The first part of this study focused on the morphology of different early life stages and their further development. This second part of the study demonstrates that the different types of polyps, strobilations/metamorphoses, their products (residues/ephyrae/medusae) and their further development (e.g. gastric system development of ephyrae) explain well the evolutionary development of the Rhopaliophora.

By using the strobilation rates and information on the way morphological characters changed during the process of strobilation/metamorphosis, the strobilation and metamorphosis types were newly defined. Species were allocated to types by the combination of this information. The incomplete metamorphosis in Carybdea xaymacana is re-affirmed as a modified monodisc strobilation according to the newly investigated monodisc strobilation type in carukiid species. Complete metamorphosis is a form derived from the modified monodisc strobilation, as well as the metamorphosis type observed in Staurozoa.

A clear separation of “Rhizostomeae” into two paraphyletic groups (Kolpophorae and Rhizostomatidea) and the polyphyly of Semaeostomeae is demonstrated. The results lead to a new and reorganised taxonomic system for the class Scyphozoa, which once more includes Cubomedusae and Stauromedusae. Scyphozoa now comprises two subclasses: Coronamedusae and Discomedusae. The latter subclass includes the newly-created infraclasses Pelagimedusae and Centrifugalia. Two new superorders, each containing two new orders (Stipeslongida: Sanderiida, Metamorphida; Opposita: Circulumcanalida, Cyaneida), are contained within the Centrifugalia.

The new order Circulumcanalida now contains two newly erected suborders: Antherhizostomatida and Phacellophorida. The former new suborder contains the newly erected infraorders Kolpophorae and Centripetalida. The family Sanderiidae was further erected within the new order Sanderiida.

The conservation of an endangered horseshoe crab, Tachypleus tridentatus, has been increasingly emphasized worldwide. Although monitoring of T. tridentatus reproduction is effective for its conservation, the conventional monitoring system for spawning has several drawbacks. Thus, our study aimed to examine the environmental DNA (i.e., eDNA) approach to assess T. tridentatus spawning events. Six survey sites were set at the sandy beaches of the Sone tidal flat, Japan. Visual observations of the spawning events, i.e., the presence/absence of adult pairs of T. tridentatus or that of foams, occurring because of their spawning behavior, and water sampling for eDNA assay were conducted 10 times at each site between 2019 and 2020. The spawning events were observed at a total 16 sites during the surveys, and the estimated numbers of T. tridentatus adult pairs were 1–5 per site. T. tridentatus DNA was detected in 15 of the 16 samples at the sites where the spawning events were observed and in 13 of 44 samples where the spawning events were not observed. The estimated number of pairs and the eDNA concentration showed a significantly positive correlation. Moreover, the presence/absence of the spawning events was distinguished with excellent accuracy based on the eDNA concentration. Therefore, our results showed that the spawning events of the species and the eDNA concentration were closely related. In conclusion, we expect that the population of T. tridentatus in spawning grounds can be evaluated more complementarily by combining the conventional observation method with the eDNA approach.

Mytilopsis sallei (Récluz, 1849), an invasive bivalve, has been established for more than two decades in Pawong Canal, which is a part of the largest lagoon system in Thailand. A recent survey in 2019 revealed the presence in the canal of another invasive species, the mytilid mussel Mytella strigata (Hanley, 1843). Both species are byssally attached bivalves that can change soft bottoms into hard substrata composed of bivalve shells, with consequent effects on benthic communities. Data of benthic macro-invertebrate species collected 25 years ago, before the arrival of both invasive species, were compared with data from a recent survey. Although 28 different species were observed in each study, only five were present in both. However, there is no significant change in the diversity and evenness of this benthic community. A few species of hard-bottom dwellers appeared in the canal after invasion while some soft-sediment inhabitants were absent. Given that the full dataset from the previous study was not available and a long-term series of observations was not made, it is difficult to distinguish the effects of bivalve invasion from the effects of physical change in the environment. However, we documented the current condition of this complex estuarine ecosystem and discussed some possible effects of non-native bivalve invasion on the local benthic macroinvertebrate assemblages.

The cotylean polyclad flatworm Bulaceros porcellanus Newman & Cannon, 1996 has so far only been recorded in a few tropical locations in the Indo-Pacific Ocean, including the Lakshadweep Islands (India), the Great Barrier Reef (Australia), and the Marshall Islands (Republic of the Marshall Islands). In this study, we report B. porcellanus for the first time from Japanese waters based on specimens collected in Misaki (Kanagawa) along the Pacific coast of Honshu. This new record expands the species’ known geographic distribution northward to include the warm temperate region of Japan. Our molecular phylogenetic analysis revealed that B. porcellanus formed a clade with Pseudoceros harrisi Bolaños, Quiroga & Litvaitis, 2007, and was separated from the monophyletic group composed of 19 other species of Pseudoceros Lang, 1884, including its type species Proceros velutinus Blanchard, 1847 (now Pseudoceros velutinus). Based on the morphological and phylogenetic closeness between P. harrisi and B. porcellanus (type species of Bulaceros Newman & Cannon, 1996), we transfer P. harrisi into Bulaceros with a revision of the diagnosis of Bulaceros.

The coastal waters off Tokoro, Okhotsk Sea have been characterized by a substantial decrease in primary production from spring to summer season which will have repercussions on the maintenance of the Japanese scallop Patinopecten yessoensis (Jay, 1857) culture. This study examined the seasonal changes in scallop growth, condition and organ indices in relation to environmental conditions, and compared the sinking particulate organic carbon (POC) flux as an estimate of food availability to scallop food requirements. Monthly specimens were obtained in July 2005, and from May 2006 to December 2008 off Tokoro, Okhotsk Sea. Growth parameters derived from the modified von Bertalanffy growth function of 885 scallop individuals were the mean asymptotic shell height H_∞=126.51 mm, Brody growth coefficient K=0.081 year⁻¹, and t₀=6.19 year. Condition (CI) and gonadosomatic indices (GSI) increased in April and May, decreased from June to September, but increased again from October until May. The adductor muscle index (AMI) increased from June to October, but declined from November until May when CI and GSI began to increase. Primary production (PP) and chlorophyll a biomass decreased from spring (25.75–29.74 µg C L⁻¹ h⁻¹ and 1.74–5.37 µg L⁻¹) to autumn (0.90–10.83 µg C L⁻¹ h⁻¹ and 0.29–0.75 µg L⁻¹) concurrent with decreasing CI and GSI, but no deleterious effect on AMI was evident during these periods. The sinking POC flux (25–68 mg C m⁻² day⁻¹) was low during summer when gonad activity was also low, but started to increase in autumn when gonad activity started to peak. Although a deficit in food availability (lower sinking POC fluxes than scallop food consumption) was observed during summer, the potential advection of organic matter from offshore to nearshore region may have provided additional source of food to bottom-cultured scallops, contributing in part to the increase in shell growth and somatic tissues.

Interstitial areas of coastal pebble sediment in the Japanese Archipelago are inhabited by extremely elongated gobies of the genus Luciogobius, which are characterized by an increased number of vertebrae and reduction of scales, eyes, and fins. To explore the little-known interstitial life and feeding biology of Luciogobius gobies, we investigated the diets of two interstitial Luciogobius species, L. elongatus and L. grandis, and the interstitial organism communities of the gobies’ microhabitats in an exposed gravelly coast in Shirahama, southern Japan. The interstitial organism community in pebbly sediment was dominated by minute arthropods such as harpacticoids, isopods, and ostracods, presenting a marked contrast to the communities in sandy sediments, which are dominated by nematodes and turbellarians. The gut contents of the two goby species were composed exclusively of interstitial organisms, especially harpacticoids and isopods. Although each prey assemblage was roughly similar to the interstitial organism community in the corresponding microhabitat, marked preferences for harpacticoids and flabelliferan isopods were detected in L. elongatus and L. grandis, respectively. Irrespective of their intense feeding on harpacticoids, rare catches of large isopods were suggested to be nutritionally important for the gobies. Seasonal and diurnal changes in these feeding habits were not detected, while the percentage of gravid females peaked in October in both the species. These results suggest that the Luciogobius gobies are the first-known fishes that depend exclusively on interstitial organisms, and that selective feeding upon meiobenthos may facilitate the coexistence of several interstitial goby species in pebbly sediment.

Recent technological innovations have introduced a new category of inexpensive, compact and shock-proof digital camera called the action camera (AC). An AC manufactured by Casio Computer Co., Ltd., and fitted with a LED ring light specially designed for this camera, was used to record coral spawning in a seawater aquarium during the night. A colony of Acropora digitifera from the reef flat off Sesoko Island, Okinawa, Japan was transferred to a flow-through aquarium located nearby the collection site. The photographing was conducted at 1 min intervals from around 18:00 until LED batteries were exhausted after midnight. The camera with the LED was re-set daily until the spawning was confirmed. The spawning was observed on 5 June 2017, four days before the full moon. The “setting,” appearance of gamete bundles on the colony surface, was first observed near the apical part of the colony around 20:10–20:20 and extended to the lower part around 20:50–21:00. The first bundle particles in the water column were detected at 22:17 and their number peaked to 100 at 22:27. The “setting” in the lower part was still recognized around 22:40. Around 22:50, the bundle particle number in water decreased to <5. Thus, the intensive spawning lasted ≤30 min. The present study shows that an action camera equipped with LED effectively elucidates the details of coral spawning. Considering the stable environment of aquarium tanks, ACs set up in an aquarium located close to the reef will be suitable for remote monitoring of coral reproduction.

Cloridopsis scorpio (Latreille in Latreille, Le Peletier, Serville & Guérin, 1828) was collected from the intertidal zone of Hiroshima and Okayama Prefectures, western Japan in 2017–2020. This is a rediscovery of the stomatopod crustacean after almost a century in Japanese waters. Morphological characters and coloration of the species are described based on the newly collected specimens. The rostral plate lengthens, and the lateral process of fifth thoracic somite becomes blunted and inclined ventrolaterally with growth. This fact supports that C. aquilonaris Manning, 1978 is a junior synonym of C. scorpio as shown by the previous study.

This study investigates the distribution, growth, and reproduction of the fluvial, regional, and vulnerable species, Macrobrachium shokitai, on Iriomote Island in southwestern Japan. Prawns were captured at seven stations along a small, 4.0-km-long river, among ca. 40 rivers on the island. Animals were collected monthly during 2019 and abundance, carapace length (CL), sex, and the presence or absence of eggs attached to pleopods were documented. Prawns were primarily collected at stations >2.7 km above the river mouth at >270 m elevation. Stable, dense populations were observed above the waterfall. The sex ratio was significantly female-biased between January and August. Later in the year the ratio was 1 : 1. A bimodal CL distribution was recorded; with the emergence of larger individuals in some months. Lifespan appeared to be >1 year. Recruits, including several juvenile instars, were found between August and December. Ovigerous females were encountered from April to October but were more abundant from June to August. The phenology and duration of the breeding season were consistent with generic characteristics of the genus throughout subtropical and temperate East Asia.

The genetic structure of the vestimentiferan Lamellibrachia satsuma (Annelida: Siboglinidae) distributed in Kagoshima Bay, southwestern Japan, and the Nikko and Daikoku seamounts on the northern Mariana Arc, was analyzed using 14 newly developed microsatellite markers. We detected large genetic differences between the L. satsuma distributed in Japanese and Mariana waters, whereas no significant divergence was detected between local populations inhabiting the calderas of the two Mariana seamounts, which are separated by a distance of approximately 270 km. STRUCTURE analysis revealed that two genetically distinct clusters are dominant and no genetic differentiation was detected between sampling years or sites in Kagoshima Bay. We speculated that the Kagoshima Bay population is derived from individuals that have immigrated relatively recently from two undiscovered habitats in waters outside of the bay.

This is the first record of an epibiotic association between an acontiate sea anemone and a fish. We collected unidentified epibiotic sea anemones living on the necto-benthic fish Inimicus japonicus (Cuvier 1829) from the Seto Inland Sea, Japan. Molecular analyses revealed that the anemones were most closely related to Verrillactis paguri (Stimpson in Verrill 1869) sensu England (1971), which has a mutualistic relationship with hermit crabs. Therefore, we tentatively assigned the collected species as Verrillactis sp. (Actiniaria: Sagartiidae). This finding of sea anemones using fish as substrate is unprecedented. Future research into this relationship to determine if this is an incidental occurrence or facultative symbiosis is warranted. Our findings contribute to a wider understanding of symbiosis in sea anemones.

The polychaete genus Tomopteris is an important component of the zooplankton and is also known for its communication using bioluminescence. However, there have been few studies about the species diversity and ecology of Tomopteris because of taxonomic and handling difficulties resulting from the fragility of their bodies. In this study, we performed DNA barcoding and species delimitation analysis to better understand the species diversity of Tomopteris using 17 specimens collected on a research cruise conducted off the eastern Kii Peninsula, Japan. Due to damage to the detailed morphological characteristics necessary for identifications based on the current taxonomy, none of the specimens was morphologically identifiable at the species level. The analyses of the mitochondrial cytochrome c oxidase subunit I (COI) and nuclear large ribosomal subunit (28S) RNA gene sequences revealed 7–8 molecularly delimited species among them, which was greater than the number of species currently recognized in Japan. These results suggest that the traditional morphology-based taxonomy overlooks the taxonomic complexity of Tomopteris, thus a combinatory analysis of both morphology and DNA barcoding will be desirable for assessing the true species biodiversity of Tomopteris.

The Paracalanus parvus (Claus, 1863) species complex is a common marine calanoid copepod found in the world’s oceans. Recent genetic analyses of its specimens from the world oceans revealed that the complex in the western North Pacific consists of three species, i.e., P. indicus Wolfenden, 1905, P. tropicus Andronov, 1977 and an undescribed species. We assign the last one to be Paracalanus orientalis n. sp. by comparing it with previous morphological descriptions of related species. Both sexes of the new species are fully described based on genetically identified specimens from the surface layer of the south of Japan. The new species can be reliably distinguished from P. indicus and P. tropicus by the hunchback shape of the female if it is clearly present. Among the three species, the high length:width ratio of the third exopod segment of leg 4 is also characteristic of the new species, which would be identifiable, in most cases, by having a ratio of 5.4 or more.

Freshwater jellyfish comprising the genus Craspedacusta are thought to have originated in the Yangtze River, China and have since spread to all continents except Antarctica. In this study, jellyfish were collected from Haruta-ike, an artificial pond in Chikuma City, Nagano (Japan). Medusae were identified as Craspedacusta sowerbii using morphological and molecular techniques. Despite the existence of Haruta-ike since prior to the Edo era (ca. 1603), this work represents the first published record of freshwater jellyfish in this pond. Herein, we report on the novel swimming behavior documented in this population, which includes both male and female C. sowerbii medusae. Additionally, we discuss the life cycle of polyps reared in culture from Machikane-ike, a pond in Osaka for which we have published the first complete mitochondrial genome of C. sowerbii from Japan. Finally, we report on the morphology and life cycle of the rare Japanese freshwater jellyfish Astrohydra japonica in Lake Biwa (Shiga), documented only a few times in the 40 years since its original discovery in Japan. The results of our robust phylogenetic analysis using the 16S rRNA gene and COI markers of C. sowerbii and A. japonica in this study and for C. sowerbii material from Singapore, together with all publicly available sequences for these markers for the two species worldwide, revealed two major C. sowerbii clades suggesting the Nagano and Osaka populations originated from two distinct introduction events. This collaborative research was made possible through international collaborations among multiple research facilities, museums and one wildlife reserve.