Other than the vesicomyid bivalves that were already reported elsewhere, additional interesting bivalve and gastropod species were found among the molluscan samples collected from the Nankai Trough by the ROV Kaiko during April-May, 2001. These include two new gastropods Calliloncha nankaiensis n. sp. and Phymorhynchus turris n. sp. A protobranch bivalve, Nucinella viridis was re-discovered for the first time since the original description fom a locality some distance away from the type locality.
An ovulid gastropod collected from off the Boso Peninsula in Honshu, Japan, is described as a new species Aperiovula aurora. It is morphologically most similar to Aperiovula jeanae Cate, 1973, from which it can be distinguished by its thinner and translucent shell and callously produced posterior terminal ; the sharper and stronger denticles on the narrower outer lip ; the wider aperture, and the lack of both a yellowish marginal line and a funiculum.
A new shallow water Chama species is described based on shells collected from Melanesia, Polynesia, southern Japan and Aldabra Island, Indian Ocean. The species attaches itself to the substrate by a narrow portion of the left valve. The right valve is subcircular, slightly inflated, and bears three dark brown spiral streaks on the outer surface. The outer surface is ornamented with commarginal fluted lamellae, giving a honeycomb-like appearance. The left valve is nearly smooth and well inflated. The shell is composed of two aragonitic layers. This new species clearly differs from Chama japonica Lamarck by its characteristic honeycomb-like ornamentation.
An octopus collected by a bottom trawl survey off Tajima Region in the western part of the Sea of Japan, was identified as Octopus longispadiceus (Sasaki, 1917). A total of 506 specimens were examined either in raw, defrosted or formalin fixed conditions. Both females and males were collected from depths of 175 m to 300 m, and mature individuals appeared all year round. The sex ratio is nearly 50 %. The maximum size is 458 mm in TL(total length) in males and 485 mm in females in raw condition. This species is mediumsized and mature at 240 mm in TL in males and 265 mm in females. The mantle surface is generally smooth. The dorsal mantle length is about 25 % of TL. The mantle opening extends about half of the body circumference. The funnel organ is thickly W-shaped with a long central ramus in formalin fixed condition. The total number of gill lamellae is 20-23. The arms are slender, with a formula of I>II>III>IV, and the longest arm is about 75 % of TL. Suckers are biserial, about 130 in number on arm I of a male specimen of 300 mm in TL. Males have enlarged suckers at 15-30th from the base on all arms. The right third arm in mature males is hectocotylized and longer than the left third arm. The hectocotylus is slenderly conical and of about 6 % of TL. The terminal organ is crescent-shaped in raw condition. The maximum length and diameter of the spermatophore are 90 mm and 1.4 mm, and those of the egg are 22 mm and 9 mm, respectively. An ovary posesses 30 to 50 eggs. Judging from egg size, newly hatched juveniles crawl on the seabed, with no planktonic phase.
Density in relation to habitat of the alien freshwater snail Potamopyrgus antipodarum and the indigenous Semisulcospira spp. were investigated for a year in a channel in Moriyama City, Shiga Prefecture, central Japan. Water depth, velocity, water temperature, DO, pH, EC, Ca^<2+>, COD, NH^<4+>, NO^<3+> and substratum were measured and analyzed as environmental factors. As a result of variable selection following Hayashi's Quantification Theory Type I, velocity and substratum were selected most frequently by both P. antipodarum and Semisulcospira spp. The density of P. antipodarum was low throughout the year on a pebble substratum, whereas other combinations of environmental factors and species varied between sampling seasons. Laboratory experiments showed that adult Semisulcospira had no preference for any kind of substratum, whereas juveniles preferred a tile bottom with algal cover to a sandy bottom without cover. Adult P. antipodarum preferred a tile bottom with pebble cover or a sandy bottom with algal cover whereas the juveniles preferred substrates with any kind of cover. Adults and juveniles of both species preferred a slow current of less than 10 cm/s. The large gap between field survey and laboratory experimental results suggests that the tolerance ranges of substratum and velocity are much broader than the preferred ranges of both snail taxa. Juvenile P. antipodarum was less tolerant of water flow, suggesting that populations cannot establish themselves in turbulent flow on a coarse substrate.
The relationship between body length and color of Elysia ornata (Swainson, 1840) was examined by laboratory experiment and field observation. The results indicated that the difference in body color is not an individual variation but is related to growth. Body coloration follows a sequence from transparent, pale green, dark green, greenish white and to beige, with the length reaching maximum size at dark green, and decreasing thereafter. Based on these results, it was inferred that E. ornata has an annual life cycle beginning in late spring.
We collected and observed Corbicula samples in the Lake Biwa-Yodo river system and Yamato river system to determine whether invasion of the exotic Corbicula fluminea has occurred. In the Lake Biwa-Yodo river system, C. fluminea has invaded, and the indigenous C. leana has disappeared in the Yodo River. C. fluminea consisted of diploids and triploids. These samples were hermaphrodites and produced non-reductional sperm, suggesting androgenetic reproduction. We could not find C. fluminea in the Yamato river system, and only the indigenous C. leana exists. The present study suggests that the invasion of exotic C. fluminea may result in the extinction of the indigenous C. leana.