Stenoplax hernandezi, Dell’Angelo & Schwabe & Gori & Sosso & Bonfitto, 2014

Dell’Angelo, Bruno, Schwabe, Enrico, Gori, Sandro, Sosso, Maurizio & Bonfitto, Antonio, 2014, Chitons (Mollusca, Polyplacophora) from São Tomé and Príncipe Islands, African Invertebrates 55 (2), pp. 171-200 : 178-181

Stenoplax hernandezi sp. n.

Figs 3A–U , 9A–B

Etymology: Named in honour of José María Hernández, malacologist from Gran Canaria, who passed away recently. He had frequently been a companion of Sandro Gori on trips and dives.

Description:

General shell shape: Animal small, up to 8 mm long, holotype 7.6 × 3.3 mm ( Fig. 9A ), elongate oval, much longer than twice the width, moderately elevated (dorsal elevation 0.31), back evenly rounded with convex side slopes. Colour of tegmentum uniformly light orange, somewhat variegated, with sandy brown streaks or flecks. Girdle of the same colour, with lighter bands present.

Head valve semicircular ( Fig. 3F ), posterior margin widely V-shaped. Intermediate valves ( Fig. 3I ) with anterior margin slightly convex and sinuate at both sides, side margins rounded, posterior margin more or less straight, apices not evident, lateral areas little raised, poorly defined. Tail valve semicircular ( Fig. 3L ), length less than ⅔ of width, anterior margin straight and directed backwards laterally, mucro central, poorly evident, antemucronal slope slightly convex ( Fig. 3M ), postmucronal slope slightly concave just behind the mucro.

Head valve, lateral areas of intermediate valves, and postmucronal area of tail valve sculptured with many close-set, irregularly undulate concentric grooves, better defined towards periphery ( Fig. 3H ); central area of intermediate valves and antemucronal area of tail valve smooth, some longitudinal grooves present but hardly visible.

Articulamentum: Apophyses triangular, widely separated by a straight sinus ( Fig. 3J ), insertion plates short, slit formula 15 / 3 / 11, slit rays neatly indicated by distinct rays of pores, teeth sharp and not equidistant, eaves narrow.

Girdle: Densely covered with small, rectangular, slightly bent, imbricating scales dorsally ( Fig. 3S ), rounded at the top, up to 55 μm long and 65 μm wide, with 14–15 narrow riblets, interstices wider. Compared to congeners, the scales are smaller, more rectangular and distally straight along a narrow strip at the margin, among which arise some conical, weakly longitudinally striated, pointed spines, up to 100 μm long ( Fig. 3Q ). Marginal fringe composed of small, longitudinally striated, pointed spines, ca 47 × 8 μm ( Fig. 3P ). Ventrally, the girdle is paved with radiating rows of small, cylindrical, smooth spicules ( Fig. 3R ), up to 60 μm long.

Radula: Central tooth small and narrow, first lateral tooth elongate ( Fig. 3T ), with a narrow blade and a subapical wing, major lateral tooth ( Fig. 3U ) bears a bicuspid head with two equally strong denticles.

Holotype: SÃO TOMÉ AND PRÍNCIPE: São Tomé, Minerio, ST 06, 7.6 × 3.3 mm, Fig. 9A ( MZB 49751 ) . Paratypes: Same data as holotype : 1 specimen, length 5.5 mm, slightly curled ( BD 109 ) ; 1 specimen, length 7 mm, slightly curled ( MNHN IM-2012-2701) ; 1 specimen, length 6.8 mm, disarticulated and coated for SEM analysis ( MZB 49752 ) ; 1 specimen, length 4.9 mm, slightly curled ( SG) ; 1 specimen, length 5.6 mm, slightly curled ( ZISP 2197 ) . Other paratypes: ST10, 1 specimen in alcohol, length 5.7 mm ( ZSM Mol-20034146); ST10 , 1 specimen in alcohol, length 5 mm ( ZSM Mol-20034144); ST10 , 1 specimen in alcohol, length 5.7 mm ( ZSM Mol-20034150), disarticulated and coated for SEM analysis ( Figs 3A–B ); PR03 , 1 specimen in alcohol, of a darker colour, Indian red with lighter streaks, length 6.8 mm ( MHNS 100601 ) .

Other material examined: ST03: 1 specimen, length 8 mm, disarticulated for SEM analysis ( Fig. 9B ), and 76 valves (15 head, maximum width 3 mm, 12 intermediate, maximum width 3.4 mm, and 49 tail, maximum width 3 mm) ( BD 110 , ER); ST06 : 2 specimens, maximum width 6.7 mm, and 11 valves (3 head, maximum width 3 mm, 5 intermediate, maximum width 3 mm, and 3 tail, maximum width 2.3 mm) ( BD 111 ); ST08: 2 specimens, maximum length 5 mm ( ZSM Mol-20034145) .

Distribution: São Tomé and Príncipe Islands.

Comparison and remarks: This species was assigned to the genus Stenoplax mainly because of the elongate shape, with the length 2 to 3 times the width. On their own, not all characters of the new species match those of the genus Stenoplax . The lateral areas of Stenoplax species are generally rather elevated (Kaas & Van Belle 1987: 124; Vendrasco et al. 2012: 34), but are little raised and poorly defined in our species. However, the same characteristic is seen in other Stenoplax species, e.g. S. marcusi ( Righi, 1971) .

About half of the approximately 22 living species of Stenoplax worldwide, occur in the temperate or tropical eastern Pacific, but a few species are found in the Caribbean, and two species are known from Brazil, namely Stenoplax kempfi ( Righi, 1971) and S. marcusi ( Righi, 1971) . The finding of Stenoplax hernandezi sp. n. on the Atlantic African coast greatly extends the distribution of this genus.

Stenoplax kempfi differs from the present species mainly in respect of the different sculpture (radial riblets on the head valve, lateral areas of intermediate valves, and postmucronal area of tail valve, longitudinal riblets on the central area of intermediate valves, and antemucronal area of tail valve, vs. concentric undulate grooves and no riblets in the new species); the greater dimensions (maximum length from 44 to 120 mm vs. 8mm); and the dorsal girdle formations (always elongate scales/spicules, not rectangular scales as in the new species).

Stenoplax marcusi , originally placed in Ischnochiton , and later attributed to Stenoplax by Kaas and Van Belle (1994) following study of the type material, has a tegmental sculpture more similar to that of S. hernandezi sp. n., a similar colour, and it is of comparable size (maximum length 11.8 vs. 8 mm). It differs because of the higher elevation (dorsal elevation 0.47 vs. 0.31), the number of intermediate valve slits (1 vs. 3), and the different girdle formations (see Righi 1971; Kaas & Van Belle 1994).

Only a single species of Stenoplax is known from the Indian Ocean, namely S. madagassica (Thiele, 1917) from Madagascar. This species differs from the present species in having greater dimensions (up to 65 mm long vs. 8 mm), being more highly elevated (dorsal elevation 0.42 vs. 0.31), having different dorsal girdle scales (elongate and up to 145 μm long with interspersed whitish ringshaft-needles vs. rectangular up to 55 × 65 μm), and in respect of the geographical distribution ( Madagascar vs. São Tomé).

Stenoplax hernandezi sp. n. is generally similar to some of the small Ischnochiton species reported from West African coasts (i.e. I. goreensis , I. paessleri , and I. nicklesi ), but they all have a tegmental sculpture that is mainly smooth or finely granulose.

Figures

Fig. 3. (A–U) Stenoplax hernandezi sp. n., São Tomé Island: (A–E), (P–Q), (T–U) specimen from ST10 (ZSM Mol-20034150), length 5.7 mm, disarticulated and coated for SEM analysis, (A–B) dorsal and frontal views, (C) detail of the head valve, (D) detail of the sculpture of an intermediate valve, (E) detail of the tail valve, (P–Q) girdle formations in situ, (T–U) radula, (T) complete view, (U) detail of major lateral tooth; (F–O), (R–S) paratype MZB 49752, specimen from ST06, length 6.8 mm, disarticulated and coated for SEM analysis, (F–H) disarticulated head valve, (F–G) dorsal and ventral views, (H) detail of the sculpture, (I–K) disarticulated intermediate valve, dorsal, ventral and frontal views, (L–O) disarticulated tail valve, (L–N) dorsal, lateral and ventral views, (O) detail of the sculpture, (R–S) detail of girdle formations, (R) ventral spicules, (S) dorsal scales. Scale bars = 1 mm (A, B); 800 μm (C, E, I, L); 600 μm (F, G, J, K, M, N); 100 μm (D); 80 μm (O, Q); 60 μm (H, T); 40 μm (P); 30 μm (R, S, U).

Fig. 9. (A–B) Stenoplax hernandezi sp. n., São Tomé Island: (A) ST06, Holotype, length 7.6 mm, MZB 49751, (B) ST03, length 8 mm, specimen disarticulated for SEM analysis; (C–E) Callochiton saotomensis sp. n., São Tomé Island: (C) ST03, Holotype, length 4.6 mm, MZB 49761, (D) ST06, length 4.9 mm, Paratype NHMUK, (E) ST06, length 4.4 mm, Paratype ZISP 2198; (F–G) Chiton lyratus Sowerby, 1840, NHMUK 1979176, two Syntypes, type locality: unknown (restricted to Príncipe Island);(H–J) Chiton canariensis d’Orbigny,1839,NHMUK 1854.9.28.152,three Syntypes, type locality: Canary Islands; (K–N) Rhyssoplax canariensis (d’Orbigny, 1839), colour variations, (K) ST03, length 20.5 mm, (L) ST02, length 8 mm, (M) ST03, length 13.2 mm, (N) ST02, length 8.8 mm; (O) Acanthochitona crinita (Pennant, 1777), São Tomé Island, ST06, length 6.5 mm; (P–S) Craspedochiton foresti (Leloup, 1965), São Tomé Island, ST03, (P–Q) length 16 mm, dorsal and ventral views, (R–S) length 10 mm, dorsal and lateral views.