Stenoplax bahamensis Kaas & Van Belle, 1987

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

Stenoplax bahamensis Kaas & Van Belle, 1987 Figures 2 I–M, 4 A–H

Material examined.

15 specimens; 11-25.4 mm long, 3-8.1 mm wide. Playa Norte (CNMO4942, CNMO4956, CNMO4973, CNMO4976), Isla Perez (CNMO4943, CNMO4944, CNMO4977), Pared Arrecifal (CNMO4957), Blanca y Pajaros (CNMO4961), Laguna Desterrada (CNMO4974, CNMO4975), Laguna Arrecifal (CNMO4978).

Description.

Medium-sized, elongate-oval chitons, around three times longer than wide. Color of tegmentum variable, mostly cream, dark and lighter green, light brown, with scattered dark brown spots (Figure 2 I–M). Tegmentum with nodulose ribs, arranged concentric to the apex. Head valve (Figure 4A), semi-circular, slightly notched; sculpturing with nodulose ribs, break into numerous and regular nodules, particularly to the periphery. Tail valve (Figure 4B), with elevated postmedian mucro; postmucronal area sculptured as head valve, antemucronal area with 37-42 nodulose ribs. Intermediate valves (Figure 4C), with side margins somewhat rounded; lateral areas elevated, with 14-18 concentric nodulose ribs; pleural areas with 16-19 longitudinal ribs; the ribs next to lateral areas developed few small irregular to rounded lobules or pustules. Articulamentum slightly light blue; apophyses narrow and long twice as wide, wing shaped: jugal laminae smooth and wide (Figure 4D); slit formula 9 –13/1/9– 13. The ribs next to the postmucronal region tend to form more lobule or pustule like formations than intermediate valves (Figure 4E). Megalaesthetes small and surrounded by 4-6 smaller micraesthetes (Figure 4F). Girdle covered with tiny scales (Figure 4G), each with 11-13 wide, somewhat flat ribs on its surface. Radula (Figure 4H), with tricuspid major lateral tooth, cusps pointed and rather irregular in shape and length, the central and minor lateral teeth about the same length and as narrow and short spatulated shaped.

Habitat.

Found in intertidal to shallow subtidal to 12 m depth, on sunken wood, rock and turtlegrass, T. testudinum .

Remarks.

According to Arce and García-Ríos (2015), the sculpturing of small adults of S. floridana (18.6 mm) is very similar to S. bahamensis adults (31.8 mm), with somewhat continuous ribs in lateral areas. And the juvenile forms of both species (9.6 and 8.6 mm respectively) are almost identical, making their identification quite difficult.

PNAA S. bahamensis juveniles (ranging from 11-14.2 mm length), showed continuous ribs in lateral and central areas, with few granular formations next to the diagonal ridge; and our comparison with S. floridana was limited to one adult specimen.

Our observations of adult specimens agree with previous authors ( Bullock 1985, Kaas and Van Belle 1987), that distinguish S. bahamensis from S. floridana by the absence of inconspicuous pustule formation next to the lateral area (Figure 4E). Examination of S. floridana (26.7 × 9.2mm) (CNMO5557) from Banco Chinchorro, Quintana Roo (Figure 14 E–G), revealed central areas with granulate ribs, and lateral areas raised higher, with strong tuberculated discontinuous ribs, also present in head valve and postmucronal area of tail valve, whereas S. bahamensis develops lower lateral areas with more continuous ribs.

Bullock (1985) contributed on the knowledge of Caribbean Stenoplax s.s. species. He presented an exhaustive review of species aesthete density, major lateral tooth outline and sculpturing morphology. Based on his observations, he grouped S. bahamensis and S. floridana in a single lineage, defined on the reduced rib width, fewer aesthetes and elongate denticle cap. In addition to the examination, he also included the basal spot of the major lateral tooth, and an outline of the denticle cap, which according to him was distinctive, and useful as taxonomic character to distinguish among other chiton species. However, the differentiation of Stenoplax Caribbean species we have used here relies more on adult sculpturing differences (on how broken the ribs can appear, and if there is pustule development (Figure 14G) in the diagonal ridge of intermediate valves).