Darina solenoides (King & Broderip, 1832)

(Figures 14–16)

Erycina solenoides King & Broderip, 1832: 335; Mabille & Rochebrune, 1889: 108.

Mesodesma solenoides Griffith & Pidgeon, 1834: pl. 22, fig. 1; Hanley, 1843: 39, pl. 10, fig. 3.

Lutraria tenuis Philippi, 1845: 50; Stempell, 1899: 238.

Darina solenoides King & Broderip. Gray, 1853: 42; Adams & Adams, 1856: 381, pl. 101, figs. 2-2a; Chenu, 1862: 60, fig. 244; Conrad, 1868: 46; Martinez y Saez, 1869: 14; Dall, 1895: 212; Ihering, 1907: 458; Lamy, 1912: 315; Carcelles, 1944: 284; Carcelles & Williamson, 1951: 344; Castellanos, 1970: 239, pl. 23, fig. 4–6; Bernard, 1983: 41; Pastorino, 1995: 14, pl. 5, fig. 27 a–d.

Lutraria kingi Fischer, 1887: 1119 .

Darina tenuis Philippi. Carcelles & Williamson, 1951: 344 .

Diagnosis. Shell elliptical, fragile, thin, inequilateral, laterally compressed; gaped; umbos posterior of midline; external surface smooth; periostracum brownish; hinge with weak lateral teeth placed very close to cardinals; pallial sinus deep; siphons completely separated; ctenidia formed by two demibranchs different in size and shape.

Description. Shell elliptical, fragile, inequilateral, young specimens pellucid; with a maximum length about to 70 mm; external surface smooth, with irregular, concentric growth lines only (Fig. 14 A–C). Anterior dorsal margin straight, larger than posterior margin, which is curved; posterior end with siphonal gap larger than anterior, both ends rounded; ventral margin almost straight, curved at distal portions. Lunule and escutcheon poorly defined; small umbos placed behind midline of shell, not inflated, weakly prosogyrous (Fig. 14 A–B). Internally white, hinge with one short, fragile anterior lateral tooth in left valve (AII), two fused cardinal teeth (2a & 2b) forming Vshaped tooth. Tooth 2b larger than 2a, flanked by a lamella (4b); posterior lateral tooth (PII) similar in size to anterior, very close to the cardinals (Fig. 15 B); right valve with two anterior lateral teeth (AI & AIII) different in size, ventral (AI) more elongated and stronger; cardinal teeth (3a & 3b) unfused, 3a larger; both posterior lateral teeth (PI & PIII) similar in size and shape, short, very fragile (Fig. 15 A). Pallial sinus deep, approximately half of shell length; pallial line very well defined separated from ventral margin (Fig. 14 H). Anterior adductor muscle scar ellipsoidal, larger than more subcircular posterior; internal ligament well developed in very large chondrophore, ventrally projecting; external ligament rudimentary, posterior to umbos (Fig. 15 C). Shell ultrastructure formed by two layers: external with single cross lamellar, internal with complex cross lamellar structure (Fig. 15 D).

Mantle cavity organs. External demibranch trigonal, not elongated, very high (Fig. 16 A); internal demibranch elongated in anteroposterior axis (Fig. 16 B, D). Internal surface of external demibranch plicate and covered by cilia to facilitate food current (Fig. 16 C); this demibranch unfused at any point with labial palps. Labial palps small, trigonal, elongated, with plicate internal surfaces(Fig. 16 E); outer labial palp fused to mantle, inner fused internal demibranch. Posterior portion of mantle completely arched to the inside, leaving siphon naked. Siphons unfused, only united at base very close to the siphonal retractor muscle (Fig. 16 F); incurrent and excurrent siphons with very well developed tentacles at apertures, all equal in size and position (Fig. 16 G–H); siphonal papillae present longitudinally (Fig. 16 I–J).

Type material. [ D. solenoides] NHM Reg. 1837.12.1.879/884, 1968507 and 1859.9.19.59, 9 syntypes (six broken shells) (figs. 14A–F); [ L. tenuis] ZMB 112740 and 112741, one pair of valves from " ad fretum magellanicum ", ex coll. Philippi and 3 pairs of valves from Magellanic Strait, coll. Dunker, ex coll. Philippi (figs. 14G–L).

Type locality. Puerto El Hambre in the Magellanic Strait, collected in sandy beaches.

Other material examined. Argentina. Punta Alta, Buenos Aires (MORG 33271); San José Gulf (MORG 44813); Puerto Pirámide (MORG 39183); Puerto Madryn (MLP 2322, 8457); Comodoro Rivadavia (USNM 348257, MLP 4498); Puerto Deseado (MLP 4876-2); Punta Medanosa (MLP 1868); Bahía Gregory, Magellanic Strait (USNM 96843); San Sebastián, Tierra del Fuego (NHM 1905,8,11,19-21); Río Grande, T. del Fuego (NHM 12613); Ushuaia (MZUSP 66674). Chile. Puerto Montt (MLP 84509).

Distribution. Punta Alta, Argentina, to Puerto Montt, Chile.

Remarks. Fischer (1887, 1119) considered the genus Darina as a section of the genus Lutraria . For this reason, he proposed Lutraria kingi to remove the supposed homonymy with Lutraria solenoides Lamarck, 1818, living in European waters. However, both genera are clearly distinct and valid.

Darina solenoides is distributed along the Patagonian coast. It is a fragile but very common species found in muddy and sandy intertidal zones in low energy environments. Although the shell characters are sufficient to recognize this form from other mactrids living in the Magellanic area, the organs of the mantle cavity also differ notably from all species previously analyzed. Lutraria tenuis Philippi, 1845, is a synonym (Fig. 14 G–L).