Pupillary response under varying conditions of bright light and darkness was compared in three species of Cubozoa with differing ecologies. Maximal and minimal pupil area in relation to total eye area was measured and the rate of change recorded. In Carukia barnesi, the rate of pupil constriction was faster and final constriction greater than in Chironex fleckeri, which itself showed faster and greater constriction than in Chiropsella bronzie. We suggest this allows for differing degrees of visual acuity between the species. We propose that these differences are correlated with variations in the environment which each of these species inhabit, with Ca. barnesi found fishing for larval fish in and around waters of structurally complex coral reefs, Ch. fleckeri regularly found acquiring fish in similarly complex mangrove habitats, while Ch. bronzie spends the majority of its time in the comparably less complex but more turbid environments of shallow sandy beaches where their food source of small shrimps is highly aggregated and less mobile.

Monthly phytoplankton monitoring with primary productivity estimation using pulse amplitude modulation (PAM) fluorometry was conducted from May 2014 to March 2018 in Bingo-Nada, center of the Seto Inland Sea, Japan, where oligotrophication has been a concern. The average chlorophyll a concentration, 7.24 µg L⁻¹, was even higher than the values of previous studies; therefore, it should not be assumed that the primary productivity has decreased. However, some seasonal trends seemed to drastically change; one of them was notable diatom blooms from December to January. PAM fluorometry revealed that high Fv/Fm of the diatoms due to rich nutrients and high α values (initial slope of ETR versus light) enabled them to actively photosynthesize even under low-light conditions in winter. This winter bloom further caused severe nutrient depletion in spring, which resulted in notable drops in overall photosynthesis and the ETR-based primary production rate (PPR_ETR) in March or April. From May, Fv/Fm and rETR_max, which began to increase from their minimum values in April, peaked in July or August when the highest PPR_ETR was recorded. However, over 70% of the total phytoplankton population consisted of dinoflagellates, raphidophytes and silicoflagellates, not diatoms. The PPR_ETR then drastically decreased from August to September, regardless of the fact that there were rich nutrients in the entire water column. Rather, decreased light level, as well as increased turbidity caused by non-phytoplankton particles, especially those abundant in this season, might hinder photosynthesis in autumn, further resulting in nutrient-rich conditions leading to the winter bloom.

The evolutionary history and diversity of unionoid mussels in East Asia need to be clarified and would shed light on the formation process of the unique fauna of Japan. Unionoid mussels (Mollusca: Bivalvia) are unique models for understanding the process by which organisms have diversified before and after the formation of the Japanese archipelago. Unionoid mussels have poor dispersal ability, so it is thought that they would have been strongly influenced by the archipelago’s formation. Therefore, the speciation and diversification processes of mussels before and after the archipelago’s formation were investigated by analyzing the nuclear and mitochondrial DNA of a wide range of species, particularly those inhabiting East Asia. The evolutionary history and divergence time of these mussels were examined. Unionoid mussels were found to have higher endemicity than other freshwater organisms. Although most of the endemic unionoid mussels of Japan are likely to have diverged before the formation of the Japanese archipelago, some other Japanese unionoid mussel species, including species endemic to Lake Biwa, an ancient lake in Japan, potentially diverged after the Japanese archipelago began to separate from the continent. This suggest that adaptation to the unique habitat of the ancient lake has caused diversification in the mussels endemic to it.

This study aimed to estimate the feeding pressure of a Diadema setosum population on a barren ground in Kata Bay, Mie Prefecture, Japan, which is a temperate region. We combined data of the feeding rate of D. setosum from tank experiments and of the D. setosum population dynamics obtained from monthly surveys. We conducted tank experiments to clarify the relationships between the feeding rate of D. setosum and the water temperature and test diameter of the sea urchins. The feeding rate and water temperature were positively correlated over the range of 15°C to 30°C. The test diameter composition of the D. setosum population in Kata Bay was stable throughout the study period (June 2014 to May 2015). The results revealed distinct seasonal changes in the feeding pressure of the D. setosum population owing to temperature variations.

Spatial, temporal and vertical variation of distribution in Asian mussels (Arcuatula senhousia) were described via belt transect observations by scuba diving, and the mussel bed coverage in a brackish river was observed over a 3-year period (2012–2014). The spatial changes in the mussel bed distribution showed a similar pattern over the 3-year period: the mussel bed spread from the deeper part of the river toward the shallow part of the river. Temporal changes in the mussel bed distribution followed two patterns: one was expansion from spring to summer and retreat during autumn, and the other was expansion from late autumn to the following summer and retreat during autumn. The relationship between the mussel bed coverage and environmental parameters (salinity, dissolved oxygen, water temperature, and water depth) was determined using Pearson’s correlation analysis. To identify the major habitat of the mussels, the median values of environmental parameters were compared with six levels of biomass categories of the mussel bed coverage, by the Steel–Dwass test. We presumed that major habitats are found in areas with a salinity of 9.2–17.3. This result was obtained from continuous field data (e.g., mussel distribution, water environments, and depth) indicating that caused by the patchy spatial and temporal distribution of the mussel.

Shell-boring gastropod molluscs and crabs are important predators of many bivalve molluscs, and impact bivalve populations and food webs. When gastropods and crabs target the same bivalve prey, complex interactions between them arise that mediate their impacts. Kleptoparasitism and scavenging (K-S) interactions result when one predator, usually the gastropod, loses or abandons its prey, which is then stolen or scavenged by the other. Such interactions have thus far only been reported among nine crab-gastropod species groups when they fed on shared bivalve prey, but potentially occur among many others. This study used the Global Biotic Interactions (GloBI) database to search for records of shell-boring gastropods and crabs preying on the same bivalve to identify tritrophic bivalve-crab-gastropod systems in which K-S interactions may occur. In total, 871 such systems were found, which included 58 bivalve species that were each eaten by 1–8 species of shell-boring gastropods and 1–19 species of crabs. In the vast majority of these systems (>99%), no previous report of K-S interactions has been published, although in 334 cases the gastropod and crab have been reported to undergo competitive or predator-prey interactions, which may indicate that K-S and other complex interactions occur among them. The tritrophic systems identified in the present survey should be investigated in future studies to determine whether the identified crab and gastropod taxa do in fact undergo K-S interactions. Doing so will enable the development of a theoretical framework for assessing the occurrence, evolution, utilization, and impacts of these interactions in natural systems.

The community structure and life cycle of dominant chaetognath species were studied in a 0–1000–m water column in the Oyashio region, western subarctic Pacific, six times during the year from March 2003 to February 2004. The abundance of chaetognaths ranged from 3235 to 6691 inds. m⁻². Throughout the study period, ten chaetognath species belonging to nine genera were identified. Among the chaetognaths, Eukrohnia hamata accounted for 39–62% of the abundance, followed by E. hamata/Eukrohnia bathypelagica juveniles, Parasagitta elegans, and E. bathypelagica. The species diversity (H') of chaetognaths varied between 1.12 and 1.50. For E. hamata, individuals had a body length ranging from 2.3–23.8 mm. The abundance of juveniles fluctuated from 22.0–48.3% and was higher in June 2003. Based on cohort analysis, recruitment of E. hamata juveniles occurred from spring to summer, and they reached a body length of 8 mm in one year. The overwintered population showed rapid growth from June to October, when they reached 15 mm in body length. Eukrohnia hamata showed little growth in winter and reproduced the subsequent spring. Thus, a two–year life cycle of E. hamata is proposed for the population in the Oyashio region. For comparable information about the life cycle of E. hamata, an eight to ten–month generation length was reported for the population in the eastern subarctic Pacific. These regional differences in the generation length of E. hamata are attributed to differences in habitat temperature, with much higher temperatures in the eastern (3.8–6.0°C) than the western (2.3–2.9°C) subarctic Pacific.

Beach strandings of 31 Keesingia gigas Gershwin, 2014, occurred in Exmouth Gulf, north-western Australia, between 16 and 25 March 2016, with 17 strandings recorded in just two days. Between 28 March and 31 May 2017 there were another 54 reports of K. gigas, many of multiple individuals, mostly on the Ningaloo coast west of Exmouth. These events provided an opportunity to examine this large but rarely observed jellyfish, as the only previous specimen collected was the holotype, off Shark Bay in 2012. Of 19 specimens collected and/or photographed on the beach in 2016, overall medusa length ranged from 190 to 345 mm, except for two 80 mm individuals. All but one of the smallest lacked tentacles, consistent with the original description of the species, photographs and field observations recorded in this study. The tentacles observed on the smallest animal examined may represent ontogenetic variability. Clusters of nematocysts were observed on the velarial margin of the bell. Three confirmed envenomations of adult people in 2016 did not result in them developing symptoms of Irukandji syndrome, however among six confirmed envenomations in 2017, two stings to adults resulted in some systemic Irukandji syndrome symptoms. In 2016, most sightings of K. gigas were in Exmouth Gulf, while in 2017 almost all were on the ocean side of North West Cape, indicating distribution was not solely restricted to Exmouth Gulf. Prevailing winds in the five days leading up to strandings in 2016 also suggest the jellyfish had been transported from the ocean north of Exmouth and not from within Exmouth Gulf. Similarly in 2017, prevailing winds and waves were consistent with currents moving jellyfish from north to south, with the time course of observations beginning in the northern part of Ningaloo in March and finishing in the southern section in May. All known sightings of K. gigas have been made in the March to May period in five separate years. Public awareness campaigns by the Shire of Exmouth in 2016 and Department of Biodiversity, Conservation and Attractions in 2017 were successful in raising awareness of the potential threat to swimmers and beach users and also proved effective in obtaining reports of the distribution of K. gigas.

With a focus on building an inventory of subtropical species of epifaunal holothurians and completing an environmental assessment, SCUBA surveys were carried out in the Amami Islands of Kagoshima Prefecture in southern Japan. The following 12 nominal species belonging to seven genera (subgenera), as well as two as yet undescribed species belonging to two genera, were observed: Bohadschia argus Jäger, 1833; B. bivittata Mitsukuri, 1912; B. vitiensis (Semper, 1867); Holothuria (Halodeima) atra Jäger, 1833; H.(H.) edulis Lesson, 1830; Holothuria (Microthele) nobilis (Selenka, 1867); Personothuria graeffei (Semper, 1867); Stichopus chloronotus Brandt, 1835; S. hermanni Semper, 1867; S. naso Semper, 1867; Thelenota anax H.L. Clark, 1921; Euapta sp.; Synapta maculata (Chamisso et Eysenhardt, 1821); and Opheodesoma sp. In the semi-closed waters around the Amami Islands, epifaunal holothurians showed what seemed to be species-specific distribution patterns related to topographical features. Furthermore, ignition loss values of feces and sediments suggested that selective deposit feeding may be predominant in at least several of the species sampled.

Neutrodiaptomus formosus (Kikuchi, 1928) is a calanoid copepod found in freshwater ponds in Japan. In the original description of this species, the illustration of the female habitus and the diagnostic description of the genital double somite were based on the copepodid V stage. Since there have been no studies describing the adult female of N. formosus, we redescribed fully both sexes of adults. The female genital double somite has a prominent projection at the right posterior corner, which is unique in this genus. Some differences between N. formosus and a previous description of the congener N. tumidus were found in the segmentation and setation of the cephalothoracic appendages. A key to females of 11 diaptomid species occurring in Japanese freshwaters is provided, because the previous keys so far were based mainly on male characters.

The zooxanthellate scleractinian species Goniopora stokesi is widely distributed across the Indo-Pacific Ocean, and in Japan the northernmost records of this species are from Tatsukushi, Kochi on Shikoku, although these records are not associated with specimens deposited in museums. The species is unique among Goniopora in that it lives on soft bottom sediment, forming free-living colonies, and produces asexual daughter colonies, or ‘polyp balls,’ via budding from parent colonies. Here we report on a large G. stokesi community from Otsuki, Kochi, Japan, representing the northernmost specimen-based record of the species. Specimen-based records are important as verifiable baseline data in light of global warming and climate change, which is expected to drastically effect the marine flora and fauna of Kochi and surrounding areas.