Scolionema suvaense is a medusa belonging to the order Limnomedusae, and commonly lives on seaweed and seagrass. The benthic polyp and planktonic medusa are both stages of the life cycle of S. suvaense. Most of the planulae produced by sexual reproduction by the medusae do not metamorphose into polyps but to motile frustules moving to the bottom, which then metamorphose into polyps. The polyps can asexually produce not only medusae but also motile frustules. At a low temperature (15°C), polyps mostly asexually produce frustules. Extension of the habitat area by motile frustules is suggested to occur during winter and early spring when the temperatures are low. With increasing temperature, the reproduction type of S. suvaense polyps changes to asexual production of medusae. In the medusa stage, S. suvaense can reproduce both sexually and asexually. The relative percentages of asexually-reproducing medusae vs. sexually-reproducing medusae were highest at 20°C. This coincides with the highest abundance recorded for S. suvaense medusae, which showed an abrupt increase between May and June. Moreover, with increasing temperature (over 25°C), the reproductive mode of S. suvaense medusae changes from asexual to sexual, and asexually reproducing medusae are not observed at such high temperatures. Changes in the reproduction type and fluctuations in the subsequent population dynamics of S. suvaense depend on the in situ temperatures.

Ammonium excretion by planktonic copepods is an important process supplying nitrogen to primary producers and other microbial activity in pelagic ecosystems. Because of methodological constraints, conventional analyses using sealed chamber methods have required the adoption of unnatural experimental settings, such as a small container, high density, or a long incubation for ammonium excretion measurements. Therefore, most of the estimated ammonium excretion rates are potentially linked to stresses on the experimental animals during incubation. In this study, the effects of container size, density, and incubation period on the ammonium excretion rates of five copepod species (Calanus sinicus, Eucalanus californicus, Metridia pacifica, Pleuromamma abdominalis, P. gracilis) were examined using a highly sensitive analytical method that has recently been developed to measure ammonium concentration at the nanomolar level. The results indicate that responses to experimental stresses are species-specific, while incubation in a small container was generally the most significant factor causing overestimation (up to 2.6 times) of the excretion rates. This study shows that this highly sensitive analysis method, which allows short incubations with a single individual in a relatively large-sized container is an appropriate method to estimate ammonium excretion of pelagic copepods in the field using the sealed chamber method.

The reproductive season and life span of the common estuarine polychaete Simplisetia erythraeensis (Annelida: Nereididae) was investigated by monthly sampling during the years 2015 and 2016 in two tidal flats at two different regions in southern Japan: Shigetomi in Kagoshima Bay, Kyusyu Island, a warm-temperate region and Ichi on Amami-Oshima Island, Ryukyu Islands, a maritime subtropical region. At both sites, one or two cohorts were distinguished simultaneously using monthly body-width histograms. Recruitment of new cohorts occurred from August in Kagoshima Bay and from July on Amami-Oshima Island. Ovigerous females were observed from March to August in the former area and from April to July in the latter area, with the highest female maturity rate (number of females with oocytes/total number of adults larger than the smallest mature size) in July in both areas. The maximum value of the mean oocyte size of each female increased with increasing female maturity rates. The density of each cohort decreased from May onward because of the death of adults after reproduction, which resulted in the complete disappearance of the cohort in early October. Our results showed that the life span of S. erythraeensis was one year, and the reproductive period occurred in summer.

The vertical distribution of eggs and larvae of Maurolicus japonicus were described based on discrete depth oblique tows of a larva net seasonally collected during both day- and nighttimes at a 130 m-depth station near the continental shelf of Tosa Bay. The abundance and diel differences in the vertical distribution of eggs and larvae in relation to physical properties of the water column were also examined. Both eggs and larvae occurred only in spring and neither occurred in other seasons. In spring, water temperature, salinity and specific gravity were less stratified in the water column with a modest upwelling to ca. 100 m-depth, around which the seasonal temperature hardly changed. Unlike the Japan Sea, with a year-round reproduction of M. japonicus, the limited occurrence of eggs and larvae in Tosa Bay was likely related to upwelling in April. Eggs were twice as abundant at night- than daytime, suggesting that this species spawns at night. Most of the eggs were in A and B-stages and aggregated at 30–70 m depth during both the day and night. Larvae that were mainly in preflexion without yolk and flexion stages, were found in the 110–70 m and 130–90 m depth layers during day- and nighttimes, respectively. Earlier larval stages were dispersed abundantly from 130-m to the surface during daytime, with little correspondence to light attenuation or chlorophyll-a concentrations. Compared to the Japan Sea, Tosa Bay yielded various mesopelagic fish species that inhabit and breed in the mesopelagic zone, so that the reproductive niche would be temporally and spatially limited for M. japonicus.

In order to identify the food sources of the intertidal snail, Batillaria multiformis, stable isotope (δ¹³C, δ¹⁵N) and gut content analyses were conducted. Snails and their possible food sources were collected from the river-sea transects at the mouths of the Iroha and Katsura rivers in northeastern Kyushu, Japan, which differ in land use parameters (e.g., the ratio of agricultural area in the catchment area). There was a shift in the δ¹³C of the snail along the transects, showing higher values at the upstream stations (−15.8 and −14.8‰) and lower values (−17.5 and −17.1‰) at the seaward stations. The δ¹⁵N showed no significant spatial gradient along the transects, but a significant difference was observed (t-test, p<0.001) between the Iroha (mean, 10.0‰) and Katsura (11.1‰) rivers. A Bayesian mixing model and the biomass of possible food sources on the tidal flats showed that (1) the major food sources were marine phytoplankton, seaweeds, and benthic microalgae, and that (2) the dependency on marine phytoplankton increased in the seaward stations. However, gut content analysis revealed that most of the identifiable dietary items across all stations were benthic diatoms, which is considered to be due to the short-term result of ingestion on the sediment surface at low tide. Increasing dependency on phytoplankton at seaward stations was considered to be due to consumption of suspended particles in the water column at high tide by filter feeding. Differences in the δ¹⁵N between the two rivers suggest the possibility of using the δ¹⁵N of snails as an indicator of watershed characteristics.

Specimens of the intertidal snail, Batillaria multiformis were collected from 25 river mouths in the Kunisaki Peninsula and the Nakatsu and Usa districts in southwestern Japan to assess the relationships between their isotopic compositions (δ¹³C and δ¹⁵N) and watershed characteristics. Among 22 watershed parameters, which were classified into the land use, human population, topography, geology, and vegetation in the catchment area of each river, five parameters were significantly correlated with the δ¹⁵N of snails (Pearson’s correlation coefficient r: all p<0.05). The δ¹⁵N values increased with a decrease in the inclination of the ground, volcanic rock rate, and forested land rate and with an increase in the cultivated land and paddy field rates. The δ¹³C and δ¹⁵N analyses showed minor contributions of terrestrial plants to the diet of snails and their dependence on estuarine and marine producers, mainly on benthic microalgae, for food. These findings suggest that the δ¹⁵N values of intertidal snails were determined by the consumption of estuarine and/or marine producers, which assimilated nutrients derived from the watersheds.

The release of antibiotics to the environment after use by fish farms could have potentially negative effects. Here, we assessed how the antibiotic oxytetracycline (OTC) affect the meiofaunal communities of Jeju Island in Korea. We assessed how: 1) nematodes responded to 0–2000 ppm OTC over 48 h; 2) Metoncholaimus spp. (the most abundant group) responded to 0–500 ppm OTC over 48 h; and 3) the meiofaunal community responded to 0–200 ppm OTC for 1-month. In experiment 1, all nematodes died at high antibiotic concentrations (1000 ppm, 2000 ppm), but death rates rapidly decreased at 500 ppm (42%) and below (0–9%). In experiment 2, Metoncholaimus spp. death rates were similar to those documented in experiment 1 for OTC concentrations of 500 ppm (45%) and decreased at 125 ppm and 250 ppm (to 30% and below). In experiment 3, long-term exposure to low concentrations (200 ppm or less) of OTC did not have a strong effect on meiofaunal density, with meiofaunal density in the experimental group showing no decline with OTC concentration. However, we confirmed that differences in OTC concentration affects the structure of meiofaunal communities, making it important to regulate their use by fish farms.

The annual macroalga Sargassum horneri often forms a dense canopy at the sea surface. To evaluate the effects of the vertical structure and presence of the sea-surface canopy of S. horneri on the distribution of its epifauna, we collected epifauna from four different vertical portions of S. horneri at Kitsunezaki, west coast of Oshika Peninsula, Miyagi, Japan. The vertical portions of the thalli were defined as follows: sea surface (S), the portion that was always lying on the sea surface; intermediate (I), which was periodically lying on the sea surface with tidal changes; underwater (U), which was always submerged; and bottom (B), the portion around the holdfast. The mean total density of epifauna in the sea-surface portions (S and I) was significantly higher than that in the underwater portions (U and B). Harpacticoid copepods and bivalve spat accounted for more than 90% of the epifauna in all portions. It seems that the canopy of S. horneri collects bivalve larvae and other epifauna from the water column by sweeping the sea surface with the changing of the tide. We proposed the ‘Seasonal Coincidence Hypothesis’ to explain the dense occurrence of bivalve spat on S. horneri in this study.

Caprella hakuhoae sp. nov. (Crustacea: Amphipoda: Caprellidae) is herein described based on specimens collected from the landward slope of the 522 m deep Japan Trench, in the northwestern Pacific Ocean. This species is closely related to Caprella fimbrillata Vassilenko, 1993, recorded from the continental slope (605–1200 m) of the Chishima/Kuril-Kamchatka Trench, also in the northwestern Pacific Ocean. Caprella hakuhoae sp. nov. is distinguished from C. fimbrillata by the lack of paired mid-dorsal projections on pereonite 2, the presence of paired dorsal projections on pereonite 6, the presence of two pairs of dorsal projections on pereonite 7, and the merus being shorter than the propodus on pereopod 7.

The rock-boring sea urchin, Echinostrephus molaris, excavates soft rocks, and the pits are later used by non-boring sea urchins such as Echinometra mathaei and Anthocidaris crassispina. The pits used by these non-boring sea urchins are also characteristically inhabited by a limpet-like algivorous trochid snail, Broderipia iridescens (Gastropoda, Trochidae). To determine how this unique symbiotic association is maintained, we observed reproduction and larval development of the snail, and performed assays to detect what induced settlement in the snail larvae. As candidates of settlement inducers, a habitat biofilm plate, the host and the non-host sea urchins, and conspecific snails were kept in separate glass bowls, and newly hatched B. iridescens veliger larvae were introduced to each bowl. Sixty to 80% of larvae settled in bowls that contained the non-boring host sea urchins and conspecific snails. On the other hand, only approximately 8% of the larvae settled in bowls containing the boring non-host sea urchins. This is the first report demonstrating that larval settlement of an algivore is induced not by the habitat biofilms, but by the symbiotic hosts.