Ferrocina brunei, Taylor, John D. & Glover, Emily A., 2013

Taylor, John D. & Glover, Emily A., 2013, New lucinid bivalves from shallow and deeper water of the Indian and West Pacific Oceans (Mollusca, Bivalvia, Lucinidae), ZooKeys 326, pp. 69-90 : 76-77

Ferrocina brunei sp. n. Figs 7, 8, 9C

Type material.

Holotype: whole shell NHMUK 20130122 L 8.2 mm, H 6.7 mm, T 1.6 mm; Paratypes: NHMUK 20130123, figured L 8.4 mm, H. 7.3 mm, T 2.1 mm; L 8.9 mm, H 7.8 mm, T 2.1 mm, non figured 19 v.

Type locality.

Brunei, 05°21'12"N, 111°26'21"E, 63 m, muddy sand near oil drilling rig.

Description.

Shell small, H to 7.8 mm, L to 8.9 mm, T to 2.1 mm, longer than high H/L = 0.89 ± 0.034 (n=13). Colour grey-white with patches, streaks or stripes of rusty red, more pronounced dorsally, including lunule, occasionally whole shell red-brown; internally red brown particularly at anterior. Sculpture of numerous (ca 40) low, radial ribs that divide and intercalate, crossed by very fine, widely spaced commarginal lamellae. Shallow posterior sulcus and posterior dorsal area with slightly elevated commarginal lamellae, radial ribs absent. Anterior dorsal area also without ribs. Protoconch (Fig. 7K) PI = 82 µm, PI+ PII = 159 µm, PII with many growth increments. Lunule short lanceolate, slightly impressed, asymmetrical, greater part in right valve, brown coloured. Ligament set in shallow groove. Hinge plate thin (Figs 7L, M), RV with single cardinal tooth and small anterior and posterior lateral teeth; LV with two cardinal teeth, anteriormost is larger and faint sockets for anterior and posterior lateral teeth. Anterior adductor muscle scar broad, short, detached for ½ of length at an angle of 25°. Pallial blood vessel trace prominent, terminates ventral to anterior adductor scar. Pallial line entire. Inner shell margin denticulate.

Anatomy.

Ctenidia comprising inner demibranchs (Fig. 8A), pink, thick, occupying about ½ of mantle cavity. Foot cylindrical with small heel. Labial palps small ridges. Visceral mass anterior to foot laterally extended into pair of ramshorn-like coiled structures (Fig. 8B). Posterior exhalant aperture with inverted tube (Fig. 8C), inhalant aperture with small papillae, short section of fused mantle ventral to aperture. Ctenidial filaments with thick bacteriocyte zone with bacteriocytes packed with short rod-shaped bacteria 2-5 µm long and 1-2 µm wide (Figs 8 D–F). Bacteria aligned with long axes normal to apical surfaces of bacteriocytes.

Distribution.

Known only from type locality (Fig. 4).

Etymology.

Named for Sultanate of Brunei. Noun in apposition.

Remarks.

Ferrocina brunei is similar to the type species Ferrocina mutiradiata from Fiji but has much less prominent radial ribs, a less strongly denticulate shell margin and is smaller (shell length to 9 mm compared to 18 mm).

The anatomy is similar to most Lucinidae and bacterial symbiosis is confirmed by the presence of abundant bacteria in the ctenidial filaments. A distinctive feature of the anatomy is the bilateral ramshorn-like extension of the visceral mass anterior to the foot. We have observed similar structures only in Bathyaustriella thionipta Glover, Taylor & Rowden, 2004 from a hydrothermal vent on the Kermadec Ridge and in a Ferrocina species recently discovered off the southern USA in the Western Atlantic (unpublished observations). The function of this structure is unknown but thin sections of the structure in Bathyaustriella thionipta showed that it consisted of diverticula of the digestive gland.