Benthomodiolus Dell, 1987

Oliver, P. Graham, 2015, Description and morphology of the " Juan de Fuca vent mussel ", Benthomodioluserebus sp. n. (Bivalvia, Mytilidae, Bathymodiolinae): " Phylogenetically basal but morphologically advanced ", Zoosystematics and Evolution 91 (2), pp. 151-165 : 151

Taxon classification Animalia Mytiloida Mytilidae

Genus Benthomodiolus Dell, 1987

Type species.

Benthomodiolus lignocola Dell, 1987

Definition.

To 43 mm, Thin, umbonate, narrow modioliform, weakly arcuate with beaks about ¼ distance from the anterior, anterior margin rounded only a little narrower than rounded posterior margin. Median area slightly sulcate, widest part behind the umbos. Hinge margin lacking crenulations, ligament sunken, very long. Periostracum persistent, smooth or with sparse hairs. Pedal/byssus musculature in two groups, a posterior set close to the posterior adductor muscle and a median set attached to rear of the umbo. Mantle edge mostly free, poorly frilled, posterior junction short. Hind-gut with a short or very short loop. Ctenidial filaments linear or laminar. Symbiotic bacteria are extra-cellular.

Species included.

Benthomodiolus lignocola Dell, 1987; Benthomodiolus geikotsucola Okutani & Miyazaki, 2007; Benthomodiolus erebus this paper; Benthomodiolus abyssicola (Knudsen, 1970).

Distribution.

Described species are restricted to the Pacific Ocean from New Zealand, Japan, British Columbia and Panama at bathyal to abyssal depths. An undescribed species from the South Atlantic is reported by Thubaut et al. (2013)

Remarks.

Benthomodiolus lignocola , Benthomodiolus geikotsucola and Benthomodiolus erebus are regarded as congeneric, based on a combined analysis of COI mtDNA and 28S rRNA ( Thubaut et al. in press). In the tree by Thubaut et al. (in press) Benthomodiolus lignocola is shown as the sister taxon to Benthomodiolus erebus with Benthomodiolus geikotsucola and an un-named species from the South Atlantic as sister taxa on a separate branch. Kyuno et al. (2009) and Lorion et al. (2013) show Benthomodiolus erebus and Benthomodiolus geikotsucola as sister taxa with Benthomodiolus lignocola on a separate branch. The tree by Kyuno et al. (2009) is based on the mitochondrial ND4 gene while that of Lorion et al. (2013) is based on combined COI, NADH4, 16S, nuclear 28S and histone 3 data. Modiolus abyssicola lacks supporting molecular data and is placed in Benthomodiolus primarily on the disposition of the pedal/byssus musculature ( Dell 1987, Thubaut et al. in press). Bouchet in WoRMS ( 2015) also includes Modiolus abyssicola in Benthomodiolus . However Gustafson et al. (1998) and Kenk and Wilson (1985) reported that the gill of Modiolus abyssicola was like that of Modiolus and thus probably a filter feeder and suggested that it should be placed in Modiolus . This species is revisited below and shown to have a more typical wedge shaped modioliform shell with sparse hairs. The gill filaments are linear and unlike the laminar filaments in Bathymodiolus as reported by Kenk and Wilson (1985). However it is shown below that the filaments of Benthomodiolus lignocola are also linear but that in both Benthomodiolus lignocola and Benthomodiolus abyssicola that the abfrontal surface bears polygonal microvillar structures typical of species with symbiotic bacteria. Regardless of the shell form it seems most likely that Modiolus abyssicola does belong in Benthomodiolus but it remains tentative awaiting confirmation from molecular data.

Consequently the generic diagnosis is based on a combination of characters from Benthomodiolus lignocola , Benthomodiolus geikotsucola and Benthomodiolus erebus . If Modiolus abyssicola is included, then the shell form needs to be expanded to include the more wedge shaped form.