Tonicia schrammi (Shuttleworth, 1856)

Reyes-Gomez, Adriana, Ortigosa, Deneb & Simoes, Nuno, 2017, Chitons (Mollusca, Polyplacophora) from Alacranes Reef, Yucatan, Mexico, ZooKeys 665, pp. 1-36 : 2-4

Tonicia schrammi (Shuttleworth, 1856) Figures 6 A–C, 7 A–K

Material examined.

Two specimens; 27.5 and 28.5 mm long, 14 and 14.2 mm wide. Cabaña CONANP (CNMO4992).

Description.

Medium-sized chitons with an oval outline. Live specimens mostly pink, dark purple and white (Figure 6B), turning to orange when preserved; creamy with large dark brown spots (Figure 6A, C); girdle with narrow white bands, and rounded small spots scattered without any clear pattern. Head valve (Figure 7A) semicircular and slightly notched; sculpture pattern smooth, with fainted radial irregular knobs. Tail valve (Figure 7B) oval; mucro elevated and slightly postmedian; postmucronal area bear few knobs radially oriented to the mucro; antemucronal area smooth. Intermediate valves (Figure 7C) with strongly bluntly pointed apex. Lateral areas (Figure 7D) strongly indicated by a diagonal rib of semi rectangular knobs, also present on the posterior margin, which gives the appearance of dentations, and in between them a third rib with fewer knobs that are closer to the posterior margin. Ocelli arranged in irregular radial bands. Micraesthetes small, grouped with no apparent number and ar rangement, others seem to be scattered (Figure 7E). Tegmentum with scattered small black rounded ocelli, forming radial irregular bands (Figure 7F). Girdle covered with tiny spicules. Apophyses short and wide, semi triangular shaped (Figure 7G). Articulamentum white and thick, insertion teeth hardly dentate, slit deep (Figure 7H); slit formula 9 –10/1/10– 13. Radula (Figure 7I) with central teeth long and distally narrow, minor lateral tooth somewhat arched, long, smaller than central tooth, distally somewhat pointed (Figure 7J); major lateral tooth as one single, rounded, wide plate, not dentate (Figure 7K).

Habitat.

Found in the intertidal on rocks and turtlegrass, T. testudinum .

Remarks.

García-Ríos (2003: fig. 109) described Puerto Rico specimens, and figured the radula microstructure of a 21 mm specimen, which showed the medium tooth narrow and bent outward on its posterior end, and the minor lateral teeth somewhat straight. Our examination of a PNAA specimen of 28.5 mm length revealed the minor lateral tooth arched inwards, and the central tooth is rounded on its distal end (Figure 7J). Microstructure examination of PNAA specimen (28.5 mm) showed the ocelli aligned in irregular bans, somewhat in quincunx towards to the apex, and less abundant than the previous descriptions (Figure 7D, F).

The examination of one paralectotype (NMBE19115/5a) (Figure 14D), revealed a higher density of ocelli somewhat aligned in groups on the lateral areas, head valve and postmucronal area of tail valve. Kaas et al. (2006) described a specimen from Puerto Rico (La Paguera, Media Luna) with similar ocelli arrangement pattern and density. Our specimens showed less density of ocelli and although they appear to be aligned, their arrangement is more irregular (Figure 7D, F). The knowledge of this species in the Mexican Caribbean is limited, and only further studies will corroborate the morphologic distinctions of specimens from this area. In this study, we extend the distribution range of T. schrammi to the PNAA.