Pleurolucina harperae n. sp.

Glover, Emily A. & Taylor, John D., 2016, Pleurolucina from the western Atlantic and eastern Pacific Oceans: a new intertidal species from Curacao with unusual shell microstructure (Mollusca, Bivalvia, Lucinidae), ZooKeys 620, pp. 1-19 : 4-10

Taxon classification Animalia Lucinida Lucinidae

Pleurolucina harperae n. sp. Figs 1, 2, 3, 4, 5

Lucina leucocyma : Daccarett and Bossio 2011: 177, fig. 1243.

Pleurolucina leucocyma : Huber 2015: 433, fig. p. 85.

Type material.

Holotype: 1 whole shell L 8.8, H 8.5 T 3.2 mm (NHMUK 20160338), southwestern Curaçao, channel into Spaanse Water, opposite Hyatt Resort, 12°03 ’57” N 68°51 ’13” W. BivAToL stn Cur-5-15-009, 22 May 2015.

Paratypes: 92 valves (NHMUK 20160339), 2 paired valves (RMNH 5003991-50003992), 3 paired valves (FMNH344698), 2 paired valves (USNM 1411553). Same locality as holotype.

Other material.

19 ethanol preserved specimens (NHMUK), same locality as holotype.

Description.

Shell subovate, slightly anteriorly extended, L to 9.6 mm, H to 9.7 mm, H/L 0.99, moderately inflated, sculpture of flat, closely spaced commarginal lamellae, with four prominent, broad ribs with interspaces variable in width, but always narrower than ribs themselves; microsculpture of tight rows of shallow pits (Fig. 1 P). Umbones low, situated on midline. Anterior dorsal area arcuate. Protoconch: PI 217 µm, PI + PII 228 µm, PII a narrow rim with fine increments (Fig. 1 O). Lunule short, semicircular, slightly impressed. Ligament short, set in shallow resilifer. Hinge teeth: LV with two cardinal teeth; a robust anterior lateral tooth and smaller posterior lateral. RV with a single large cardinal tooth and anterior and posterior lateral teeth. Anterior adductor muscle scar short, broad, widely divergent from pallial line (60-70 µm) for about half of length (Fig. 2 A), posterior scar ovate; pallial line entire, pallial blood vessel scar sometimes visible. Shell margin finely beaded, sinuate with anterior sinus deeper. Shell within pallial line often patchily eroded to expose inner shell layers. Colour grey-white.

Anatomy.

General anatomy resembles most other described lucinids (Fig. 3). Mantle fusion ventral to the posterior apertures is very short. Foot short and broad when retracted but can be vermiform when extended (Fig. 3 A) with a small heel. Visceral pouches absent. Distinct mantle gills are absent but the inner mantle ventral to the anterior adductor muscle is thickened (Fig. 3 C) and may be a respiratory area with blood space as seen in other lucinids ( Taylor and Glover 2000). Labial palps are very short. In common with all other studied Lucinidae , Pleurolucina harperae has ctenidia comprising inner demibranchs only; these were pink in life, large, thick and occupying much of the mantle cavity (Fig. 3 B). Ctenidial filaments are approx. 40 µm thick and 380 µm deep with a narrow 45 µm ciliated zone and a deep bacteriocyte zone (Fig. 3 D). Bacteriocytes were packed with ‘potato-shaped’ bacteria 3-5 µm long and 1.5-2.0 µm wide (Figs 3 G, H). The surface of the microvilli-covered bacteriocytes and intercalary cells were colonised by abundant spirochaetes 2.5 µm long and 0.2 µm wide (Fig. 3 F) similar to those re ported by Ball et al. (2009) from Euanodontia ovum (Reeve, 1850). In comparison the symbiotic bacteria of Clathrolucina costata collected at the same time and same habitat were longer and rod shaped, 8-10 µm in length and approx. 1 µm wide.

The sperm of Pleurolucina harperae were 9 µm long and 1.2 µm wide at the base, tapering and curved distally (Figs 3 J, K). From the same locality, sperm of Clathrolucina costata were shorter, 4.8-5 µm and 1-1.2 µm wide with blunt tips. Oocytes of Pleurolucina harperae were approx. 200 µm in diameter (Fig. 3 I). Comparative sperm data is available for a few other western Atlantic lucinids ( Bigatti et al. 2004); sperm of Codakia orbicularis were 14-15 µm long, tapering with a width of 0.8 µm; Ctena orbiculata were cylindrical, slightly curved, 7.5 µm long and 1-1.2 µm wide at base and Lucina pensylvanica were 15.5 µm long, with curved tapering heads and 1.1 µm wide at the posterior.

Shell microstructure.

Within a very thin (ca 1 µm) periostracum, Pleurolucina harperae has a basic four layered shell (Figs 4 A,B); an outer composite prismatic layer, followed inwards by a thin crossed-lamellar layer, then a thicker layer of irregular spherulitic prisms and within the pallial line a complex crossed-lamellar layer with sublayers of irregular prisms. The shell layers are interrupted by sheets of conchiolin around 20-90 µm in thickness, each with repeated sublayers of small discrete ‘tulip-shaped’ calcified spherulites approx. 5 µm in diameter (Figs 4 D, F). Each spherulite is joined to those of the layer below with a narrow (0.5 µm) semicalcified channel through the conchiolin (Figs 4 E, F). At the shell surface, the conchiolin sheets correspond to major depositional halts (Fig. 4 A) visible as notches in the shell with the conchiolin appearing contiguous with the invaginated periostracum. In each shell there may be between 1-5 of such sheets.

Drill holes in Pleurolucina harperae produced by predatory naticid gastropods were observed with full penetration in 14 out of 114 single valves, but with 12 records of incomplete drill holes that terminated at an internal conchiolin layer (Fig. 5). In one shell there were three failed drills and in another two failures before successful penetration. Incidences of apparent multiple completed drill holes in dead shells may have resulted from post-mortem degradation of organic layers in failed drill holes.

Similar conchiolin calcified sheets were identified in Pleurolucina hendersoni (Figs 6 A, B) and Pleurolucina undata (Figs 6 C–E) but not in Pleurolucina leucocyma (2 shells examined) or Pleurolucina sombrerensis (2 shells examined). Also conchiolin sheets with multiple layers of calcareous spherules were observed in Lucina pensylvanica from the Florida Keys (Figs 6 F,G), apparently confined to the inner shell layer within the pallial line. This is distinct from the calcified periostracum of this species (Fig. 6 H) as described by Taylor et al. (2004). No conchiolin sheets were observed in a single Cavilinga blanda examined. For comparison, the repeated conchiolin sheets reported in Cardiolucina species by Ishikawa and Kase (2007) were studied in Cardiolucina quadrata from the Philippines. These sheets were approx. 10-15 µm thick and only lightly calcified with sporadic spherulitic crystal aggregations (Figs 6 I-K) with no multiple sub-layers.

Habitat.

Pleurolucina harperae is an intertidal to shallow subtidal species collected from sand amongst seagrass rhizomes (largely Thalassia testudinum , Halodule sp.) in contrast to Pleurolucina leucocyma that is usually recorded from deeper water, for example 30-180 m around the Florida Keys ( Britton 1970). Records of Pleurolucina harperae from Atlantic Panama (USNM below) are also from shallow water seagrass habitats. At Curaçao it co-occurred with several other lucinid species: Clathrolucina costata ( d’Orbigny, 1845), Ctena imbricatula (C.B. Adams, 1845), Anodontia alba Link, 1807, Codakia orbicularis (Linnaeus, 1758), Lucina roquesana J. & W. Gibson-Smith, 1982 and Divalinga quadrisulcata ( d’Orbigny, 1845).

Distribution.

Southern Caribbean: Panama (USNM 759784; 620716, 759825) Colombia -Taganga ( Daccarett and Bossio 2011), Curaçao. The distribution of Pleurolucina harperae in the southern Caribbean is uncertain but it may be restricted to the southwestern area. There have been no records from the Antilles and intensive sampling of molluscs around Guadeloupe by Muséum national d’Histoire Naturelle (KARUBENTHOS 2012, 2015) recorded only Pleurolucina hendersoni and Pleurolucina sombrerensis ( Taylor and Glover submitted). Similarly, only Pleurolucina sombrerensis was recorded from a recent survey of the marine molluscan fauna of French Guiana (MNHN - GUYANE 2014).

Etymology.

Named for Elizabeth (Liz) Harper, University of Cambridge, bivalve researcher, colleague and friend, who helped collect the new species.

Comparison with other species.

Pleurolucina leucocyma (Fig. 7) was thought to be widespread across the tropical Western Atlantic but we now consider it to be restricted to Florida and the Gulf of Mexico with the southern Caribbean records representing Pleurolucina harperae . The new species differs from Pleurolucina leucocyma (mean L 6.2 mm, H 7.4 mm, H/L 1.13) in being larger, less inflated and usually longer than high in the adult (Fig. 8). The radial folds are usually lower and the anterior adductor muscle scar is shorter and more divergent from the pallial line (Fig. 2 B). In shape and sculpture, it is most similar to the somewhat larger Pleurolucina undata (Figs 9 E-G) (mean L 15.1 mm, H 15 mm, H/L 0.95) from the eastern Pacific, Gulf of California, intertidal zone to 60 m ( Coan and Valentich-Scott 2012).

Other less similar species are: Pleurolucina hendersoni (Figs 9 A, B) an offshore to deep water species (to 300 m) from the southern Caribbean (Cuba, Lesser Antilles) that reaches about 12 mm in length and resembles the eastern Pacific Pleurolucina leucocymoides . Compared with other Pleurolucina , the sculpture of broad radial folds is less pronounced and the commarginal lamellae are widely spaced and prominent. Pleurolucina sombrerensis (Figs 9 C, D) lives in deeper water to 200 m from the Florida Keys to Brazil. The shell reaches about 6-7 mm in length and is rounded in outline, with a shallow radial anterior sulcus and prominent close commarginal lamellae, sometimes separated by deep interspaces. It does not closely resemble other Pleurolucina but shares some shell features including dentition and adductor scar shape. The larger Pleurolucina leucocymoides (Figs 9 H–J) is known from shallow water to 150 m and ranges from Baja California to Ec uador and Galapagos Islands. The sculpture of broad prominent commarginal lamellae and absence of prominent radial folds distinguish it from other Pleurolucina . Lastly, Pleurolucina taylori (Figs 9 K–M) is known from the intertidal zone to 183 m in the Gulf of California; it is distinguished by the highly inflated shell and closely spaced, low commarginal lamellae with four to five radial folds and resembles the extinct late Pliocene - mid-Pleistocene Floridian species Pleurolucina amabilis .