new polyps develop from free-swimming buds. The number of ephyrae formed per strobila differs between taxa with monodisc and polydisc strobilation in the Kolpophorae and Dactyliophorae, respectively. Given the low number of studied species it is expectable that new reproductive strategies will be discovered when additional species are investigated. We recommend increasing (1) descriptions on life cycles and reproductive strategies for a greater number of species, (2) attempts to locate the polyps in the field, (3) the study of species in their natural environment, to understand the population dynamics of Rhizostomeae and to clarify the potential of artificial structures to increase medusa populations. In addition, experimental studies are needed to improve our understanding of the factors affecting transitions between life cycle stages and medusa production rates.

Beard worms from the family Siboglinidae, are peculiar animals and are known for their symbiotic relationships with sulfur bacteria. Most Siboglinids inhabit the deep-sea floor, thus making difficult to make any observations in situ. One species, Oligobrachia mashikoi , occurs in the shallow depths (24.5 m) of the Sea of Japan. Taking advantage of its shallow-water habitat, the first ecological survey of O. mashikoi was performed over a course of 7 years, which revealed that its tentacle-expanding behavior was dependent on the temperature and illuminance of the sea water. Furthermore, there were significantly more O. mashikoi with expanding tentacles during the nighttime than during the daytime, and the prevention of light eliminated these differences in the number of expending tentacles. These results confirmed that the tentacle-expanding behavior is controlled by environmental light signals. Consistent with this, we identified a gene encoding a photoreceptor molecule, neuropsin , in O. mashikoi , and the expression thereof is dependent on the time of day. We assume that the described behavioral response of O. mashikoi to light signals represent an adaptation to a shallow-water environment within the predominantly deep-sea taxon.

Ephyrae, the early stages of scyphozoan jellyfish, possess a conserved morphology among species. However, ontogenetic transitions lead to morphologically different shapes among scyphozoan lineages, with important consequences for swimming biomechanics, bioenergetics and ecology. We used high-speed imaging to analyse biomechanical and kinematic variables of swimming in 17 species of Scyphozoa (1 Coronatae, 8 "Semaeostomeae" and 8 Rhizostomeae) at different developmental stages. Swimming kinematics of early ephyrae were similar, in general, but differences related to major lineages emerged through development. Rhizostomeae medusae have more prolate bells, shorter pulse cycles and higher swimming performances. Medusae of "Semaeostomeae", in turn, have more variable bell shapes and most species had lower swimming performances. Despite these differences, both groups travelled the same distance per pulse suggesting that each pulse is hydrodynamically similar. Therefore, higher swimming velocities are achieved in species with higher pulsation frequencies. Our results suggest that medusae of Rhizostomeae and "Semaeostomeae" have evolved bell kinematics with different optimized traits, rhizostomes optimize rapid fluid processing, through faster pulsations, while "semaeostomes" optimize swimming efficiency, through longer interpulse intervals that enhance mechanisms of passive energy recapture.

The grapsoid crab Xenograpsus inhabiting shallow-water hydrothermal vents shows a limited distribution in the western Pacific, thus providing a good opportunity to observe how a species expands its distribution and establishes and maintains a metapopulation. We analyzed four Japanese populations of Xenograpsus (Kueishan-dao, Showa Iwo-jima, Shikine-jima, and Omuro-dashi) distributed along the Kuroshio Current to assess whether population connectivity is affected by the current. The reconstructed phylogenetic tree indicated that the specimens analyzed belong to a single clade, including X. testudinatus in Kueishan-dao. We could not include genetic data of X. novaeinsularis and tentatively refer to the specimens analyzed as X. testudinatus. All four populations shared the two major haplotypes. Statistically significant population differentiation was nevertheless recognized between Showa Iwo-jima and Omuro-dashi, about 1,000 km apart. The number of individuals analyzed in Shikine-jima (N = 11) and Kueishan-dao (N = 5) might not be enough to detect differences. The relative age of population expansion seemed to correlate with the direction of the Kuroshio Countercurrent. Together with knowledge on the distribution of the planktonic larvae, larvae of X. testudinatus stay and mostly contribute to its source population. The Kuroshio Current system occasionally transports the larvae and juveniles, or even adult individuals on floating pumice and other materials, to eventually settle into newly formed habitats after volcanic eruptions, thus expanding the distributional range of the species.

Two specimens of pandeid species, Eutiara decorata Berberian, Michenet and Goy, 2021 were collected from Kumejima Island in Okinawa Prefecture, Japan. These specimens with a small white apical process were not yet fully-grown adults. A hyperiid amphipod, Brachyscelus crusculum, was attached to Eutiara decorata in situ. Eutiara decorata has only been reported in Tahiti Island, French Polynesia, in 2021. This is the first record from Japan and the second record of occurrence since its original description.

In order to investigate possible differences and patterns of body-plan morphogenesis among Scyphozoa, we described the morphology and development of the bell and oral arms of 11 species. We built a model for the development of the group and provide a guide for recognizing development stages. In general, the bell diameter was more efficient than age (days) to identify the stage of development, which suggests that the events of morphogenesis depend more on body size than on age. We used measurements of bell and oral arm to build growth curves and described the timing of developmental events (e.g., appearance of digitata, velar lappets and oral arms, filling of interlobe gaps) and patterns of morphogenesis (contour of marginal lappets, branching of oral arms, and presence of structures/appendages on the oral arms). The time until the appearance of digitata and velar lappets, the marginal lappet contour, and the presence of lateral folds in oral arms supported a phylogenetic relationship between the "Semaeostomeae" and Rhizostomeae (Dactyliophorae and Kolpophorae) lineages. "Semaeostomeae" jellyfish with digitata (family Ulmaridae) and differentiated velar lappets developed such structures at larger bell diameters than rhizostomes; only Dactyliophorae rhizostomes had serrated contour of the lappets; only species of Rhizostomeae showed development of the lateral folds; and only Ulmaridae and Rhizostomeae had differentiated appearance of the velar lappets. We propose a specific definition of oral arms that is based on morphological and functional features. Therefore, estimating development timing and describing the way that these structures develop can help to understand how jellyfish feeding strategies change during initial growth and how the changes in these structures can reflect the differences in feeding habits among the groups.

Ephyrae are produced through the strobilation of polyps in the general life cycle of Aurelia coerulea . However, it has been reported that planulae can also metamorphose directly into ephyrae, without passing through the polyp stage. There is a mixture of ephyrae developed from planulae (planula-strobilated ephyrae) and ephyrae developed from polyps (polyp-strobilated ephyrae) in the ephyra population. However, the effect of the planula-strobilated ephyrae on the ephyra population is yet to be determined, since their morphological characteristics have not yet been elucidated. This study aimed to determine the morphological characteristics to distinguish between planula-strobilated and polyp-strobilated ephyrae. The differences in body dimensions, such as total body diameter (TBD), central disc diameter (CDD), lappet stem length (LStL), rhopalial lappet length (RLL), and total marginal lappet length (TMLL) were compared between the two types of ephyra. Thus, we show that body proportions can be used to identify planula- and polyp-strobilated ephyrae. The ranges for identifying planula-strobilated ephyra were 35.0–38.3% for CDD/TBD, 56.7–64.9% for LStL/CDD, 84.7–99.5% for TMLL/CDD, and 31.0–37.5% for RLL/TMLL. This method could be an important basis for devising countermeasures for jellyfish blooms in areas where ephyrae deriving from planula strobilation occur.

The edible jellyfish Acromitus hardenbergi Stiasny, 1934 is harvested throughout the year at the mouth of the Perak River, Malaysia. Although this species is an important fishery resource in the local area, limited biological studies have been carried out on it. The aim of the present study was to elucidate the life cycle of this unique brackish-water jellyfish in order to conserve the species and develop sustainable jellyfish fisheries. Mature medusae were collected at the mouth of the Perak River. Embryonic and larval development after fertilization was completed within 24 h until the planula stage and within 48 h until the polyp stage. Primary polyps had a long stalk with a small stolon at the base of the calyx. Fully developed polyps were bowl-or goblet-shaped but became an elongated stalk under starved conditions. Asexual reproduction was accomplished only by means of budding, and no podocysts were produced. Strobilation was mono-disc type. These characteristics may be adaptations to the dynamic environmental conditions in the estuary of the Perak River, where salinity fluctuates widely due to strong inflows of highly turbid freshwater coupled with tidal changes. This study suggests that polyps of A. hardenbergi expand their population not by podocysts, but by budding as quickly as possible and forming one large ephyra by mono-disc strobilation without the residuum, because the polyp cannot remain for a long time at its settlement place in the sediment-rich environment with drastic salinity change.

Kleptoplasty or acquisition of chloroplasts from ingested photosynthetic organisms, is thought to be a key factor in determining the trophic requirements and carbon mineralization of foraminifera, consequently influencing their ecology, evolution, and calcification. However, the acquisition, maintenance, and functions of “stolen” chloroplasts (kleptoplasts) in foraminiferal cells have not been well characterized. Using molecular phylogeny and experimental measurements of oxygen flux and intracellular (vacuole) pH, we characterized aspects of kleptoplast origin and function in Planoglabratella opercularis (d’Orbigny), a rocky-shore benthic foraminifera. We determined that the kleptoplasts in P. opercularis are derived from various species of epiphytic diatoms. The acquisition of epiphytic diatoms is congruent with the behavior of P. opercularis, which crawls on the thalli of coralline algae and grazes on the microalgae on the algal surface. Kleptoplasts were located near the host foraminiferal cell periphery, just under the pore plug, mainly along the dorsal side, which is important for oxygenic photosynthesis. The lifespan of kleptoplasts varied according to food availability and to the intensity and duration of light irradiation. Planoglabratella opercularis was able to capture not only diatom chloroplasts but also those from Chlorella. Kleptoplastids of diatom origin decreased in autofluorescence and were digested within 11 days under light in the absence of food. This suggests that host foraminifera frequently capture new diatoms to sustain kleptoplasts. Kleptoplastid autofluorescence was prolonged in the dark or 24:24-h light:dark cycle, and when food was available. Host foraminifera metabolized food to maintain the activity of kleptoplasts and effectively utilized photosynthetic products from both organic and inorganic materials in response to the ambient environment. This suggests that P. opercularis behaves as a mixotroph. Simultaneously, kleptoplasts maintain a high intracellular pH environment, indicating that foraminifera capture kleptoplasts not only to gain their photosynthates but also to control the intracellular pH levels to construct a high magnesian calcite test.

Abstract In Thailand, two species of rhizostome jellyfish, Rhopilema hispidum and Lobonemoides robustus, are commercially harvested. The cnidomes, nematocyst size and toxicities were compared between these species. Rhopilema hispidum and L. robustus each had four types of nematocysts on their oral arms. For R. hispidum, these nematocyst types included two types of isorhiza and two types of rhopaloid, while in L. robustus, there were three types of isorhiza and one type of rhopaloid. For R. hispidum, tubule lengths of the largest nematocyst type (large round isorhiza; mean ± SD = 313.8 ± 62.2 μm) were significantly longer than those of L. robustus (large ellipsoid rhopaloid; 162.1 ± 38.5 μm). Using the freshwater shrimp, Palaemon paucidens, in a bioassay, we determined that the lethal nematocyst concentrations for R. hispidum and L. robustus were 5705.3 ± 1118.1 and 3408.3 ± 1032.9 unit g−1 wet weight, respectively, and that these concentrations were significantly higher in the former than in the latter.

The stings of box jellyfishes can be fatal, so knowing the fauna of a certain area is important to save lives. Five described and two still-undescribed species of Cubozoa have been reported from Thailand: Chironex indrasaksajiae, Chironex sp., Chiropsella sp., Chiropsoides buitendijki, Copula sivickisi, Morbakka fenneri, and Tripedalia cystophora. We made detailed observations of the morphology of two of the species newly recorded in Thailand: Alatina morandinii and Tripedalia binata. The molecular phylogeny of these species is also discussed. Additional investigations are needed to understand the diversity of Cubozoa in Thailand.

The benthic ctenophore Vallicula multiformis Rankin, 1956 (Ctenophora, Platyctenida) was originally described from Jamaica. We found the species in an aquarium during our survey of jellyfish from Palawan Island, the Philippines. Molecular analysis of the species confirmed the morphology-based identification of the samples. This discovery is the first report of the occurrence of species from the western Pacific area.

Relationships between foundation species and their associating animals provide baseline knowledge towards elucidating biodiversity. Here, we report epibiotic animals associating with a foundation species, the squat lobster Shinkaia crosnieri, in a deep-sea hydrothermal vent field in the Okinawa Trough, Northwest Pacific, and discuss their relationships. The epibiotic fauna recorded on S. crosnieri included Amphisamytha sp., dirivultid copepod, and Lepetodrilus nux. Of note, the number of individuals of Amphisamytha sp. increased with increasing size of S. crosnieri. In particular, one S. crosnieri individual had more than 300 individuals of the dirivultid copepod, L. nux, and more than 30 individuals of Amphisamytha sp. attached to it. The results suggested that older individuals host more epifauna, contributing to greater biodiversity. Furthermore, aggregations of individuals increased habitat heterogeneity, resulting in their hosting more species.

In this study, we investigated the trophic interactions between the Japanese giant box jellyfish, Morbakka virulenta, and fish in the central part of the Seto Inland Sea, western Japan, in autumn and winter. Occurring in the surface waters at nighttime, these cubomedusae, regardless of their size (1.5 to 22.5 cm in bell height), were found to be piscivorous, feeding mainly on the Japanese anchovy, as shown by their stomach contents analysis. This finding was supported by a stable isotopic analysis and by an unchanged cnidome, irrespective of the bell height of medusae. Their nocturnal occurrence near the surface often took place around the slack tide, during which the medusae were foraging with tentacles fully extended. Other associations between the medusae and fish were also observed at this time: presumed commensalism with juvenile Japanese horse mackerel, and predation by black scraper.

Envenomation by toxic box jellyfish species is known to be a serious problem to public health. In order to elucidate the problem, it becomes necessary to predict the occurrence of box jellyfishes, as well as understanding their ecology and life cycle. Mature medusae of Carybdea brevipedalia (Cubozoa: Carybdeida), which is a common species of box jellyfish in Japan, were collected from northern Japan to observe its early life history, including polyp formation. Fertilization occurred externally, and blastulae developed into planulae. Free swimming planulae settled and metamorphosed into tiny primary polyps with two forms, i.e. settled and creeper. Adult polyps formed cysts at temperatures below 15 degrees C or when water replacement and/or feeding was stopped. Budding occurred in four-tentacled polyps, and the buds were released after commencement of budding. Complete metamorphosis of a whole polyp into a single medusa occurred at stable temperatures between 18 to 25 degrees C (18, 20, 23, 25 degrees C, respectively) or when temperatures were raised from 20 to 25 degrees C. Newly released medusae had four tentacles. Our study demonstrated that polyps of C. brevipedalia survive and propagate over a wide range of water temperatures and that developmental features resemble closely those of some tripedaliid species, namely Tripedalia cystophora and Copula sivickisi, rather than Carybdea marsupialis. The morphological affinities of polyp in C. brevipedalia, T. cystophora and C. sivickisi support recent molecular results. However, further studies are needed to confirm the morphological contradiction between C. brevipedalia and C. marsupialis in the future.