A Mussel Colonizing Deep-Sea Whale Bones in the Northwest Pacific (Bivalvia: Mytilidae)
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VENUS 66 (1-2): 49-55, 2007 Benthomodiolus geikotsucola n. sp.: A Mussel Colonizing Deep-sea Whale Bones in the Northwest Pacific (Bivalvia: Mytilidae) 1Takashi Okutani and 2Jun-Ichi Miyazaki 1Japan Agency for Marine-Earth Science & Technology (JAMSTEC), 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan; [email protected] 2Faculty of Education & Human Sciences, University of Yamanashi, Kofu, Yamanashi 400-8510, Japan Abstract: A new mytilid mussel found colonizing a whale skeleton at a depth of 4020 m on the Torishima Seamount in the Northwest Pacific is described. It is the third named species in the genus Benthomodiolus Dell, 1987. Only minor superficial characters separate it from the two other known species; these are a lack of both periostracal hairs and radial lines and the fact that this new taxon is the only species in the genus living attached to whale bones. In addition, however, the morphology of the byssal muscles in this species is different from the other two. Keywords: new species, whale bone, Mytilidae, deep sea Introduction The skeleton of a whale of the species Balaenoptera edeni was discovered on the summit of the Torishima Seamount at a depth of 4036 m in 1992 during the Shinkai 6500 Dive 148 (Wada, 1993; Fujioka et al., 1993). Since then this site has been revisited from time to time, such as in 1993 (Dive 174: Naganuma et al., 1996), in 1997 (Dive 369), and again in 2005 (Dive 895: this paper). Colonies of a mytilid mussel observed on every part of the whale skeleton were sampled on each occasion, but they have been left unstudied up to this date. On the most recent dive by the manned submersible Shinkai 6500, the second author (JM) succeeded in collecting new biological samples, including this mussel. Morphological observations and preliminary molecular analyses (our unpublished data) of the present material led to the conclusion that this mussel represents a new taxon that may be allocated to the genus Benthomodiolus Dell, 1987. The description of new species is given below. Materials and Methods The Shinkai 6500 Dive #895 (July 13, 2005) took place at 30˚55´N, 141˚49´E, 4020 m in depth, on the flat summit of the Torishima Seamount (Fig. 1: see Fujioka et al. (1993), Wada (1993) and Naganuma et al. (1996) for topographical details and environment of the site). More than 150 specimens of the mussel were collected. Most of them were detached from two vertebrae (probably BB-8 and BB-10 of Fujioka et al., 1993), which were grabbed by the manipulator of the Shinkai 6500 and taken on board the support ship R/V Yokosuka. Some additional specimens were sucked up with a slurp-gun from the surface of the other bones in situ. All the material examined is preserved in JAMSTEC, except for the type material, which is deposited in the National Museum of Nature and Science (NSMT). 50 T. Okutani & J. Miyazaki Taxonomy Family Mytilidae Rafinesque, 1815 Genus Benthomodiolus Dell, 1897 Benthomodiolus geikotsucola n. sp. (Figs. 1-3) Type materials: Holotype NSMT-Mo 76703; paratypes NSMT-Mo 76704a-j (10 specimens), all collected by Shinkai 6500 Dive 895. Type locality: The flat summit of the Torishima Seamount, 30˚55´ N, 141˚49´ E, 4020 m in depth. Description: Shell modioliform, low anteriorly, higher towards posterior, rather thin, ornamented by irregular growth lines, covered by straw-colored periostracum. Periostracum in umbonal region usually eroded, remaining only around marginal portion in grown specimens, no periostracal hairs present. Umbo situated at about 1/3 anteriorly, weakly prosogyrous. Prodissoconch not observed as umbonal region usually heavily eroded. Antero-dorsal margin weakly convex. Anterior margin round. Ventral margin nearly straight in young specimens, but slightly concave in grown shells. Posterior margin nearly round, slightly slanting. Postero-dorsal margin nearly straight with obtuse postero-dorsal angle. Interior surface shining with pearly luster, with deep umbonal cavity. Anterior adductor scar oval, but both posterior adductor scar and pallial line indistinct. Hinge plate very thin, edentulous. Ligament long, opisthodetic, occupying about 75-80% of postero-dorsal margin. Periostracum slightly inrolled along anterior, ventral and posterior margins. Both anterior and posterior adductors oval. Foot short, conical with slender byssal groove. Anterior retractor slender, originating from dorsal side of foot. Posterior pedal retractor very thin, branched near base of former, close to termination of anterior bundle of posterior byssal retractor, which is thick and cylindrical. Posterior bundles of posterior byssal retractor very long, thin, running axially with ramus of both sides set very close to attachment point above dorsal rim of posterior adductor. Gill thin, rather low in height. Mantle margin frilled in anterior part, but less undulating posteriorly. Inhalant and exhalant siphons barely separated by delicate septum. Measurements (in mm): Holotype: SL (shell length) 42.5 × SH (shell height) 18.5 × SW (shell width) 13.6. Selected paratypes (SL × SH × SW in mm): 31.7 × 13.9 × 10.1; 28.9 × 11.7 × 9.8; 27.7 × 12.0 × 10.2; 27.5 × 11.3 × 9.3; 26.1 × 10.2 × 9.0. Distribution: Torishima Seamount (whale remains) － 4036 m (Shinkai 6500 Dive #148), 4037 m (Dive 174) and 4020 m (Dive 895). Etymology: The species name is comprised of geikotsu-, Japanese for “whale bone”, and -cola, the Latin suffix meaning “attached to”. Discussioin The genus Benthomodiolus was established by Dell (1987) with the type B. lignocola Dell, 1987, a deep-sea species from New Zealand. The genus until now has included one more species, B. abyssicola (Knudsen, 1970) from the Gulf of Panama. Both species were found attached to sunken wood. The present species is the third member of this genus and first one byssated to whale bone. Dell (1987) emphasized the generic characters separating this genus from related genera such as Adipicola Dautzenberg, 1927, Idasola Iredale, 1915 (= Idas Jeffreys, 1876) and Mytella A New Mytilid Species from Deep-sea Whale Bones in NW Pacific 51 N 40˚ 35˚ 30˚ 25˚ 20˚ 15˚ E130˚ 135˚ 140˚ 145˚ 150˚ km 0 200 400 Fig. 1. Map of Torishima Seamout ( ★ : Diving site of Shinkai 6500 Dive # 895). 52 T. Okutani & J. Miyazaki A Ｄ B C D E D E Fig. 2. Benthomodiolus geikotsucola n. sp. A. Holotype, NSMT-Mo 76703. B, C. Paratype, NSMT-Mo 76704. D. Specimens attached to a vertebra taken on board the R/V Yokosuka. E. In situ view of whale bones colonized by B. geikotsucola n. sp. Soot-Ryen, 1955, noting that its species have a unique arrangement of pedal and byssal muscles, possess periostracal hairs and lack minute dentition along the hinge line. The present new species agrees well with the definition of Dell’s genus, particularly in regard to the muscle configuration, such as the offshooting of the thin posterior pedal retractor from the root of the anterior retractor (Dell 1987: figs. 51-53). This species is, however, different from B. lignocola and B. abyssicola in the following ways: (1) It has very long posterior bundles of the posterior byssal retractor, the right and left rami of which are set very close to each other. (2) It lacks periostracal hairs. A New Mytilid Species from Deep-sea Whale Bones in NW Pacific 53 pr apb l ar vm ppb es aa pa f mm b is g Fig. 3. Configuration of muscles of Benthomodiolus geikotsucola n. sp., in sagittal section. Abbreviations: aa – anterior adductor; ar – anterior retractor; apb – anterior bundle of posterior byssal retractor ;b – byssus; es – exhalant siphon; f – foot; g – gill; is – inhalant siphon; l – ligament; mm – mantle margin; pa – posterior adductor; ppb – posterior bundle of posterior byssal retractor; pr – posterior byssal retractor; vm – visceral mass. (3) It lacks delicate radial lines on the shell surface. (4) It is found attached to whale bone, not on wood. According to Bottjer & Carter (1980) and Ockelmann (1983), the hairs of mytilids are not of periostracal origin but created by the foot, and should be called “byssal hairs”. Such hairs are occasionally present and sometimes lacking among Idasola and Bathymodiolus species. Thus the presence/absence of hairs is probably less significant as a “generic” character. The present habitat information shows that the species of Benthomodiolus and related deep- sea genera are associated with two different kinds of organic remains, wood and whale bones, but it is not clear if the substratum preference is consistent or species-specific. Dell (1987) wrote that although the type series of B. lignocola was originally discovered in borings in a large waterlogged log (whence the name) at 1075-1100 m, another specimen possibly belonging to this species was observed in a photograph (but not collected) at the same site as an Adipicola osseocola sample found attached an abandoned trawl cod end filled with fish bones, at a depth of 880 m. But it is unclear whether that B. lignocola was attached to the fish bones or to the trawl- net material. Assuming the specimen in the photograph was correctly identified, that record would be evidence that B. lignocola may attach to bones as well as wood. The origin of the type specimens of Idasola washingtonia is not known (Bernard, 1978). All nine lots of this species examined by Dell (1987) were from sunken wood between 800 m and 3100 m. However, Smith et al. (1989) and Bennett et al. (1944) reported it from whale bones in the Santa Catalina Basin, at a depth of 1240 m. Distel et al. (2000, Fig. 1) listed it from both whale bones and sunken wood. From these reports and gene database, I washingtonia seems to colonize both whale bones and sunken logs. However, such an alternative substratum selection seems to be uncommon.