Acanthochitona mahensis Winckworth, 1927

Thomas, Liju, Anseeuw, Bruno, Kutty, Ranjeet & Parambil, Sooraj Kandathil, 2023, Redescription of Acanthochitona mahensis Winckworth, 1927 (Polyplacophora, Acanthochitonidae) from Indian Coasts, Zootaxa 5244 (1), pp. 41-50 : 43-48

Acanthochitona mahensis Winckworth, 1927

( Figures 2–7 )

Acanthochitona mahensis Winckworth, 1927: 207 , pl. 29; Leloup, 1941: 4; Satyamurti, 1952: 5; Rajagopal & Rao, 1974: 401; Sarma, 1974: 161; Wellens, 1988: 20; Subba Rao & Dey, 2000: 6; Slieker, 2000: 50; Hylleberg & Kilburn, 2002: 21; Subba Rao, 2003: 56; Ramakrishna et al., 2007: 3; Sirenko & Schwabe, 2011: 118; Anu et al., 2017: 42; Kazmi, 2018: 12; Baiju et al., 2019: 5; Edward et al., 2022: 469; Thomas & Kutty, 2022: 6.

Acanthochitona cf. mahensis: Anseeuw & Terryn, 2004: 19 ; Dell’Angelo et al., 2020: 385.

Acanthochiton ashbyi Leloup, 1937: 1 ; Leloup, 1941: 1.

Materials examined. Holotype, No. ZSI M14376/2 , Near Chombala, Mahe , South India. It consists of three complete valves (Valve I, Valve II and an additional intermediate valve) ( Fig. 6 ); the remaining valves are broken and fragments of these valves are present. Kerala, Chombala : 3 specimens ( ZSI / WGRC /I. R.- INV. 21907 , BL 11.14 mm; ZSI / WGRC /I. R.- INV. 21908 , BL 14.09 mm; ZSI / WGRC /I. R.- INV. 21909 , BL 22.36 mm), 11°39′41.5″N 75°32′53.2″E, natural rocks near the rock bund, Dec 2021, Liju Thomas and Jithin K; Vizhinjam : 1 specimen ( ZSI / WGRC /I. R.- INV. 21912 , BL 26.00 mm), 8°22′51.9″N 76°58′58.6″E, intertidal rocks, Jan 2018, Liju Thomas and Ramvilas Ghosh A; Karnataka, Honnavar : 1 specimen ( ZSI / WGRC /I. R.- INV. 21913 , BL 14.32 mm), Oct 2018, Liju Thomas; Goa, Anjuna : 1 specimen ( ZSI / WGRC /I. R.- INV .21910 , BL 16.87 mm), 15°35′03.8″N 73°44′12.3″E, intertidal rocks, Oct 2018, Liju Thomas; Tamil Nadu, Kovalam : 1 specimen ( ZSI / WGRC /I. R.- INV .21911 , BL 16.90 mm), 12°47′24.4″N 80°15′15.9″E, intertidal rocks, April 2021, Liju Thomas and Judith Das.

Diagnosis. Tegmentum sculptured with closely packed obovate, apically flat pustules, arranged in quincunx. Each pustule with a single macraesthete pore surrounded by up to 34 micraesthete pores. Longitudinally striated jugal area. Posterior mucro with convex postmucronal slope and tegmentum extending beyond the articulamentum. Sutural tufts with 50 or more bristles.

Description. Chiton of medium size, largest specimen 26.00 mm (ZSI/WGRC/I.R.-INV.21912). Specimen used for description, BL 11.1 mm (ZSI/WGRC/I.R.-INV.21907). Body oval; valves beaked, moderately elevated (dorsal elevation quotient of valve IV 0.35); back evenly rounded. Colour of tegmentum as combinations of brown, dull white and dark green, jugum dark brownish in the middle and much lighter on the sides.

Head valve semi-circular, front slope convex, posterior margin almost straight with a small projecting apex. Intermediate valves rather wide, trapezoidal, moderately elevated; front margin slightly sinuate; hind margin slightly concave at both sides of the beak; side margin rounded. Tail valve triangular with posterior mucro; postmucronal slope convex; width of tegmentum ca 78 % of that of the head valve.

Tegmentum sculptured with closely packed obovate, apically flat pustules, arranged in quincunx. Each pustule with a single macraesthete pore, located towards the posterior end of the pustule. Macraesthete pore is surrounded by up to 34 micraesthete pores. No pores on tegmental plain. Jugal area with vertical raised lines, with aesthete pores throughout. Tegmentum in the posterior margin of tail valve extends beyond the articulamentum.

Articulamentum well developed, shade of white and blue in colour; apophyses rounded, widely apart from each other in intermediate valves, truncated in tail valve. Slit formula 5/1/2.

Girdle rather wide, ca. 2 mm from valve V, densely covered by small pointed needles (ca 30 µm) interspersed by larger ones (ca 250 µm). Girdle dull white, interspersed needles with greenish brown tips, base of the tufts with light green colour. Sutural tufts with many sharply pointed long smooth needles (130–513 µm). Marginal needles similar to sutural tufts but smaller (ca 200 µm). Ventral spicules flattened, bluntly pointed (ca 50 µm).

17 gills on each side in examined specimen (BL 11.1 mm) starting from valve IV to VII.

Radula 4 mm long, with 36 transverse rows of mature teeth in specimen with BL 11.14 mm. Symmetrical radula, central tooth broadly rectangular, posterior surface slightly concave at distal half; first lateral tooth with nodulous antero-dorsal corner; major lateral tooth with tricuspid head with sharply pointer denticles, central one little longer than others; major uncinal tooth with fairly wide blade.

Notes. Acanthochitona mahensis resembles several other species of Acanthochitona such as A. biformis ( Nierstrasz, 1905) from Indonesia, Philippines and Vietnam, A. intermedia ( Nierstrasz, 1905) from Indonesia, A. jugotenuis Kaas, 1979 from Comoros and Mozambique, A. leopoldi ( Leloup, 1933) from Indonesia, Spratly Islands and Sri Lanka, A. penicillata ( Deshayes, 1863) from the Indian Ocean and Philippines, and A. woodwardi Kaas & Van Belle, 1988 from Kuwait and Qatar.

Acanthochitona mahensis differs from A. biformis in having obovate pustules), one macraesthete and up to 34 micraesthete pores (vs one macraesthete pore or occasionally two pores near centre), and a longitudinally striated jugal area (vs jugum with fine growth lines) ( Sirenko & Saito 2017).

Acanthochitona mahensis differs from A. intermedia in having obovate pustules (vs drop-shaped pustule), one macraesthete and up to 34 micraesthete pores in pustules (vs one macraesthete and no micraesthete pores) and a longitudinally striated jugal area (vs smooth jugal area) ( Sirenko & Zhang 2019).

Acanthochitona mahensis differs from A. jugotenius in having obovate pustules (vs round granules) and smooth marginal needles (vs ribbed marginal needles) ( Kaas 1979).

Acanthochitona mahensis differs from A. leopoldi in having a longitudinally striated jugal area (vs smooth jugal area), one macraesthete, up to 34 micraesthete pores (vs one macraesthete pore and no micraesthete pores) and a posterior mucro (vs central mucro) ( Sirenko & Tai 2020).

Acanthochitona mahensis differs from A. penicillata in having obovate pustules (vs drop-shaped granules), a longitudinally striated jugal area (vs smooth jugal area), one macraesthete and up to 34 micraesthete pores in pustule (vs one macraesthete pore and one to three micraesthete pores in pustule) and a sutural tuft with 50 or more bristles (vs 20 or more bristles) ( Dinapoli & Janssen 2009).

Acanthochitona mahensis differs from A. woodwardi in having closely packed pustules (vs separated pustules), a convex postmucronal slope (vs concave) and an extended posterior margin of the tail valve beyond the articulamentum (vs not extended beyond articulamentum) ( Kaas & van Belle 1988).

Distribution. India, Andaman and Nicobar Islands, Gulf of Mannar, Pakistan, Sri Lanka, Jordan and Eritrea. Present study provides extended distribution range of the species to the north and south of the type locality along the west coast and from east coast of India.

Figures

FIGURE 2. Acanthochitona mahensis. ZSI/WGRC/I.R.-INV.21907, BL 11.14 mm. A. Middle part, dorsal view. B. Lateral view. C. Anterior part, dorsal view.

FIGURE 3. Acanthochitona mahensis ZSI/WGRC/I.R.-INV.21907, BL 11.14 mm. A. Valve I, dorsal view. B. Valve II, dorsal view. C. Valve V, dorsal view. D. Valve VIII, dorsal view. E. Valve V, tegmentum sculpture in jugal and pleurolateral areas. F. Valve V, rostral view. G. Valve IV, ventral view. H. Valve VIII, lateral view. Scale bars: 500 µm (A–D, F, G, H); 200 µm (E).

FIGURE 4. Acanthochitona mahensis. A. Piece of perinotum with three tufts of needle, and with marginal spines, ZSI/WGRC/ I.R.-INV.21907 BL 11.1 mm. B. Part of perinotum showing the dorsal, marginal and ventral spines, ZSI/WGRC/I.R.-INV.21908 BL 14.1 mm. Scale bars: 200 µm.

FIGURE 5. Acanthochitona mahensis. A, B. ZSI/WGRC/I.R.-INV.21907, BL 11.1 mm. C, D. ZSI/WGRC/I.R.-INV.21908, BL14.1 mm. A, C. Part of radula. B. Central and lateral tooth of radula. D. Central tooth of radula. Scale bars: 40 µm (A, B); 100 µm (C); 25 µm (D).

FIGURE 6. Acanthochitona mahensis. Holotype M14376/2 (ZSI, Kolkata). A. Valve I, dorsal view. B. Valve II, dorsal view. C. Valve VI, dorsal view. D. Valve VI, ventral view. Scale bar: 1 mm (A–D).

FIGURE 7. Type locality of Acanthochitona mahensis, Chombala, Mahe, India.