Callochiton saotomensis sp. n.

Figs 5A–S, 9C–E

Etymology: Named for the São Tomé Island, the type locality of the species.

Description:

General shell shape:Animal of small size, maximum length 6.5 mm, elongate, moderately elevated, dorsal elevation 0.36, subcarinated. Tegmentum evenly reddish, more or less dark; the surface areas of the intermediate valves sometimes lighter laterally, and the girdle evenly of a lighter tone.

Head valve semicircular (Fig. 5A), with a straight anterior slope and a wide, V-shaped posterior margin, unnotched in the middle. Intermediate valves more or less rectangular (Fig. 5D); anterior margin slightly concave in the jugal area; side margins somewhat rounded; posterior margin concave on both sides of the beaked apex; lateral areas raised. Tail valve more than semicircular (Fig. 5J); mucro elevated and situated anteriorly, antemucronal and postmucronal slopes straight (Fig. 5K).

Tegmentum roughly covered with elevated dorsal papillae (Fig. 5I), on which the apical caps are situated with no apparent association with the surrounding subsidiary caps (Fig. 5H). Dorsal papillae arranged in irregular rows, radially in the head valve, the lateral areas of intermediate valves and the postmucronal area of tail valve, longitudinally in the central area of intermediate valves and the antemucronal area of tail valve. The rows have an irregular pattern, in some cases with the dorsal papillae assembled in series not connected among them (Fig. 5L), looking like segments sometimes slanting in the central area of intermediate valves.Apical caps circular, diameter ca 6 μm. Black pigmented shell eyes present in the head valve, the lateral areas of intermediate valves, and the postmucronal area of tail valve.

Articulamentum: Red-brownish and thin. Apophyses short and wide with anterior edge broadly rounded, connected by a wide V-shaped sinus. Insertion plates long, slit formula:? / 2–3 / 12–13. Teeth of insertion plates broad and solid. Slit rays indicated in all valves by radial rows of elongate holes (Fig. 5E). Eaves spongy.

Girdle: Densely covered dorsally with long, round-topped, inwardly directed and smooth rectangular spicules (Fig. 5N), measuring 110 × 25 to 200 × 40 μm (Fig. 5O). Ventrally, the girdle is paved with radiating rows of small, sharply pointed, imbricating scales (Fig. 5P), up to 50 μm long.

Radula:Asymmetric central tooth elongate (Fig. 5R), tulip-shaped. First lateral tooth with broad shaft and wing-shaped extension in front (Fig. 5S), blade sharply pointed, with a shallow depression in the middle. Major lateral tooth (Fig. 5S) bears a broad tridentate cusp with pointed denticles.

Holotype: SÃO TOMÉ AND PRÍNCIPE: São Tomé, Lagoa Azul, ST 03, length 4.6 mm, Fig. 9C (MZB 49761).

Paratypes: Same data as holotype: 1 specimen, length 6.5 mm, slightly curled (BD 115); 1 specimen, length 3.4 mm (MNHN IM-2012-2702) . Other paratypes: ST06: 1 specimen, length 3 mm (BD 116); ST06: 1 specimen, length 4.2 mm (MSNG 57363); ST06: 1 specimen, disarticulated and coated for SEM analysis (MZB 49761); ST06: 1 specimen, length 4.9 mm, Fig. 9D (NHMUK); ST06: 1 specimen, length 4.1 mm (SG); ST06: 1 specimen, length 4.4 mm, Fig. 9E (ZISP 2198); ST10: 1 specimen in alcohol, length 4.5 mm (ZSM Mol-20034148) .

Other material examined: ST03: 1 specimen, length 4.3 mm, and 4 valves (3 intermediate, maximum width 2.8 mm, and 1 tail, width 2.5 mm) (BD 117, ER); ST06: 8 specimens, maximum length 4 mm, and 4 intermediate valves, maximum length 2.3 mm (BD 118) .

Distribution: São Tomé and Príncipe islands.

Comparison and remarks: A rather uniform species. Also, the colour is prevalently as described, evenly reddish. Only a few specimens have a lighter tone, evenly evident on some valves, either on the entire valve or only some parts thereof, e.g. pleural areas (Fig. 9D, Paratype NHMUK). Some inconspicuous, oblique coloured striae in the central areas of the intermediate valves give the impression of real streaks.

Only one congener, Callochiton septemvalvis (Montagu, 1803), is common throughout the Mediterranean Sea and also occurs along all of the European Atlantic coast, from Norway and the Shetland Islands as far south as the Canary Islands (Kaas & Van Belle 1985 b; Dell’Angelo & Smriglio 1999; Rolán 2011), the Azores (Avila & Albergaria 2002), the West African coast and the Gulf of Guinea (Leloup 1968 a). Another species, C. calcatus Dell’Angelo & Palazzi, 1994, is known from scattered localities in the Mediterranean Sea, and is characterized by the strongly angulated valves, with a dorsal elevation of ca 0.7.

C. septemvalvis differs by the larger size (length up to 30 mm), the greater number of slits in the insertion plate of the tail valve (14–18 vs. 12–13), the central mucro of the tail valve (vs. situated anteriorly), some differences in radular teeth (an elongate first lateral tooth), and mainly by the different tegmental sculpture, with the very regular arrangement of non-elevated dorsal papillae (cf. Baxter & Jones 1984: pl. 2, figs a–d), and the larger diameter of the apical caps (8.5 μm vs. 6 μm).

C. septemvalvis has a very complicated taxonomic history (Kaas 1978; Kaas & Van Belle 1985 b; Dell’Angelo & Smriglio 1999). Many authors separated the typical Atlantic form from the Mediterranean one, which is characterized by its smaller size and the presence of longitudinal grooves on the pleural areas, at subspecific or specific level (Kaas 1978; Carmona Zalvide et al. 2002). Specimens from the African coast and the Canary Islands invariably have longitudinal grooves (see Kaas & Van Belle 1985 b: 14). Occurrence records include “Cap de Naze et Cap de Somone” (1 specimen, 7.5mm), Dakar (1 specimen) and the Gulf of Guinea (“Calypso”, northeast of Príncipe and Mosteiros Islands, 37 m, 1 specimen, 3.5 mm). These last records, all reported by Leloup (1968 a), should be checked to clarify whether the material is C. septemvalvis or the new species.