Ischnochiton erythronotus (C. B. Adams, 1845)

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

Ischnochiton erythronotus (C. B. Adams, 1845) Figures 2 A–H, 3 A–G

Material examined.

20 specimens; 0.5-16 mm long, 0.3-8.5 mm wide. Laguna Arrecifal Desterrada (CNMO4980), Isla Perez (CNMO4981, CNMO4982, CNMO5002), Cabaña y Playa CONANP (CNMO4983, CNMO4984, CNMO4989, CNMO4998, CNMO5000, CNMO5003, CNMO5004), Cabezas entre Blanca y Pajaros (CNMO4985), Playa Norte (CNMO4986, CNMO4988, CNMO5001), Playa Arrecifal (CNMO4987).

Description.

Small-sized chitons, broad oval shape. Color of tegmentum and girdle very variable, mostly creamy, red, purple or light green and mottled with dark brown dots or patches (Figure 2 A–H). Tegmentum sculptured with irregular concentric riblets and longitudinal narrow ribs. Head valve (Figure 3A), semicircular, not notched, sculpturing of irregular concentric ribs, broken into numerous riblets, forming fine radial indicated grooves. Tail valve (Figure 3B), wider than long, mucro postmedian, somewhat elevated; antemucronal area with 9-11 narrow, sometimes branched longitudinal narrow ribs; postmucronal area with concentric riblets and nodules, forming 19-23 radial grooves. Intermediate valves (Figure 3C), semi-rectangular outlined, side margins rounded and posterior margin from slightly concave to straight; lateral areas somewhat elevated, sculptured as the head valve, with two to three radial grooves; pleural areas with 12-14 longitudinal ribs, which fade towards the jugal area. Articulamentum laterally short; apophyses narrow, subtriangular shaped, jugal sinus wide and smooth (Figure 3D), slit formula 8 –10/1/7– 10. Megalaesthetes surrounded by 7-8 large micraesthetes (Figure 3E). Girdle variable in color as tegmentum, with alternating irregular bands of dark and lighter color, dorsally with wide, short scales (Figure 3F), with 10-12 wide and flat ribs. Radula (Figure 3G), with major lateral teeth tricuspid, the outer cusp is shorter than the others, central tooth very narrow anteriorly wider and spatulated.

Habitat.

Found in intertidal to shallow subtidal to 12 m depth. Found on dead coral, wood, rock, and sunken turtlegrass, Thalassia testudinum Banks ex König.

Remarks.

This species displays a considerable intraspecific variation in color and valve sculpturing; also observed in chitons from Cozumel Island and in Banco Chinchorro reef (in the most southern region of Quintana Roo) (CNMO5558), which was also noted by García-Ríos (2003) for specimens from Puerto Rico and Ferreira (1978a) for Jamaica specimens. This variability was found in both, the juvenile and adult chitons morphology. In general, specimens reaching a length from 8-12.1 mm showed more regular and less broken ribs, whereas animals between 13.5-16 mm length developed branched ribs, particularly in pleural areas, and occasionally showed nodule formations on the head and the postmucronal area of the tail valve. This is the most common and abundant chiton species in the PNAA.