Styela canopus (Savigny, 1816)

Monniot, Françoise, 2018, Ascidians collected during the Madibenthos expedition in Martinique: 2. Stolidobranchia, Styelidae, Zootaxa 4410 (2), pp. 291-318 : 311-314

Styela canopus (Savigny, 1816)

Figs 19-20-21

Cynthia canopus Savigny, 1816

Stations: AB 195; AB 197; AM 20; AM 34; AR 101. (MNHN S1 STY 324)

The specimens from 6mm to 3.5cm have different aspects. The smaller ones have a clear tunic with red pigment on the siphons but the larger ones are totally bright red. White bands alternate with brown ridges on the rims of the siphons. The tunic is leathery and corrugated without true scales and especially lumpy at the siphons with chalky pigment ( Fig. 21). The body is ovoid and fixed by a large part of the ventral side. The body wall is pale with red pigment on the siphons ( Fig. 20A ). The musculature is dense but not thick with a continuous sheet of crossed fibres. A velum is present in the short oral siphon. The number of oral tentacles varies from 20 to 50 in the largest specimens. The prepharyngeal area is narrow with a deep dorsal V. The dorsal tubercle opens anteriorly with horns slightly curved in the largest specimens. The dorsal lamina is straight and long increasing in height posteriorly ( Figs 19B , 20B ). There are 4 branchial folds well separated on each side ( Figs 19B , 20B ). The number of longitudinal vessels on the folds and between the folds depends on the body size. The formula for a specimen of 2cm is:

R E- 4 (12) 4 (14) 4 (14) 4 (15) 4–DL–3 (15) 4 (14) 4 (17) 5(14) 4 –E L

For a specimen of 3.5cm on the right side:

R E-10 (34) 14 (38) 14 (30) 14 (32) 10 –D L

It is difficult to determine if the longitudinal vessels next to the folds belong to a fold or to the interspaces. There are 2 to 4 stigmata in a mesh. Parastigmatic vessels are present.

The digestive loop occupies about half of the left body side ( Figs 19A , 20A ). The oesophagus is narrow followed by a long stomach with numerous folds. There is a small button-like caecum. The intestine forms a first long loop and curves in a second more opened loop ending in lobed anus near the atrial opening. In several specimens the rectum is inflated ( Fig. 20A ). The gut is loosely attached to the body wall by thin trabeculae. There are 2 elongated gonads on each side ( Figs 19A , 20A ); the ovary is tubular and bordered by testis lobes on each side. The testis lobes have a round shape in small specimens ( Fig. 19A ) but they are lobed in larger specimens ( Fig. 20A ). The internal side of the body wall is covered by numerous endocarps ( Figs 19A , 20A ). The atrial aperture and the dorsal side of the body wall have a dense band of thread like filaments extending also over the atrial velum.

Styela canopus is a common and widely distributed species (Rocha, Moreno & Metri 2005). It has been described under several species names with variable characters. In the Caribbean area this common Styela has often been named S. partita (Stimpson 1852) a species considered by Kott and Goodbody (1982) a synonym of Styela canopus , a status difficult to accept for so variable and worldwide specimens. Allthough no sufficient discriminating characters could be found between the different samples, they may represent more than one species but only a genetic study would solve this question.

Figures

FIGURE 19. Styela canopus station B 197. A, body ventrally opened scale bar = 1cm; B, branchial sac stained, scale bar = 5mm.

FIGURE 20. Styela canopus station AM 20. A, body ventrally opened, scale bar = 1cm; B, branchial sac stained, scale bar = 5mm.