We report the presence of the coral-killing sponge species Chalinula nematifera (de Laubenfels, 1954) in southern Japan via photographic records and specimens. The species appears to be a substrate generalist on scleractinians and occasionally octocorals, and is found across a wide variety of depths (0 to 24 m), as has been recently reported for this species from Indonesia. From our information, the species is found in southern Japan across the southern and central Ryukyu Islands, with a northern limit of Amami-Oshima Island, although future work will likely expand the range. As our records stretch back more than 20 years, we do not believe Chalinula nematifera is a recent arrival to the region. Due to the potential for this species to harm coral reefs, we suggest field surveys are needed to establish a baseline distribution and abundance database to aid in future monitoring.

Gastropods of the genus Mucronalia (Eulimidae) are known ectoparasites of brittle stars (Ophiuroidea). However, the identities of host species are only known for three of some ten described species in the genus. Here, we report associations of Mucronalia snails, including one new species, with ophiuroid hosts from Japan and New Caledonia: M. spp. aff. alba with Ophiomastix annulosa, Ophiomastix mixta, and Ophiocoma cynthiae; M. sp. cf. exquisita with Amphioplus (Amphioplus) sp.; Mucronalia ryukyuensis n. sp. with Ophionereis porrecta; and Mucronalia sp. with Amphiura velox. Mucronalia ryukyuensis n. sp. is described from Okinawa, southwestern Japan. The new species is characterized by its slender shell with a large body whorl and aperture compared with other species of the genus. Among Japanese congeners, the shell of the new species is most similar to that of M. exilis, but differs in having a teleoconch with less-convex whorls, regularly increasing in diameter. The present study corroborates Warén’s (1983) general rule that eulimids in a single genus parasitize hosts of the same echinoderm class, and our records suggest that closely related species of Mucronalia tend to exploit hosts of a more restricted lineage. We also briefly discuss the divergence processes of Mucronalia snails, which likely involve host shifts and geographic isolation.

The coral-killing sponge, Terpios hoshinota, has great potential to cover corals and threaten coral reefs. This sponge has numerous endosymbiotic cyanobacterial cells, silicate spicules in the tissue, and particles on the tissue surface, which could have an important role in the growth and defense of the sponge. However, little is known about the morphological traits of these components. Here we report the estimated density measurements of cyanobacteria by grinding the coral branch covered by the sponge without sponge tissue remains. Density was estimated to be 1.2 ×10⁷ cells cm⁻². We also obtained the densities for the spicules and particles which were 4.7×10⁵ cm⁻² and 1.3×10⁶ cm⁻², respectively. The spicule length of the Sesoko sample was shorter than those reported in previous studies. Size frequency distribution of the particles was measured for the first time, showing that the sponge selected silt-size particles with a mean size of 22.3 µm. We conclude that this study provides basic information on measuring the important components of T. hoshinota and is useful for comparing the measurements of those components in other coral reefs overgrown by this coral-killing sponge.

We demonstrated the genetic variation and population structure of a direct developing marine gastropod, Littorina horikawai which is endemic species distributed in the western coast of Kyushu, Japan and is found mainly on the adjacent islands. We conducted the field survey and sampling covering most of the distribution area of L. horikawai. In total, we collected 173 individuals from 13 localities (between Tsushima Island and Yakushima Island) and determined the sequences of mitochondrial DNA, the cytochrome b (CYTB) gene. We detected a significant genetic population differentiation in L. horikawai despite the limited distribution area of the western coast of Kyushu. Also, its haplotype distribution is mainly consistent with the geographical structure: the northern part of Kyushu, the central islands, and the southern islands. However, some haplotypes obtained from the Goto Islands which is located in the border between the northern part of Kyushu and the central islands do not correspond to the above geographic distribution. The high level of genetic differentiation of L. horikawai compared to other low dispersal Littorina species despite its narrow distribution range may be due to its predominantly fragmented island habitats. Although it is necessary for understanding the detailed evolutionary history of L. horikawai to perform the high-resolution genetic analysis, our present findings indicated that a high level of genetic differentiation of L. horikawai occurred even in a narrow distribution range because of not only the ecological traits linking to dispersal ability but also its discontinuous pattern such as the island inhabitation.

To assess the distribution of macrozoobenthos and environmental conditions after river flooding events offshore of the Mimi River, the water quality, benthic conditions and macrozoobenthos communities were investigated at five stations over a 6-year period. At the station closest to the river mouth, sediment median grain size was coarser than that at the other stations. Also, the macrozoobenthos community at this particular station was dominated by crustaceans and differed from that observed at other stations. Because these changes in sediment and macrozoobenthos were related to the size of floods caused by typhoon rains, it is suggested that the outflow of relatively coarse sand from the river affected benthic communities at this station by changing the sediment conditions. The three offshore stations detected relatively fine sediment and polychaete-dominated communities, and they found no evidence of flood effects on both their environment and macrozoobenthos. At the remaining station, the environmental conditions and the macrozoobenthos community were somewhat changed as a result of the flood events, and the median grain size and community composition differed from those of the other stations. These changes may have depended on the transport capacity of coastal currents for coarse sediments at each flood event. Our results show that sediment supply derived from river flooding could significantly modify benthic conditions, resulting in increased macrobenthic biodiversity offshore a river mouth.

In November 2020, a high biomass multi-species algal bloom caused heavy water discoloration in the fish and mussel farm areas of the Johor Strait, Malaysia. A total of 19 microalgal taxa were identified from the plankton samples collected during the bloom event. Eleven genera were diatoms, and eight genera were dinophytes. The microalgal composition was dominated by the diatom Guinardia sp., with an average cell density of 1.7×10⁶ cells L⁻¹, making up 65–80% of the phytoplankton composition. Concomitantly, high densities of the dinophytes Heterocapsa minima (3.8–5.3×10⁵ cells L⁻¹) and Karlodinium spp. (3.5–6.6×10³ cells L⁻¹) were found. This is the first record of the occurrence of H. minima in Malaysian waters. Detailed morphological observations of H. minima based on scanning electron microscopy are presented in this study. To have a better insight into the Heterocapsa species assemblages in the Johor Strait, the diversity of Heterocapsa species assemblages along the strait was investigated based on a metabarcoding approach. Environmental DNA collected between 2018 and 2019 was used for high throughput amplicon sequencing targeting the small subunit (SSU) ribosomal RNA gene marker. The metabarcoding analysis detected three rare Heterocapsa species in the waters, H. niei, H. rotundata, and H. steinii. The results showed that Heterocapsa species assemblages varied temporally across the strait, with higher species diversity and amplicon sequence variant (ASV) read abundances detected in the Eastern Johor Strait. Although no fish/shellfish kills were sighted during the 2020 bloom event, the presence of harmful microalgal species, such as Heterocapsa minima and Karlodinium spp., urged the need for a comprehensive HAB monitoring program in the Strait to safeguard the aquaculture industry in the areas.

Calanus sinicus is one of the most dominant planktonic copepods and plays a key role as a secondary producer in the mid-latitude coastal shelf areas of the Northwestern Pacific. In this study, we investigated seasonal changes in in situ egg production rate (EPR) of C. sinicus collected from March 2014 to February 2015 in Sagami Bay, Japan. Simultaneously, we collected formalin-preserved samples to measure reproductive index (RI), which was calculated as the proportion of individuals with matured gonad to total females in each sample. Monthly averages of EPR and RI varied from 2.4 to 44.9 eggs female⁻¹ day⁻¹ and 0% to 75.9%, respectively. RI showed a significant positive effect on EPR. Moreover, EPR decreased with increasing temperature, which was the inverse of the effect of temperature on EPR reported in a previous study in the Seto Inland Sea. The difference between the studies suggests that the local population may adapt to different temperature environments in each location. Using a stepwise multiple regression model, EPR estimates for C. sinicus in Sagami Bay were explained by the average water temperature and RI with high probability (R²=0.71). Although local water temperature ranges must be considered before applying our model to other areas, the model would be useful to reconstruct past secondary production of C. sinicus from preserved samples collected in other neighboring coastal areas.

Once-to-twice weekly sampling at nocturnal high tide for a year enabled us to detail the seasonal population dynamics, production, and feeding of the chaetognath Aidanosagitta crassa in a eutrophic inlet of the Inland Sea of Japan. The population density fluctuated from 1.0 ind (or 0.0029 mg C) m⁻³ to 3,370 ind (or 14.69 mg C) m⁻³, with a small, protracted peak in winter and a large, sharp peak in summer. The population consisted of 3 subpopulations, each of which produced 6–7 generations, yielding a total of 19 generations. In most generations, the mortality was notably higher in the egg to small juvenile (body length: ca. 2 mm) stage and in the adult stage than in the large juvenile stage. Basic population parameters, e.g., time to first maturity (range: 32–71 d), minimum size for reproduction (4.0–8.9 mm), mean body length of mature adults (5.5–10.4 mm) and life span (34–96 d), were negatively correlated with temperature. The specific growth rates (0.07–0.22 d⁻¹) were a positive function of temperature, yielding an annual population production rate of 91.49 mg C m⁻³ y⁻¹. Only one third of A. crassa contained prey in the gut, and their feeding habit was completely carnivorous, with preference for progressively larger prey with their growth. Estimated carbon weight-specific feeding rates (0.01–0.81 d⁻¹) sustained net growth with a mean gross growth efficiency of 45% in warm seasons (≥ca. 15°C). This highly intensive survey of the A. crassa population serves as a benchmark for intra- and interspecific comparisons of ecophysiological and population parameters.