Callistochiton ashbyi ( Barnard, 1963 )

Dell’Angelo, Bruno, Prelle, Giovanni, Sosso, Maurizio & Bonfitto, Antonio, 2011, Intertidal chitons (Mollusca: Polyplacophora) from southern Madagascar, African Invertebrates 52 (1), pp. 21-21 : 26-29

Callistochiton ashbyi ( Barnard, 1963)

Figs 3A–P , 4D–F

Chiton ashbyi: Barnard 1963: 343 , fig. 29m; 1974: 741; Kaas 1979: 860 (in synonymy of Callistochiton crosslandi ); 1986: 13 (in synonymy of C. crosslandi ); Leloup 1981: 14, 17; Giles & Gosliner

1983: 2; Kaas & Van Belle 1994: 138 (in synonymy of C. crosslandi ).

Description:

Animal of moderate size, up to 27× 14 mm, moderately elevated, back evenly arched, not carinated, valves not beaked. Colour of tegmentum and girdle uniform pale ochraceous to yellowish.

Head valve semicircular, little elevated, anterior slope slightly convex, posterior margin widely V-shaped with rounded notch at apex ( Fig. 3A ). 11 or 12 rounded radial ribs of different widths, not split, separated by narrow interstices, not reaching apex, tegmentum very finely granulated, ribs ornamented with large nodules (10–15), which become less pronounced towards apex ( Fig. 3B ).

Intermediate valves broadly rectangular, anterior margin convex, posterior margin straight, side margins strongly bilobate, apices usually worn away ( Fig. 3G ). Lateral areas distinctly defined, sculptured like head valve, two nodulose ribs of about equal width, separate by narrow, deep sulcus. Central area sculptured with 23–27 longitudinal, elevated, granulose riblets ( Fig. 3H ); many riblets in central part of the valve strongly ramifying, riblets on jugum are less defined, with narrow interstices, not reaching apex. If not eroded, apical region finely reticulated.

Tail valve with mucro almost central, small, sharp ( Fig. 3C ), postmucronal slope concave ( Fig. 3D ). Antemucronal area sculptured like central areas, postmucronal area sharply differentiated with 12–16 strong, nodulose, radial ribs; ribs of differing widths, some ribs splitting, bearing a few tubercles near outer margin.

Articulamentum white, insertion plates well developed, apophyses short and trapezoidal, more triangular in valve ii. Slit formula 9/1/9, slits deep ( Fig. 3J ), teeth irregular, smooth, broad, somewhat scalloped.

Girdle moderately wide, dorsally densely clothed with strongly imbricating, round-topped scales, up to 228×145 μm ( Fig. 3N ), sculptured with 12–15 very fine riblets ( Fig. 3P ). Ventral scales flat, smooth, elongate rectangular, ca 85–100×18–25 μm, arranged in radiating rows ( Fig. 3O ).

Central tooth of radula very slender, parallel-sided, first lateral tooth equally narrow, major lateral tooth with bilobed blade ( Fig. 3L ), with two denticles of same dimensions.

Ctenidia arranged holobranchially.

Comparison and remarks:Three Callistochiton species were previously recognized from Madagascar: C. crosslandi Sykes, 1907 , C. barnardi Leloup, 1981 , and C. rotondus Leloup, 1981 (Kaas & Van Belle 1994). These authors and Kaas (1979), included C. madagassicus Thiele, 1909 from Madagascar, and Chiton ashby Barnard, 1963 from South Africa as synonyms of C. crosslandi , which was described from a single specimen from Kenya (presumed lost, Kaas & Van Belle 1994).

While Kaas (1979) recognised differences between C. madagassicus (holotype MNHN, Fig. 4G ) and C. crosslandi , he regarded the former as a synonym of the latter. The unique intermediate valve type of Chiton ashbyi ( Figs 4E, 4F ) differs from either of these, but agrees completely with the Lavanono material (Table 1), which is therefore identified as Callistochiton ashbyi . This species differs from C. crosslandi in several significant characters – the radial ribs of the head valve are never bifid, the greater number of longitudinal ribs on the intermediate valves, and the smaller number of radial ribs on the tail valve.

Considering how few specimens of C. crosslandi and C. madagassicus have ever been recorded, the 20 specimens of Callistochiton ashbyi found at Lavanono are a

TABLE 2 Length and number of radial ribs of valves i, iv and viii of Callistochiton ashbyi specimens.

Specimen Length Ribs (i) Ribs (iv) Ribs (viii)

1 23.6 11 27 13

2 23.3 11 26 12 (+2)

3 19.5 11 25 12

4 21.8 11 23 13

5 15 11 25 11

6 18.5 11 25 13

7 15.8 11 26 11 (+2)

8 11.8 12 26 10

9 15.4 12 25 14

10 11.7 11 25 13

11 15.2 11 23 9

12 12.5 11 27 10 (+1)

13 14.3 11 24 10 (+1)

14 14.5 12 23 12

15 10.8 14 27 10

16 11 12 26 12 (+1)

17 16.6 11 22 12

18 13.5 10 25 11 (+1)

19 27 12 26 11 (+1)

20 21.2 11 23 10

very significant collection. Table 2 shows number of radial ribs of valves i and viii and number of longitudinal ribs of valve iv compared to specimen length of all specimens of C. ashbyi from Lavanono. It is obvious that these are consistent characters, and that the differences shown in Table 1 are significant.

Leloup (1981) identified five specimens from Tuléar, collected by P. Galenon, as Chiton ashbyi , which is the only other record of this species.

C. barnardi and C. rotondus are different species, not comparable with the Lavanono material (Kaas & Van Belle 1994; Schwabe 2004; Dell’Angelo et al. 2010 b).

Type material: Holotype: SAMC-A9337, an intermediate valve ( Fig. 4F ).

Type locality: South Africa, off Cape Infanta , 83 m, collected by R.S. Pieter Faure, 10 July 1900 .

Material examined: Lavanono ( Madagascar): 20 specimens ( GP, BD, MZB) .

Distribution: South Africa and Madagascar.

Figures

Fig.3. (A–P) Callistochiton ashbyi (Barnard, 1963),Lavanono, Madagascar, specimen n.6 (Table 1), MZB: (A, B) head valve, dorsal and lateral views; (C–F) tail valve: (C, D) dorsal and lateral views, (E) detail of the sculpture of antemucronal area, (F) detail of longitudinal, granulose riblets; (G–K) intermediate valve: (G) dorsal view, (H) detail of longitudinal, granulose riblets of pleural area, (I) ventral view, (J) detail of slit, (K) frontal view; (L, M) radula: (L) complete view, (M) detail of central, first lateral and major lateral teeth, with the bilobed blade; (N–P) girdle elements: (N) dorsal scales,(O) ventralscales, (P) a single dorsal scale;(Q–X) Cryptoplax dupuisi Ashby, 1931, Lavanono, Madagascar:(Q–V) girdle elements:(Q) “naked” dorsal girdle, in correspondance of valves i–iv,(R) “normal” dorsal girdle of valves v–viii, (S) dorsal spicules,(T) marginal spicules, (U) ventral spicules on valves i–iv, (V) ventral spicules on valves v–viii; (W, X) radula: (W) complete view, (X) detail of an half row of teeth. Scale bars 1 mm (A–D, G, I, K), 500 μm (Q, R), 100 μm (E, F, H, J, L, N, O, S–X) and 50 μm (M, P).

Fig.4.(A, B) Ischnochiton sirenkoi sp. n., Lavanono,Madagascar:(A) holotype MZB 45693, 12.2 ×6.1 mm; (B) a lot ofspecimens showing colour variations,length,from left to right, respectively 17.7,11.3, 11.2, 12, 10 and 9.5 mm; (C) Ischnochiton sansibarensis Thiele, 1909, holotype ZMB 102069, Zanzibar, ca 4 mm; (D–F) Callistochiton ashbyi (Barnard, 1963): (D) Lavanono, Madagascar, specimen n. 2 (Table 1), 23.3 mm; (E, F) holotype SAMC-A9337, an intermediate valve from Cape Infanta,SouthAfrica,dorsal and ventral views,9×3.3 mm; (G)Callistochiton madagassicus Thiele,1909, holotype MNHN,Madagascar,21×14 mm (from original description),some detached valves at left; (H) Chaetopleura chelazziana Ferreira, 1983, Lavanono, Madagascar, 47×31 mm; (I–X) Cryptoplax dupuisiAshby, 1931, Lavanono, Madagascar:(I–K) specimens with the girdle deprived of dorsal spicules on the first four valves; (I, J) adult specimen, 51×12.5mm; (K) the smallerspecimen,28.5×4 mm;(L) specimen with the girdle completely coveredof dorsal spicules, 44×10 mm; (M, N) detail of sculpture of valves i–ii, in situ; (O, P) tail valve, lateral and ventral views, length 6.8 mm; (Q–X) valves i–viii (same scale), width (plates, without apophyses), from left to right, 4.4 mm (i), 3.9 mm (ii), 1.45 mm (iii), 2.65 mm (iv), 1.45 mm (v), 1.45 mm (vi), 2.4 mm (vii), 2.75 mm (viii).