Stichodactyla helianthus (Ellis, 1768)

(Figure 8, Table 2)

Actinia helianthus Ellis, 1768: 436.

Actinia Helianthus [sic] Ellis & Solander, 1786: 6 –7.

Actinia Anemone [sic] Ellis & Solander, 1786: 6.

Discosoma anemone Duchassaing, 1850: 9.

Discosoma helianthus Milne-Edwards, 1857: 256.

Discosoma Helianthus [sic] Duchassaing & Michelotti, 1864: 28.

Discosoma Anemone [sic] McMurrich, 1898: 241.

Stoichactis anemone Haddon, 1898: 473.

Stoichactis helianthus Carlgren, 1900: 76 –77, 96–97.

Stichodactyla helianthus Dunn, 1981: 6, 78–82, 104, 106, 108.

Stoichactis Helianthus [sic] Zamponi & Pérez, 1996: 92.

Material examined.— Puerto Morelos (20°55’39.13” N, 86°49’58.93” W; 9 specimens); Isla Contoy (21°28’21.91” N, 86°47’27.25” W; 2 specimens).

Diagnosis.—Fully expanded oral disc and tentacles 60–240 mm in diameter. Oral disc 55–236 mm in diameter, flat to domed, wider than pedal disc, light brown or greenish (Figure 8 A). Tentacles very short, blunt, digitiform, in radial endocelic rows covering almost the entire oral disc; each endocelic row with dozens of tentacles but only one tentacle at the margin per exocelic row (Figure 8 B). Tentacles light brown distally, with greenish or yellowish shades proximally. Column very short, 28–170 mm in diameter and 15–25 mm in height, diabolo-shaped, smooth with longitudinal rows of brown verrucae distally; 4–7 verrucae per row (Figure 8 C). Pedal disc well developed, 35–75 mm in diameter (Figure 8 D). Pedal disc and column brownish or beige. Mesenteries irregularly arranged in four cycles (48 pairs in specimens examined): first two cycles perfect, others imperfect. No gametogenic tissue observed in specimens examined. Two pairs of directives each attached to a well developed siphonoglyph. Retractor muscles weak, diffuse, long and thin; parietobasilar muscles poorly developed (Figure 8 E). Basilar muscles well developed. Longitudinal muscles of tentacles ectodermal. Marginal sphincter muscle strong, circumscribed (Figure 8 F). Zooxanthellae present. Cnidom: basitrichs, microbasic p- mastigophores and spirocysts (Figure 8 G–N; see Table 2).

Natural history.— Stichodactyla helianthus lives in shallow waters adhered to rocks and coral rubble among patches of sand and seagrass, often between 0.5–3 m depth, but also found down to 8 m; it is often found in areas with high wave energy of the lagoon and fore-reef zones, more rarely in calm waters. It usually forms large aggregations covering great extensions of coral rocks as result of asexual reproduction by longitudinal fission (Dunn 1981; Herrera-Moreno & Betancourt 2002). It is associated with the caridean shrimp Periclimenes rathbunae or sometimes Thor amboinensis (Dunn 1981; Ritson-William & Paul 2007). This species is considered an important source of biologically active compounds (Garateix 2005; Monroy-Estrada et al. 2006; Nuñez et al. 2006; Bosmans & Tytgat 2007; Morales-Landa et al. 2007).

Distribution.—From Bermuda to Brazil, along the entire Caribbean Sea (see Table 1). In the Mexican Caribbean Stichodactyla helianthus is reported for Cozumel and Puerto Morelos reefs (INE 2000; Monroy-Estrada et al. 2006; Jordán-Dahlgren 2008), but this is the first time recorded for Isla Contoy reef.

Remarks.— Stichodactyla helianthus is the only species of the genus (of the five valid ones) distributed in the Caribbean Sea (Fautin 2011). According to Dunn (1981) this species shares the shape and color of tentacles with S. haddoni (Saville-Kent, 1893); however, the tentacles in S. helianthus are arranged in a single line for nearly the entire extent of each endocoelic row whereas those of S. haddoni have two or more tentacles per row except near the oral end.