Population traits of two species of oysters, Saccostrea cucullata (= S. mordax) and Saccostrea kegaki were investigated every year from 1991 to 2011. The study site was located near the northern limit and around the center of the geographic ranges of S. cucullata and S. kegaki, respectively. The recruitments of these species fluctuated significantly year to year. The new recruits of S. cucullata often disappeared before joining into the older-year cohort, whereas those of S. kegaki constantly merged with the older-year cohort. The growth increment in shell length of S. cucullata was 5–10 mm / year up to 40 mm in shell length and that of S. kegaki was 5 mm / year up to 15 mm, and the growth of the both species slowed down subsequently. The age of S. cucullata of 60 mm in size and that of S. kegakiof 20 mm in size were estimated to be 15 and 4 years, respectively. The ratio of dead empty shells of these species in the field decreased in winter when the density of their predator, muricid gastropods, decreased. The ratio of empty shells with holes drilled by the muricids was 36% for S. cucullata and 30% for S. kegaki. Of these shells, the proportion of juveniles (<10 mm) was 83% in S. cucullata and 8% in S. kegaki. The survivorship curves suggested that the mortality of juveniles of S. cucullata was higher than that of S. kegaki. Mass mortality of S. cucullata occurred after a period of extremely low air temperature in January, 2011, during which the temperature was the lowest in the last 30 years. In contrast, the effect of this cold wave on S. kegaki was not serious. The decrease in air temperature was drastic compared to that in the water temperature, and the mortality of S. cucullata was higher at higher littoral levels, suggesting a more serious effect from air temperature than from water temperature. The population ecology of Saccostrea cucullata thus shows characteristics of a tropical species at the northern front of its range, in that the recruitment was inconsistent and the adults were vulnerable to episodic cold weather, compared to the temperate congeneric species S. kegaki.
The karyotypes of three lotic snails, Semisulcospira kurodai, S. habei and S. fluvialis were surveyed at 3 localities in Japan, including the type locality of respective species. The observed diploid chromosome number was as follows: Semisulcospira kurodai 36, S. habei 18, S. fluvialis 26. Chromosome numbers and karyotypes were different from one another. Their karyotypes were different from three lotic species reported previously.
Here, we examined the suitability of the largemouth bass, Micropterus salmoides; common freshwater goby, Rhinogobius spp. (Rhinogobius sp. OR); and tadpoles of the American bullfrog, Rana catesbeiana as hosts for glochidia of the freshwater unionid mussel Unio douglasiae nipponensis. The hosts, to which the glochidia were attached for 2 h (density of glochidia: 500–1000 individuals/L), were cultured in tanks (6 L, 28 ± 0.5°C) for 16 days. The glochidia and juveniles that became detached from the hosts were counted daily. The juveniles appeared for 7 to 15 days after the glochidia became attached to the goby. Therefore, Rhinogobius spp. was apparently a suitable host on which over 74% of the attached glochidia metamorphosed to juveniles. However, only dead glochidia were observed between the first and forth days for the largemouth bass, and neither glochidia nor juveniles were observed for the American bullfrog. These alien species were apparently unsuitable hosts for U. douglasiae nipponensis.
Breeding season of Anemina arcaeformis was studied in a small creek at Osato-cho, in Kumagaya City, Saitama Prefecture, Japan. In September, females began to incubate eggs in their outer gills. Glochidia hatched until the beginning of January, and then females discharged them from late January to April, mainly in March.