New lucinid bivalves from shallow and deeper water of the Indian and West Pacific Oceans (Mollusca, Bivalvia, Lucinidae)

Abstract Four new species and a new genus of lucinid bivalves are described from shallow and deeper waters in the Indian and West Pacific Oceans. The new genus Scabrilucina (subfamily Lucininae) includes the little-known Scabrilucina victorialis (Melvill, 1899) from the Arabian Sea and Scabrilucina vitrea (Deshayes, 1844) from the Andaman Sea as well as a new species Scabrilucina melvilli from the Torres Strait off northeastern Australia. Ferrocina brunei new species (Lucininae) was recovered from 60 m near oil drilling activities off Borneo; its anatomy confirmed the presence of symbiotic bacteria. Two unusual deeper water species of Leucosphaerinae are described, both species included in on-going molecular analyses; Gonimyrtea ferruginea from 400–650 m in the southwest Pacific and Myrtina reflexa from 200–825 m off Zanzibar and Madagascar.


Introduction
Over the last 20 years there has been much taxonomic interest in the chemosymbiotic bivalve family Lucinidae with many new genera and species described from shallow to bathyal depths of the tropical Indo-West Pacific (Taylor and Glover 1997, 2002, 2005Glover and Taylor 2001, 2007Oliver and Holmes 2006;Bouchet and Cosel 2004, Cosel and Bouchet 2008, Okutani 2011. Despite this activity there remain many undescribed or unregarded species whose systematic position is obscure. In this paper a new genus is introduced to accommodate Lucina victorialis (Melvill, 1899) previously known from just a few valves collected in the Arabian Sea in the late 1800s but classified initially as Cryptodon (i.e. Thyasiridae) because of the deep sulcus and trigonal shape. Discovery in the Museum of Comparative Zoology, Harvard University of a large sample collected in the Arabian Sea as part of the 1963 International Indian Ocean Expedition has allowed re-description and assessment of the unusual features of this species. We also include within this genus the neglected species Lucina vitrea Deshayes, 1844, with new records from Southeast Asia, and another species with similar characters is newly described from the Torres Strait off northeastern Australia.
Benthic sampling at about 60 m depth off Brunei in north Borneo recovered from near oil drilling activities an unusual, small, mottled red-brown lucinid that on shell characters can be classified as a new species of Ferrocina a genus previously known from the rare type species F. multiradiata Glover & Taylor, 2007 from off Fiji and New Caledonia and another species from the Philippines (Glover and Taylor in press). Formalin preserved animals were available and we include some anatomical information.
In a molecular analysis of Lucinidae we included an individual of an undescribed genus and species dredged from over 600 m on the Chesterfield Bank west of New Caledonia (UGS-3 in Taylor et al. 2011, fig. 7J). This taxon grouped with a cluster of other genera in the subfamily Leucosphaerinae and we now classify and describe it as a new species of Gonimyrtea Marwick, 1929. Also in the Leucosphaerinae we describe a new species of Myrtina Glover & Taylor, 2007 from off Zanzibar and Madagascar between 200-800m and for which molecular data is available. Diagnosis. Small to medium size, L to 40 mm, thin, semi-translucent, usually slightly higher than long, ovoid to subtrigonal, strong posterior sulcus with broad sinus at posterior margin. Sculpture of sharp, fine, commarginal lamellae. Ligament short, set in shallow resilifer. Hinge with small to vestigial cardinal teeth in both valves, lateral teeth usually absent but small anterior lateral tooth may be present in juvenile shells. Anterior adductor muscle scar long, thin, ventrally detached from pallial line for ½-¾ of length, dorsal part runs on to hinge. Interior shell with translucent spots.

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Etymology. Derived from Latin scaber rough and Lucina, reference to the rough surface formed by closely spaced, sharp, commarginal lamellae. Feminine.
Comparison with other genera. Scabrilucina can be readily distinguished from other lucinids by the subtrigonal outline with a prominent posterior sulcus, the fine, regular, sharp-edged, commarginal lamellae and the absence of radial sculpture. No preserved samples of Scabrilucina were available for molecular analysis but a small lucinid from the Philippines with some similar characters was included in a prior analysis (Taylor et al. 2011) as 'Lucina' desiderata Smith, 1885. We now consider this a new species in a separate genus (Glover and Taylor in press) and related to Scabrilucina. Evidence from 18 S and 28 S rRNA genes places 'L.' desiderata in the subfamily Lucininae that by inference also includes Scabrilucina.
The little known genus Semelilucina Cosel & Bouchet, 2008, type species S. semeliformis from offshore muddy habitats in Tanimbar, SE Indonesia, may be related to Scabrilucina. The two genera differ in shell shape but have similar sculpture of fine, regularly spaced, sharp commarginal lamellae. Cosel and Bouchet 2008 considered Semelilucina closely related to Dulcina (Leucosphaerinae) and we followed this in our classification (Taylor et al. 2011). However, shell morphology suggests that Semelilucina is better placed in the Lucininae. (Melvill, 1899) http://species-id.net/wiki/Scabrilucina_victorialis Figs 1-2, 3A
Remarks. This species is known only from shells. Scabrilucina victorialis is characterised by the deep cleft of the posterior sulcus and the fine, sharp, commarginal lamellae. Scabrilucina vitrea (see below) from off Sumatra and Gulf of Thailand is smaller, taller, and thinner shelled. The shell shape and the deep posterior sulcus superficially resemble some Thyasiridae such as Conchocele and this influenced Melvill's initial placement in Cryptodon. Scabrilucina victorialis is also similar to S. melvilli (new species below) from Australia that is distinguished by its smaller size, less deeply incised posterior sulcus and more widely spaced commarginal lamellae. Many of the shells of S. victorialis from off Oman (MCZ 362493) and S. vitrea from Thailand are penetrated by narrow, straight-sided holes ca 450 µm diameter (Figs 1A, C, 5E) comparable with those resulting from octopus predation (Cortez et al. 1998, Todd andHarper 2010). Description. Shell white, L to 22.0 mm, H to 25.1 mm, slightly higher than long, H/L 1.08 ± SD 0.05 (n=5), very thin shelled, translucent, prominent posterior and anterior sulcus, umbones prominent. Sculpture of fine, thin commarginal lamellae, lamellae slightly irregular and elevated at posterior and anterior dorsal area, radial sculpture absent. Lunule broad, lanceolate, slightly impressed. Hinge extremely narrow, right valve with single cardinal tooth, lateral teeth absent, left valve with 2 small cardinal teeth, lateral teeth absent. Anterior adductor scar long, narrow, pointed, detached for 2/3rds of length, posterior scar ovoid, pallial line entire with dorsal elevations, close to shell margin, inner shell surface with many fine points of mantle attachment, shell surface glossy outside the pallial line.
Distribution. Andaman Sea, Straits of Malacca and Gulf of Thailand (Fig. 4).
Remarks. Similar to S. victorialis but smaller and less trigonal in outline of the adult shell, with finer commarginal sculpture and thinner shell.
The type material of S. vitrea has not been located but the original figures of Deshayes are clear (Figs 5 A, B). The type locality of Sumatra is close to that of the other samples mentioned here. As far as we are aware, except for a listing in Tryon (1872), this species name has not been mentioned since the original description in 1844.  Description. Shell fragile, thin, white, semi-translucent, subtrigonal, L to 26.3 mm, H to 26.3 mm, higher than long H/L = 1.05 ± SD 0.04 (n = 9), moderately inflated T/L = 0.27 ± 0.02 (n = 9). Posterior sulcus broad, shallow with marginal sinus. Anterior dorsal area weakly defined. Sculpture (Fig. 6K,L) of regularly spaced, low, thin, sharp, commarginal lamellae with faint radial folds in interspaces. Commarginals more closely  spaced in larger specimens. Commarginal lamellae elevated to scales along dorsal margin. Protoconch (Fig. 6M): 150 µm, PII a narrow rim. Lunule short, lanceolate, with more prominent growth increments in LV. Ligament short, set in groove. Hinge plate narrow (Fig. 6I, J), LV with two small cardinal teeth, anterior tooth larger, RV with single cardinal tooth, in larger specimens teeth sometimes obscure. Small anterior lateral present in RV of juveniles. Anterior adductor scar long, thin, detached ventrally from pallial line for ¾ of length at angle of 25°. Posterior adductor scar ovoid. Pallial line narrow, entire. Shell inside pallial line with small round points of mantle attachment, radial grooves, track of pallial blood vessel visible as shallow groove. Shell margin smooth.
Remarks. See above for comparison with S. victorialis. Etymology. Named for James Cosmo Melvill (1845Melvill ( -1929, British malacologist who described many IWP species. Noun in genitive case. Diagnosis. Shell to 20 mm, thin, subovate, posteriorly truncate, sculpture of numerous fine, often indistinct radial ribs crossed by fine commarginal threads. Hinge plate thin, small single cardinal tooth in RV, two cardinals in LV, lateral teeth very small or obsolete. Anterior adductor scar short, detached for 1/3 of length. Interior shell margin finely to coarsely dentate. Colour pale orange to rusty red-brown, sometimes blotchy. Remarks. The unusual and rare genus Ferrocina was first recognised from Vanuatu, Fiji and New Caledonia at depths from 80-400 m (Glover and Taylor 2007) and is known from a few shells of the type species. A second species has been identified from the Philippines (Glover and Taylor in press) also from a few shells. From shell features including the dentition and presence of a pallial blood vessel scar we classify this genus in the Lucininae. Additionally the ramshorn shaped visceral extension is also seen in the lucinine Bathyaustriella (Glover et al. 2004   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 7 L,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.
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 F. 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 B. thionipta showed that it consisted of diverticula of the digestive gland. Taylor  Definition. Shells small, subcircular to ovate, higher than long, inflated. Sculpture of closely spaced thin, low commarginal lamellae. Right valve with single cardinal tooth and small anterior and posterior laterals sometime present; left valve with two cardinal teeth and small anterior and posterior laterals sometimes present. Lunule narrow lanceolate. Ligament short, curved. Anterior adductor scar detached for about 1/5-1/2 of length.

Subfamily Leucosphaerinae
Remarks. There has been much confusion concerning the concept of this genus following from Chavan's (1969) illustration of an Eocene species with little resemblance to the type and also his assignment of Alucinoma as a synonym (see Glover and Taylor 2007). We originally placed this genus (based on shell morphology) in the Myrteinae (Taylor et al. 2011) but continuing molecular analysis (species described below as UGS-3 in Taylor et al. 2011 and unpublished data) suggests that it should be classified in the sub-family Leucosphaerinae, although the type species has not yet been investigated. As well as the genotype G. concinna from New Zealand we include G. avia and G. fidelis from around New Caledonia (Glover and Taylor 2007) and two new species from the Philippines (Glover and Taylor in press). Distribution. New Caledonia, Vanuatu, and banks and islands in SW Pacific, in deeper water between 350 and 650 m (Fig. 4).
Remarks. Compared to the genotype, G. concinna from New Zealand, at depths to 200 m, G. ferruginea occurs in much deeper water, is larger, the lateral teeth are less distinct, and commarginal lamellae are more closely spaced. Another deeper water species of Gonimyrtea is known from the Philippines (Glover and Taylor in press) occurs at depths of 500-1000 m but is smaller than G. ferruginea, more elongate, lamellae are narrower, the interspaces wider, and the lunule deeper and longer.
Both G. ferruginea and the new species from the Philippines typically have dense iron-rich encrustations on the dorsal areas of the shell (Fig. 10A-D). This also occurs in Dulcina guidoi from central Philippines at similar depths (Cosel and Bouchet 2008) but is an unusual feature of lucinids that otherwise often exhibit some brown staining around the anterior inhalant area and posterior apertures.
Etymology. Latin reflexa means bent or turned back, a reference to the form of the commarginal lamellae. Adjective nominative singular Remarks. This species was referred to Lucina inanis (Prashad) by Knudsen (1967) and the same specimens were figured and briefly discussed by Bouchet and Cosel (2008) who thought they more closely resembled species of Alucinoma. The type and other material of L. inanis from Indonesia and Philippines differ in shape and lack the regular, reflexed commarginal lamellae (Glover and Taylor in press). Characters of the new species suggest a placement in Myrtina and this position is corroborated by molecular evidence that places Myrtina reflexa in the subfamily Leucosphaerinae close to species UGS -1 (now recognised as a new Myrtina species) from the Philippines (Taylor et al. 2011). Myrtina occur in offshore habitats from 30-1200 m from the Philippines and New Caledonia and are likely present in organically enriched sediments throughout the tropical Indo-West Pacific.
Myrtina reflexa differs from the genotype, M. porcata, by the more widely spaced and reflexed commarginal lamellae. It is most similar to a new Myrtina species (Glover and Taylor in press) from 200-1200 m around the Philippines but is larger and has more prominent and reflexed commarginal lamellae.