A review of North American Recent Radiolucina (Bivalvia, Lucinidae) with the description of a new species

Abstract North American members in the genus Radiolucina are reviewed. A lectotype for the type species, Radiolucina amianta, is designated and descriptions and illustrations are provided. A description of a new species, Radiolucina jessicae, from the west coast of Mexico is presented. Key diagnostic species characteristics are outlined and compared among members of the genus.


Introduction
Members of Lucinidae have been grouped and identified incorrectly in the past because of variable shell and anatomical characteristics. Since the discovery of chemosymbiosis with sulphide-oxiding bacteria in the early 1980's, the systematics of Lucinidae has attracted more attention and many new genera and species have been described (Bouchet and Cosel 2004;Glover and Taylor 2007;Hickman 1994;Taylor and Glover 2006). Small species (less than 10 mm) have received less attention than their larger counterparts. There are currently over 400 Lucinidae species living in a variety of different habitats (Barnes and Hickman 1999;Roeselers and Newton 2012). Dall (1901) placed Phacoides amiantus Dall, 1901 and Lucina cancellaris Philippi,1846 with the Indo Pacific subgenus Bellucina (now known as Cardiolucina Sacco, 1901). Later, Britton (1972) described Radiolucina as a new subgenus of Parvilucina, and included Phacoides amiantus Dall, 1901, Lucina cancellaris (Philippi 1846), and the fossil species Phacoides waccamawensis (Dall, 1903). Recent DNA results (Taylor et al. 2011) show that Radiolucina amianta and Radiolucina cancellaris are related to Lucinisica, not Parvilucina or Cardiolucina as was thought in the past.
While reviewing the Lucinidae of the Panamic Province, morphological differences among specimens identified as Radiolucina cancellaris were discovered. After further research on the genus and examination of pertinent type specimens, a new species of Radiolucina was recognized herein named Radiolucina jessicae. The three Recent members of North American Radiolucina are described and illustrated.

Materials, methods, and abbreviations
One hundred Radiolucina specimens from Mexico and Florida were examined. Of the 100, 20 were determined to be Radiolucina jessicae, two were Radiolucina amianta, and 78 were Radiolucina cancellaris. Six Radiolucina jessicae specimens were rehydrated in water and dish soap, and reconstituted anatomy was examined.
LACM-Natural History Museum of Los Angeles, Los Angeles, USA; NHMUK-The Natural History Museum, London, UK; SBMNH-Santa Barbara Museum of Natural History, Santa Barbara, USA; USNM-Smithsonian National Museum of Natural History, Smithsonian Institution, Washington DC, USA.

Data resources
The data underpinning the analyses reported in this paper are deposited at GBIF, the Global Biodiversity  amiantus Dall, 1901. Description. Shell shape subovate; maximum length: 9.0 mm, maximum height: 8.0 mm; with an average of 13 heavy radial ribs, overlain by thin commarginal lamellae that continue through interspaces, producing a reticulate pattern; posterior end thickened, posterior dorsal area often with low spines; pallial line often discontinuous broken into large and small segments; right valve hinge with two cardinal teeth, left valve hinge with one wide cardinal tooth, one anterior lateral tooth, one posterior lateral tooth.
Comparisons. Parvilucina Dall, 1901 (type species: Lucina tenuisculpta P.P. Carpenter, 1864) attains a larger size and has fine radial ribs, and a short, broad anterior adductor muscle scars compared to Radiolucina, which has strong radial ribs and a long, narrow anterior adductor muscle scar.
Pleurolucina Dall, 1901(type species: Lucina leucocyma Dall, 1886 has heavy commarginal lamellae with few broad, weak radial ribs compared to Radiolucina. It is similar to Radiolucina in that they both have a long, narrow anterior adductor muscle scar.
Liralucina Glover & Taylor, 2007(type species: Phacoides sperabilis Hedley, 1909 has an average of 35 flat, radial ribs compared to Radiolucina, which has average 13 strong, radial ribs. There is evidence (Coan and Valentich-Scott 2012) that Radiolucina dates back to the Miocene.
Hinge. Hinge plate thick, curved on either side of cardinal teeth; beaks prosogyrate; cardinal teeth small, right valve posterior tooth thin, anterior tooth thick, left valve middle tooth wide; lateral teeth large, posterior tooth vertical, anterior tooth horizontal; ligament sunken above cardinal teeth.
Adductor scars and pallial line. Pallial line continuous; anterior adductor scar long, narrow, diverging from pallial line for about a quarter of its length; posterior adductor scar small, wide, pallial line joins at most ventral point.
Type specimens and type locality. Dall did not designate a single specimen as the holotype. To stabilize nomenclature, I herein designate the lectotype to be the right valve (USNM 64276), which is the same specimen as figured by Dall 1901, plate XXXIX fig. 10, with the type locality of Yucatan Strait, North Atlantic Ocean (approximately 21.3°N, 86.2°W), 1170 m (Fig. 4). An additional right valve (USNM 1183662) in the original lot is a paralectotype.
Distribution. Western Atlantic from North Carolina to Florida, West Indies, Gulf of Mexico, Caribbean Central America, South America south to Uruguay (Mikkelsen and Bieler 2007).
Remarks. In describing Phacoides (Bellucina) amiantus, Dall noted that it seemed to be the same species that Tuomey and Holmes (1857) had described as Lucina costata from the Pleistocene of South Carolina (non Lucina costata d'Orbigny, 1846). Boss et al. (1968: 25) misinterpreted Dall's proposal as a new name, but it is expressly a new species. Moreover, it is not at all clear that these represent the same species.

Radiolucina cancellaris
Hinge. Hinge plate thick, straight with slight curve; cardinal teeth small, right valve posterior thin, anterior thick, left valve middle tooth wide; lateral teeth large, posterior vertical, anterior horizontal; ligament sunken above the cardinal teeth.
Adductor scars and pallial line. Pallial line discontinuous, broken into small and large segments, with one small circular indentation directed ventrally; anterior adductor scar large, narrow, diverging from pallial line for about a quarter of its length; posterior adductor scar small, wide, pallial line joins at most ventral point.
Remarks. Shell shape and ribs vary at different stages of growth. Due to this, it can be difficult to distinguish variants of Radiolucina cancellaris. The number of ribs is consistent during growth (average 12); however sometimes they were thinner or thicker, and inconsistently extended to the ventral margin. Bifurcation and/or intercalary ribs are sometimes present depending on the stage of growth.
Shell shape. Subovate, long, extended anteriorly and posteriorly, length longer than height; slightly inflated; maximum length: 6.0 mm, maximum height: 5.3 mm; beaks pointed, prosogyrate. Sculpture and color. Average 13 (9-16 n=20) radial ribs, occasional bifurcate usually on larger specimens; commarginal ribs continuing through interspaces, with 6-10 thick intercalary ribs extending to ventral edge of valve, present in most specimens, more pronounced in larger specimens; interspaces shallow and thin towards beak, progressively widening ventrally; anterior side smooth with fine commarginal striae; posterior side also smooth with fine commarginal striae, with a series of average nine (4-15 n=20) thick spines of varying heights protruding from shell; exterior color tan to white, also with brown along ribs; interior color tan, white to cream, shiny; inner shell margin crenulations thin, closely spaced.
Hinge. Hinge plate thin, slightly curved on either side of cardinal teeth; right valve posterior and anterior cardinal teeth about equal in size, left valve middle tooth wide; lateral teeth large, posterior vertical and anterior horizontal; ligament long, sunken above cardinal teeth.
Adductor muscle and pallial scars. Pallial line discontinuous, broken into series of large, small segments, with one large segment directed ventrally; anterior adductor scar is large, narrow, diverging from pallial line for about half its length; posterior adductor scar small, wide, pallial line joins anteriorly to most ventral point. Anatomy from rehydrated dried specimens. Inhalant aperture usually smaller than exhalant, elongate, often narrow; tissue bridge between apertures usually narrow; ventral mantle fusion thin, narrow; mantle fusion variable, usually not fused below anterior adductor muscle; rectum curves dorsally around posterior adductor muscle and ends at exhalant aperture.
Type locality and type specimens. Etymology. Named in honor of Jessica Sanford from Santa Barbara, California for being an inspiring scientist, meaningful mentor, and wonderful friend.
Radiolucina waccamawensis (Dall 1903), from the Pliocene of the Waccamaw district, South Carolina, has about 10 strong radial ribs with deep interspaces, compared to Radiolucina jessicae which has about 13 radial ribs with shallow interspaces, and about eight intercalary ribs. The morphologic characters of Radiolucina waccamawensis are closer to Radiolucina cancellaris.
Discussion. Different morphologic characters among different ages of Radiolucina cancellaris are common, specifically ontogenetic changes in shape, rib number, and hinge teeth. Further research and use of additional characters, specifically DNA studies, (Taylor et al. 2011) could present more details and explanation for the variable characters observed, and could possibly reveal more new species within the Radiolucina cancellaris complex.