We have investigated worldwide phylogenetic relationships of deep-sea Bathymodiolus mussels and their mytilid relatives by determining mitochondrial DNA sequences. We review herein their phylogenetic relationships and the evolutionary process deduced from studies of described and unidentiﬁed species collected recently from vents, seeps, sunken whale carcasses and wood. Phylogenetic analysis of the sequence data together with those from a database showed that the mytilid mussels were divided into six clusters and that the mussels in the subfamily Bathymodiolinae are split into four groups. Neither the subfamily Bathymodiolinae nor the genus Bathymodiolus were monophyletic, suggesting that it is necessary to reevaluate their classiﬁcation. In the evolutionary process of the conventional Bathymodiolinae, the group including only Tamu fisheri split ﬁrst, and the basal trichotomous split into the remaining three groups was followed by diversification of bathymodioline mussels in each group. The first group bifurcated into two subgroups, which include Bathymodiolus and Gigantidas species, respectively. The second group was subdivided into three subclusters containing Indo-West Pacific, Atlantic and eastern Pacific species respectively. The third group included two nominal species restricted to the western Paciﬁc. Species obtained from sunken whale carcasses and wood took the outgroup position to the vent/seep mussels with only one exception, Adipicola crypta from whale carcasses. Modiolus modiolus from shallow water was positioned more distantly to the vent/seep mussels. The ﬁndings indicate an evolutionary transition from shallow water to vent/seep sites via whale carcass/wood sites, supporting the "Evolutionary stepping stone hypothesis".
Recent specimens of "Turbonilla (Cingulina)" morsei Yokoyama, 1926, which was originally described from the early Pleistocene Sawane Formation, Sado Island, were found off the Noto Peninsula in the Sea of Japan. A search in literature revealed that the species is identical to one that has been recorded as "Tachyrhynchus sp." or "Turritellopsis acicula stimpsoni Dall, 1919" in the family Turritellidae. Based on the examination of the fossil type and the Recent material, this taxon is here transferred to the genus Acirsa in the family Epitoniidae (new combination). Recent specimens have been found only in the central part of the Sea of Japan, and are probably endemic to this area. In connection with this ﬁnding, the origin of endemic species in the upper bathyal depths in the Sea of Japan is discussed.
Differences in shell shape, spiral sculpture and periostracum serve to distinguish Ovulatibuccinum Golikov & Sirenko, 1988 from Bathybuccinum Golikov & Sirenko, 1988, and the former is hereby elevated to the generic level. Bathybuccinum bicordatum Golikov & Sirenko 1988 and Ovulatibuccinum fimbriatum Golikov & Sirenko 1988 are recorded from off northeastern Honshu, hereby extending the range of these species to the south and into Japanese waters. Three species are described as new: Bathybuccinum higuchii n. sp. (northeastern Honshu, Japan), B. yadai n. sp. (East China Sea, southwestern Japan) and Ovulatibuccinum perlatum n. sp. (northeastern Japan to Russia), representing a remarkable range extending of the genus Bathybuccinum to the south.
In the concluding paper of the series, the authors address a number of names that have been cited erroneously in literature as belonging to the genus Fusinus and/or occurring in Japan. Two forms that until now had not been assigned to named species are identiﬁed as further variants of F. perplexus, a third unnamed form is ﬁgured and there is brief discussion of some related genera and species. The identity of Fusinus beckii (Reeve, 1848) is conﬁrmed by the rediscovery of its lectotype and a second specimen from the Andaman Islands. Though originally described as a subgenus of Fusinus, Chryseofusus Hadorn & Fraussen, 2003 is here accorded full generic status. Lectotypes are selected for Fusinus beckii (Reeve, 1848) and F. gracillimus (Adams & Reeve, 1848). Fusus solidulus A. Adams, 1864 is considered a nomen dubium.
An unusual new species of the cephalopod family Sepiolidae, Heteroteuthis ryukyuensis n. sp., is described from southwestern Japanese waters. The examined specimens were vomited by the ﬁsh Etelis coruscans, which had been angled from a depth of 320 m near Yonaguni Island, Ryukyu Islands, southwestern Japan. This is a new addition to the genus Heteroteuthis in the northwest Paciﬁc.
Among deep-sea gastropods, the characteristics of their egg capsules can reveal important clues regarding early life history traits. The egg capsule of P. buccinoides in a deep-sea methane seep site in Sagami Bay, Japan was identiﬁed from partial mitochondrial DNA sequences. DNA sequence comparison supported the suggestion that co-occurring egg capsules belong to P. buccinoides. The egg capsule is translucent and semicircular in shape. Its dimensions average 10.8 mm in length, 5.3 mm in width and 4.2 mm in height. Capsules contained an average of 1098 eggs or larvae. P. buccinoides deposits larger egg capsules that contain a larger number of eggs and larvae than other known cold water turrids. The distribution pattern of the egg capsules was discontinuous and irregular but biased towards the shells of Bathymodiolus.
We verified the host fish species for the freshwater pearl mussel Margaritifera laevis and compared the distributions of M. laevis and its hosts in the Abira River in central Hokkaido. Among the six species and a mutant form of rainbow trout (Oncorhynchus mykiss), mussel glochidia were found only from the gills of the masu salmon (Oncorhynchus masou masou) with an infection rate of 84.2%. Some of the attached glochidia on the ﬁsh were well grown. Although O. mykiss belongs to the same genus as the masu salmon, glochidia did not parasitize O. mykiss including the mutant form. The distribution of masu salmon was limited to a 2-km reach in the upper third of the stream where the mussel was distributed. The result suggested that in the Abira River, most of the free-living glochidia had been ﬂushed down to die without encountering their host ﬁsh.