Benthomodiolus erebus, Oliver, P. Graham, 2015

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 : 152-155

Taxon classification Animalia Mytiloida Mytilidae

Benthomodiolus erebus sp. n.

Adipicola sp. (n. sp.?) Juniper et al. 1992: 1797-1799, shell not illustrated

Bathymodiolus sp. JdeF McKiness et al. 2005: 109-116, shell not illustrated.

Adipicola MV Southward, 2008: 139-146, shell not illustrated

Material examined.

Holotype. 1 specimen, ROV ROPOS dive R682, Clam Bed, Endeavour Segment, Juan de Fuca Ridge, 47°57.8'N 129°05.5'W, 2195m, 19/August/2002. CMNML 097165

Dimensions in mm. Length 39.9; Height 15.4; Width 13.2; Anterior length 9.5

Paratype. 1 broken and partly dissected specimen, Alvin dive 2803, clam bed, Kini’s Site, Middle Valley, Juan de Fuca Ridge, 48°27.40'N 128°42.52'W, 2416m, 24/July/1994. NMW.Z.2015.013.1. This is the remainder of the material used by Southward (2008) in her description of the fine structure of the ctenidium..

Dimensions in mm. Length 39.2; Height 12.9; Width 10.3; Anterior length 10.1

Shell.

Holotype, (Fig. 1 a–d). Thin but not fragile (0.45-0.55 mm at margins). Equilateral. Umbos prominent. Inequilateral, beaks towards the anterior, just under ¼ the total length from the anterior. Outline modioliform elongate, posterior a little deeper than anterior, ventral margin slightly concave, dorsal margin gently curved. Median area sulcate, widest behind the beaks. Ligament sunken very long, 17.7 mm, no trace of underlying crenulations visible. Anterior hinge margin extending posteriorly beyond the beaks for a short distance. A small flattened lunule under the beaks. Sculpture of fine commarginal ridges increasing in size towards the margins, periostracum persistent of a golden amber shade and lacking hairs. Internally shiny, pearlescent, muscle scars indistinct. The prodissoconch and juvenile shell are too eroded to give any details here.

Paratype, (Fig. 1e). The shell is of the same proportions as the holotype, but is very much thinner (0.25-0.35 at margins) and more fragile.

Pedal byssus musculature. (Fig. 2 a–c). The foot has a long toe and a greatly reduced heel. The byssus gland opens at the base of the heel and a longitudinal groove runs the entire length of the sole. The pedal/byssus retractor is divided into two widely separated bundles; the posterior bundle (pbr1) is greatly elongate, of two primary strands and attaches to the shell just above the posterior adductor muscle; the median bundle (pbr2) is relatively short and extends dorsally attaching in the rear of the umbonal cavity. The posterior pedal retractor is slender and runs anterior and parallel to the median pedal/byssus retractor; associated are three fine strands or secondary posterior pedal retractor muscles that coalesce with median pedal/byssus retractor; the anterior pedal retractor consists of two primary bundles, is elongate and attaches above and separate from the anterior adductor muscle. A slender labial palp suspensor muscle rises from the base of the anterior pedal retractor and is inserted into the anterior adductor muscle.

Adductor muscles. The adductor muscles are of almost equal size, the posterior is circular in section while the anterior is oval (Fig. 2a).

Ctenidium and labial palps. The ctenidia (Figs 2a, 4 a–e) run almost the entire length of the mantle cavity, are thick and fleshy but not deep (Fig. 2a). The inner demibranch is a few filaments longer than the outer demibranch, both have reflected filaments the ascending arms slightly shorter than the descending arms and these fused for over half their length. The filaments are extended abfrontally, appearing as lamellae (Fig. 4a). There is a groove along the ventral edge of each demibranch (Fig. 4a). The filaments are held together by a single row of large ciliary junctions.

Scanning electron microscopy reveals that the abfrontal surfaces are extensive giving a triangular plate like form to the largely fused ascending and descending arms of each filament (Fig. 4b, c). The inter filamental junctions are very prominent formed of a large bundle of cilia (Fig. 4d). The frontal surface is ciliated while the abfrontal surface is covered by an epithelium of microvilli arranged in a polygonal structure (Fig. 4c, e). The ultrastructure was described my Southward (2008) where the ctenidia were shown to harbor symbiotic bacteria extra–cellulary among dense epithelial microvilli.

The labial palps (Fig. 3f) are small, triangular with 15 sorting ridges on each, there is a short oral tube extending to the mouth.

Mantle edge and apertures.

The mantle edges are free for their entire length (Fig. 2a) except for a narrow septum (Fig. 3b and c, pms) separating the exhalant aperture from the inhalant/pedal aperture. The mantle edge is composed of three major folds (Fig. 3 c–e); the outer mantle fold (omf) is smooth and never fused. The middle mantle fold (mmf) is narrow and weakly frilled (Fig. 3d), the stronger frills border the posterior inhalant aperture and a short median section (Fig. 3e), probably marking the pedal aperture. The inner mantle fold is finely digitate along the entire length of the pedal/inhalant aperture (Fig. 3d). The exhalant aperture is muscular and smooth edged (Fig. 3b); a pair of folds visible on the inner face probably function as a valve (Fig. 3a, eav).

Alimentary system.

The alimentary system (Fig. 3g) runs along the dorsal surface of the visceral mass. The oesophagus (oe) is flattened, relatively short, about the same length as the stomach (st). The mid gut and hind gut run in a straight line except for a very short, tight, loop (hgl) just anterior of the heart; the hind gut runs through heart and the rectum (rt) runs over the posterior adductor muscle with the simple anus (Fig. 3c) opposite the exhalant aperture.

Stomach.

The stomach (Fig. 3g) (st) is situated beneath the umbos, in front of the median pedal/byssus retractors (pbr2) and slightly to the left side. Externally it is elongate somewhat triangular in form, the oesophagus (oe) entering on the anterior face and the conjoined style sac/mid gut (ss/mg) leaving posteriorly, the right side is expanded posteriorly, there is a very slight division into anterior and posterior chambers with a shallow dorsal caecum (ca) projecting on the left anterior dorsal face and the thickening of the dorsal hood behind this (dh). Ducts to the digestive gland are prominent on the right dorsal side (ddd1-3) while a large duct exits on the lower mid left and a smaller on the ventral anterior left, the latter are not visible in Fig. 3g.

Internally the major typhlosole runs longitudinally across the floor, to its right side there is a smooth depression and leading off this are tracts to the right side ducts. There is a deep embayment running from the anterior floor and up the anterior left side into the shallow caecum and sharply curving into the dorsal hood. A small duct exits this trough on the anterior floor and medially it opens into the left pouch where a large duct exits. The gastric shield is shaped into the dorsal hood and extends posteriorly of the left posterior dorsal face. Sorting ridges are nowhere apparent except for a well defined but small area on the posterior edge of the left pouch.

Pericardium.

The pericardium (Figs 2b, 3g) (per) is situated immediately anterior of and between the posterior byssus retractor muscles (pbr1). The ventricle (vt) is muscular while the auricles (au) are relatively large and thin walled. The arrangement of the auricular veins could not be elucidated.

Etymology.

After Erebus (Greek), Noun in apposition, "place of darkness between earth and Hades" alluding to the abyssal, hydrothermal vent, type locality.

Habitat.

Located in low temperature vent flows (<20 °C) associated with other vent endemic animals but is sparse and rarely collected. At Endeavour, it was recovered in a grab of the siboglinid Ridgeia piscesae at the base of the tubeworm cluster. At Middle Valley, it was recovered nestled in crevices of a sulphide block, also colonized by Ridgeia piscesae . Overall setting at both sites featured sulphide deposits with limited high temperature venting surrounded by ponds of sediment where vesicomyid clams also occurred ( Juniper et al. 1992). Although Benthomodiolus erebus sp. n. is not visible on ROV imagery the typical habitats are illustrated here (Fig. 5 a–b).

Comparative taxa.

In the following section the other species assigned to Benthomodiolus are described with reference to Benthomodiolus erebus rather than in full detail.