Cryptoconchus floridanus (Dall, 1889)

Reyes-Gomez, Adriana, Ortigosa, Deneb & Simoes, Nuno, 2017, Chitons (Mollusca, Polyplacophora) from Alacranes Reef, Yucatan, Mexico, ZooKeys 665, pp. 1-36 : 7-9

Cryptoconchus floridanus (Dall, 1889) Figures 6 L–O, 12 A–F, 13 A–H

Material examined.

Two specimens; 7.1-9.3 mm long, 3.0-4.2 mm wide. Isla Perez (CNMO4996).

Description.

Small-sized chitons, of oval body shape, somewhat elongated. Color dark brown to black, tegmentum white or creamy (Figure 6 L–O). Valves covered by a smooth girdle; tegmentum very reduced; only the jugal area is exposed. Occasionally, tegmentum developing pustule formations, roundish or in irregular polygon shapes, opposed to oval, with a single megaesthete located on the pustule base. Head valve (Figure 12A) with a sub-quadrate outline, not notched, the posterior margin slightly concave; tegmentum semicircular and raised, flattened on its base; usually smooth, it can show small, irregularly shaped semi-oval pustules, which are larger anteriorly (Figure 13B), and arranged in a concentric pattern around the apex, covering most of the dorsal tegmentum surface (Figure 13A). Intermediate valves (Figure 12D, E), with a pointed apex; jugum smooth, slightly wider anteriorly; tegmentum when present reduced to two longitudinal narrow areas, adjacent to the jugum, and located near to the apex (Figure 13C), with small, longitudinally orientated and often irregularly rounded protruding (Figure 13D); jugum smooth with numerous megalaesthetes, distributed on its posterior end. Tail valve wider than long; jugal area narrow; mucro postmedian (Figure 12C); tegmentum (when present) bulb-shaped (Figure 13E); in juvenile specimens, the postmucronal area somewhat depressed, and slits missing (Figure 12B). Tegmentum around the mucro very limited, can bear a few pustule-like somewhat rounded to completely irregular shaped forms; the jugal area near to the mucro shows numerous megalaesthetes with no apparent arrangement (Figure 13E). Articulamentum wide, and especially in head valve, slits are somewhat deeply “u” -shaped; intermediate valves with two short and shallow slits (Figure 12F), almost absent in juveniles (Figure 12D), apophyses wide and wing-shaped; articulamentum of tail valve anteriorly wide, with short apophyses, with two “u” -shaped slits, located on the base of the valve; slit formula 5/1/2. Girdle smooth, constituted of mantle tissue, no elements present (Figure 13F). Radula (Figure 13G) with a central semi-wedge shaped tooth, rounded on its apical end and pointed posteriorly; major lateral tooth with four cusps, the outermost cusp only half of the size of the others, wider and broadened anteriorly (Figure 13H).

Habitat.

Found in the shallow subtidal to 12 m on rocks and dead coral, associated with crustose coralline red and green algae.

Remarks.

The reduced tegmentum area and its black nude girdle make the identification of this species quite unequivocal. The examination of the morphology of a juvenile (7.1 × 3.0 mm) PNAA specimen revealed the lack of slits and tegmentum pustules. In a somewhat larger animal (9.3 × 4.2 mm), the slits and pustules could be observed in all valves (explained above). Remarkably, the pustules on the head valve cover most of its area, whereas on the intermediate and the tail valve they are less numerous and more irregular in shape than on the head valve. Lyons (1988: figure 148, 149) figured some intermediate valves of a specimen of 10.7 mm length from Vaca Key, Monroe County, Florida with rudimentary pustules near the jugal area, which strongly resemble PNAA specimens. On the contrary, this condition was not observed in Puerto Rico specimens ( García-Ríos 2003).

The examination of a larger C. floridanus (CNMO5560, 20. 3 × 8.3 mm) (Figure 6O) from Banco Chinchorro revealed a lack of pustules and little tegmentum development at all. Our observations suggest that some chitons might develop tegmentum, including pustules, but this in the observed specimens was not be related to chiton size. The Banco Chinchorro specimen of C. floridanus is one of the largest animals recorded from Mexico, after the Puerto Rico species of 21 mm length ( García-Ríos 2003: figure 153), which also lacks of tegmentum formations. Regarding the radula, the fourth cusp of the major lateral tooth seems to be distinctive for the PNAA specimens, while the Puerto Rico specimen had only three cusps. The differences observed in our specimens compared to those from Puerto Rico, and the similarities with the PNAA and Florida Keys specimens is interpreted here to be due to high variability within a widespread species.