A new species of Dianadema (Clavagellidae), Dianadema ikebei n. sp., is described from Japan and comparisons are made with the other three known Indo-West Pacific species of the genus. Dianadema ikebei n. sp. has been collected from two localities in waters of between 50–100 metres depth off central to southern Japan (Honshû). The species occurs cemented to the walls of borings made by other bivalves, such as pholadids and lithophagines, in weakly consolidated sedimentary rocks composed of sand and mud. It may also cement itself to dead conspecifics and other nestling bivalves, which are secondary occupants of such borings. This is the first record of the genus in Japan and the Northwest Pacific.
From 2008 to 2012, twenty-four specimens of the lucinid bivalve Elliptiolucina ingens were collected alive by beam trawl surveys of the T/S Nagasaki-Maru, Faculty of Fisheries, Nagasaki University, from a chemosynthetically nourished community west of Amami-Oshima Island, Japan between depths of 601 and 646 m. Based on conchological examination of the new material, the species diagnosis provided in the original description was critically revised with respect to the general shell shape and conditions of the muscle scars, lunule and ligament. The gross anatomy of the mantle characters revealed the presence of a long mantle fusion below the inhalant aperture and the absence of mantle gills and apertural papillae. Histological examination of the gill filaments confirmed the presence of a bacteriocyte containing endosymbiotic bacteria; the granule cells were also found in the inner part of the lateral zone. Phylogenetic reconstruction of the Lucinidae based on two nuclear rRNA gene sequences confirmed the previous assumption that Elliptiolucina belongs to the subfamily Myrteinae. The same dataset, in combination with previously published results, suggested that the Lucinidae is divided into two reciprocal sister clades, one consisting of Lucinidae + Codakiinae, the other of (Leucosphaerinae + Pegophyseminae) (Fimbriinae (Monitilorinae + Myrteinae)).
Growth-related conchological changes in the basal shell length in 7 species and 1 subspecies of the Japanese nacellid limpets Cellana nigrolineata, C. toreuma, C. grata grata, C. grata stearnsii, C. testudinaria, C. radiata, C. mazatlandica and C. enneagona were examined. Shell measurements demonstrated frequent growth-related proportional changes; shell width, shell height and internal colored area width showed relative increases in many species and subspecies. In particular, relative increases were shown in most shell regions in C. nigrolineata and C. mazatlandica. In contrast, relative shell height decreased with growth in C. enneagona, and there were no growth-related proportional changes in C. grata grata or C. radiata. Although relative apex position shifted backward with growth in C. mazatlandica, it stayed stable in the other species and subspecies. Meristic characters of radial stripes in C. nigrolineata and radial costae in C. grata stearnsii and C. mazatlandica all showed apparent growth-related increases. Although color pattern compositions in the internal colored area varied by species and subspecies, significant growth-related changes were observed in C. testudinaria and C. mazatlandica, i.e., new light color layers were gradually formed onto the dark brown basal layers observed in smaller individuals. In conclusion, many conchological characteristics of the Japanese nacellid limpets change with growth, and their patterns differ between species.
Extensive examination of sand samples and re-evaluation of collected material allow us to present for the first time the occurrence of four genera of Polyplacophora which were previously unknown from Hawaiian waters. Leptochiton sp. and Ischnochiton sp. are identified from isolated valves only. Lucilina cf. pacifica (Leloup, 1981) and Weedingia paulayi n. sp. are also represented by numerous isolated valves but their occurrence is confirmed by complete specimens. The latter, a species new to science, is described herein. It is morphologically distinguishable from its three congeners by a round tail valve with terminal mucro, a characteristic so far unknown for this genus. With the record of these four taxa, the shallow water chiton fauna of the Hawaiian Islands is increased from previously recorded five species belonging to four genera within four families to nine species belonging to eight genera within six families. While the genus Weedingia is a typical representative of the Polynesian fauna, the genus Ischnochiton is confirmed for the first time.
Two warm-water species, Pecten (Pecten) byoritsuensis Nomura, 1933 and Oxyperas (Oxyperas) bernardi (Pilsbry, 1904) have been obtained from the Lower Pleistocene Itaya Formation in Toyama Prefecture. Based on the fossil occurrences recorded by previous studies as well as this study, these species are rare and first appeared in the Early Pleistocene in the Japan Sea borderland. P. (P.) byoritsuensis became extinct at the end of the Early Pleistocene as a result of deteriorating environments associated with glacial periods in the Japan Sea. O. (O.) bernardi survived these conditions because of its wide distribution, as with Pecten (Notovola) naganumanus Yokoyama, 1920 and P. (N.) albicans (Schröter, 1802). Detailed examination of molluscan fossils from the Itaya Formation and their ages indicates that the Tagawa assemblage from the Lower Pleistocene Omma Formation in Oyabe City can be correlated with the Itaya assemblage, not with the Zukawa.
We report the typical shape and average size of the glochidia and juveniles of Cristaria tenuis in Ishigakijima Island, Okinawa Prefecture, Japan, together with some new host species. The shape of the living glochidia was subtriangular and hooked. The average length, height, and width were 221.5 μm, 228.0 μm, and 108.3 μm. The host species for glochidia of the freshwater unionid mussel Cr. tenuis collected from Ishigakijima Island, were identified by determining whether these glochidia infected the following fish taxa: Cyprinus carpio, Carassius auratus langsdorfii, Ca. auratus, Rhodeus ocellatus ocellatus, Misgurnus anguillicaudatus, Clarias fuscus, Oryzias latipes, Poecilia reticulata, Macropodus opercularis, Oreochromis sp., Eleotris melanosoma, Hypseleotris cyprinoides, Rhinogobius spp., Rhinogobius giurinus, and Monopterus albus. The fishes were kept in tanks for 9–17 days, and the numbers of glochidia and metamorphosed juveniles detached from the hosts were counted. The juveniles were found to detach from the bodies of Cl. fuscus, Ory. latipes, Po. reticulata, Ma. opercularis, E. melanosoma, H. cyprinoides, Rhi. sp., and Rhi. giurinus. The number of glochidia infecting the fishes, the infection site, and the rate of glochidial encystment were determined using formalin-fixed fishes of the taxa Ca. auratus langsdorfii, P. reticulata, Gambusia affinis, Ore. sp., E. melanosoma, and H. cyprinoides collected from Ishigakijima Island. Glochidia were found to infect two host species: Po. reticulata and Ore. sp. Most glochidia were attached to the gills and fins of the fishes. A higher number of glochidia were found to attach and encyst on Po. reticulata than on Ore. sp. Po. reticulata was identified as a suitable host species for the glochidia of Cr. tenuis, in addition to Rhi. sp. and Ma. opercularis.
The classification of land snails of the genus Trishoplita is difficult due to large intraspecific variation in the shell and internal morphology. We studied geographic variation in four subspecies of T. eumenes (mainly T. eumenes cretacea) and T. pallens, using statistical analyses of the shell morphology and character comparisons of the soft body. Overall, three types (Groups A–C) were distinguished in the shell morphology of T. eumenes cretacea. Among them, Groups A and B also had similar characteristics in terms of the branch number of the mucous gland and the shape of the love dart. However, more black spots were present on the mantle of individuals in Groups A and C than in Group B. Based on these characteristics, geographic differentiation of the three groups of T. eumenes cretacea is discussed. The subspecies T. eumeneseumenes and the related species T. pallens share similar morphological characters.
Allozyme variability in the androgenetic freshwater clams Corbicula leana from Japan and the exotic C. fluminea were investigated. A total of 462 individuals of C. leana from 19 localities throughout the distribution range of the species were monomorphic at three allozyme loci as well as in purple inner shell color. Corbicula fluminea from three localities, on the other hand, was variable in shell color; with white, purple, white with purple flash, or deep purple examples. The present results suggest that they may include several clonal lines with different shell colors and allozyme genotypes. We discuss possible diversification of freshwater clams among bisexual species and hermaphroditic clones through clonal capture, genome capture, or ploidy elevation.
We examined the effects of the anesthetic agent MS-222 on glochidial viability in Anodonta japonica, the parasitization rate of the glochidia on the host fish Gymnogobius urotaenia and the metamorphosis rate into juveniles in vitro. Snap frequency of the glochidial valves decreased with an increase in MS-222 concentration. Likewise, parasitization success of the larvae on the gills of G. urotaenia was reduced when the host fish was exposed to an MS-222 solution(150 mg-1L)for five minutes prior to introducing the glochidia to the fish. Metamorphosis success(juveniles/free-living larvae ratio)was reduced by 50% when the host fish was anesthetized with MS-222, though the effect of the anesthetic agent was not detectable statistically. These suggest that glochidial viability and parasitization rate data obtained from previous studies using MS-222 may have contained an artifact. Therefore, future studies on the parasite-host relationship between unionid glochidia and host fishes when MS-222 is used should consider the effect of the anesthetic agent on unionid glochidia.