Two new species in the cyclophorid subfamily Alycaeinae, Awalycaeus yanoshokoae Yano & Matsuda n. sp. and A. shiosakimasahiroi Yano, Matsuda & Nishi n. sp. are described from limestone areas in Hidaka-mura, Takaoka-gun, Kochi Prefecture and in Gyokuto-machi, Tamana-gun, Kumamoto Prefecture, respectively. Awalycaeus yanoshokoae n. sp. was firstly reported from fissure deposits in a limestone cave as an undescribed fossil species, but living individuals have since been found near the cave. It can be distinguished from other Awalycaeus species by its large shell, with sparser axial ribs from the earlier whorls to end of the sutural tube, and by its parietal lip being detached from the body whorl. Awalycaeus shiosakimasahiroi n. sp. was previously recorded as "A. akiratadai", but it can be distinguished from the latter by having a non-reflexed outer lip margin, many axial ribs along the sutural tube, and a less expanded spire from the beginning of the sutural tube to the aperture.
The subtropical species Glycymeris fulgurata and the temperate species G. vestita are both mainly distributed around the Japanese islands. These species have shells that are so similar in form that their taxonomic treatments have been rather confused to date. To evaluate potential differences between these two species in the rate and pattern of their shell growth, their oxygen isotope profiles were determined using dead specimens collected from dredged sediments at a depth of approximately 40 m in Tosa Bay off the coast of Shiwa, Shimanto-cho, Kochi Prefecture. The clear growth rings located on the shell surface of both species correspond to the isotopically lighter or higher temperature peaks of the oxygen isotope profile. This characteristic suggests that the clear growth rings are formed annually. Based on this established association, we estimated the annual growth increments for both species living in Tosa Bay. The maximum shell height was approximately 43 mm for G. fulgurata, and approximately 62 mm for G. vestita, and the maximum age was 15 years for G. fulgurata and >30 years for G. vestita. The early shell growth rate was greater for G. fulgurata than for G. vestita. However, the rapid growth period lasted longer for G. vestita than for G. fulgurata, which corresponded with the longer life span of G. vestita. Thus, G. fulgurata and G. vestita found in Tosa Bay exhibit quite different shell growth rates and patterns. These observations suggest that they represent well-established, independent species with an overlapping geographic distribution originating after speciation presumably sometime in the Pleistocene Epoch.
Investigations of growth rates of marine mollusks are important for understanding paleoenvironmental conditions that influenced their evolution. Here we obtained oxygen isotope (δ18O) profiles for two Umbonium (Suchium) giganteum individuals collected from Sagami Bay on November 25, 1999, to verify the findings of a previous study that used population dynamics to determine growth rates. The oxygen isotope profiles for both individuals exhibited similar cyclic patterns of gradual increases and rapid decreases, with three maximum and three minimum values. Because this species inhabits areas that are not influenced by fresh water, these values represent seawater temperatures from spring 1997 to fall 1999. Profiles of both individuals were smooth in both summer and winter, suggesting that this species grows almost continuously throughout the year, with no major seasonal cessation of growth. The findings suggested that the prominent growth lines on the surfaces of the shells coincided with sudden decreases in the seawater temperature. Because this species mainly spawns in fall and winter when the seawater temperature decreases, these prominent growth lines can thus be considered to represent spawning events. This was supported by observations of other individuals that were collected at the same time, which suggested that spawning occurs once or twice each fall. However, spring or early summer spawning was inferred from the oxygen isotope analysis of microsamples from the older parts of one of the individuals, which would likely not have been recorded as prominent growth lines because of the greater growth rate during this season. Our results broadly confirmed those of a previous study on the rate and pattern of shell growth of U. (S.) giganteum. However, we found slightly higher growth rates than those reported by the previous study, particularly in juveniles.
Host species for the glochidia of the freshwater unionid mussel Cristaria plicata (Leach, 1815) were identified by determining whether the glochidia infected 27 fish taxa. The fishes were kept in tanks for 11–17 days after glochidial infection, and the numbers of glochidia and metamorphosed juveniles detached from the hosts were counted. Living juveniles of C. plicata detached from Tribolodon hakonensis, Pseudorasbora parva, Oryzias sp., Anabas testudineus, Trichogaster trichopterus, Trichopodus microlepis, Macropodus opercularis, Odontobutis obscura, Acanthogobius flavimanus, Gymnogobius urotaenia, G. castaneus, Tridentiger brevispinis, Rhinogobius giurinus, R. nagoyae and Rhinogobius sp. Therefore, these fishes were identified as suitable host species for the glochidia of C. plicata. Some native fishes that inhabit Anenuma Lake (e.g., Gy. castaneus) are considered to be useful local hosts. Moreover, some labyrinth fishes such as Anabas testudineus, which can climb out of the water and crawl over wet land, may disperse glochidia and juveniles over land in southern areas.
Feeding experiments on juveniles of the freshwater pearl mussel Hyriopsis schlegeli were conducted using upwelling chambers. Growth of juveniles mainly fed with a mixture of two green algae, Chlorella homosphaera and C. vulgaris, in 2013, was compared with that of specimens fed only with C. vulgaris in 2012. Juveniles in 2013 grew much faster than those in 2012 and their mean shell length exceeded 20 mm, achieving record maximum size. The result implied that using upwelling chambers and feeding small green algae are effective for successful culture of H. schlegeli.