Quantitative samplings of the macro-benthic communities were conducted on the sandy tidal flats, Sashiki Tidal Flats, that face Yatsushiro Bay in Kumamoto Prefecture, to monitor the seasonal dynamics of the macro-benthic communities and the population dynamics of the short-neck clam, Ruditapes philippinarum. This clam is one of the most dominant species of the macro-benthic communities on the sandy tidal flats of the Japanese coast. The larvae produced in the autumn breeding season settled on the tidal flats between November and December. The density of the newly settled juveniles reached about 85,000 ind. m⁻² in January 2015, and 4,760 ind. m⁻² of the juveniles with shell length of 2 to 5 mm remained until April. However, the newly recruited cohort almost collapsed between May and June. The same phenomenon was observed on the tidal flats in the spring of 2014. It was, therefore, one of the main reasons why the adult clams were sparse on the tidal flats throughout the year. Our results suggest that the predation pressure caused by ray Hemitrygon akajei and gastropod Philine argentata was a major mortality factor that collapsed the clam population in spring. In spring, numerous dips were created by the rays on the tidal flats. It is likely that the young spats newly recruited in the autumn breeding season are predated on by these predators by the spring of the next year.

The faunal composition and ecology of aquatic oligochaetes (Annelida, Clitellata) in Japanese lakes have been summarized based on a literature survey. Since Annandale’s initial benthological research in Lake Biwa in 1915, aquatic oligochaetes in Japanese lakes were studied, mainly in terms of lake typology, until the 1970s. Subsequent taxonomic and faunal studies have recorded 40 oligochaete species belonging to five families in the profundal bottom of freshwater lakes on Japanese islands. The profundal forms are generally widely-distributed species, with parallel replacement with European species. A variant form of Branchiura sowerbyi, which is almost devoid of posterior gill filaments, is restricted to the profundal bottom in the North basin of Lake Biwa, and is the only unique representative of lake profundal oligochaetes in Japan. Lim­nodrilus hoffmeisteri and Tubifex tubifex were the most common oligochaete species in profundal bottoms, both occurring irrespective of the trophic status of the lake. Unique oligochaete compositions were found in the bottom of several deep and oligotrophic lakes in northern Japan, and often comprised subterranean species. The taxonomic position of Tubifex (Peloscolex) nomurai, which was described from deep profundal bottoms in Lake Tazawa, and became extinct in the 1940s has been discussed. The composition and abundance of oligochaete communities in Japanese dam-lakes are comparable to meso- and eutrophic natural lakes in Japan. Azoic zones occurred in four lakes owing to the acceleration of artificial eutrophication and global warming recently. Oligochaete diversity in Japanese lakes is poorly understood in littoral zones and brackish lakes, and taxonomic studies will reveal many additional species.

To date, there is limited information on the taxonomic composition of enchytraeids in Japan. In this study, keys to and diagnoses for the genera of Enchytraeidae and Propappidae have been provided. The keys cover 25 terrestrial, marine, and freshwater genera that have been recorded and are potentially distributed in Japan, and some local or dubious genera were excluded. The major diagnostic characters used in the keys were as follows: 1. shape of the prostomium; 2. presence/absence and location of dorsal pores; 3. shape and numerical pattern of chaetae; 4. shape and distribution of nephridia; 5. location of the dorsal blood vessel origin; 6. shape of the chloragocytes; 7. abrupt/gradual transition of the esophagus into the intestine, 8. presence/absence, shape, and position, and size of the esophageal appendages; 9. quantity, size, shape, and texture of the coelomocytes; 10. shape of the male funnel; 11. shape of the spermatheca and its attachment (or not) to the esophagus; 12. presence/absence, size, and shape of the testis sacs or seminal vesicles. In addition, the characters of each genus have been described.

Forty taxa of aquatic microdrile oligochaetes (Annelida, Clitellata) that belong to three families were recorded in inundated rice paddies in Japan on the basis of faunistic survey in 81 localities. To our knowledge, three naidine species, Bratislavia dadayi, Dero nivea, and Aulophorus hymanae, are new to Japan. The oligochaete assemblages primarily consisted of widely distributed and thermophile species and were often dominated by desiccation-tolerant species such as Aulodrilus limnobius, Bothrioneurum vejdovskyanum, and Branchiura sowerbyi. The faunal characteristics have been discussed in relation to rice paddy environments.

We studied the environmental remediation of polluted marine sediments using annelids and demonstrated that aquatic annelids had a high potential to lower the concentration of pollutant chemicals. We investigated the seasonal variation of dominancy of a marine oligochaete Thalassodrilides cf. briani (Naididae), which has high tolerance to pollution, in the benthic community in Fukuura Bay, Ehime Prefecture, Japan. In addition, the growth rates of oligochaetes under different temperatures were examined in our laboratory to establish an efficient oligochaete culture system. In the field studies, we investigated annual changes in the abundance and biomass of the oligochaete from 2011 to 2014 in Fukuura Bay. Throughout the investigation period, T. cf. briani dominated more than 60% of the total abundance of benthic communities, except in the winter of 2013 and 2014, and reached 121,400 individuals/m² in February 2012. From spring to summer, the average body weight per individual reached a maximum, and the proportion of mature individuals was the highest in summer. These results suggested that new generations of this species occur in summer. In the laboratory experiment, T. cf. briani was cultured at 15, 20, or 25℃, and the growth and maturity rates of individuals with egg sacs were measured. The growth and maturity rates cultured at 15, 20, and 25℃ after 30 days were 130%, 138%, and 160% and 1%, 23%, and 71%, respectively. Therefore, we determined that temperatures between 20 and 25℃ triggered T. cf. briani maturation, and culture at 25℃ was the most effective for inducing maturation.