Styela paessleri (Michaelsen, 1898)

Turon, Xavier, Cañete, Juan I., Sellanes, Javier, Rocha, Rosana M. & López-Legentil, Susanna, 2016, Ascidian fauna (Tunicata, Ascidiacea) of subantarctic and temperate regions of Chile, Zootaxa 4093 (2), pp. 151-180 : 173-174

Styela paessleri (Michaelsen, 1898)

Fig. 13 D–F

References and synonymy: Styela paessleri Millar (1960) p. 115; Kott (1969) p. 121.

Localities: 1S, 4S, 8S.

The individuals of this species tend to be cylindrical, up to 35 mm in height, fixed posteriorly and with the siphons close together in the distal part. The tunic is firm, brownish, with wrinkles and tubercles. The internal morphology is typical of the genus. There are over 40 simple oral tentacles in large individuals. The aperture of the neural gland is variable (“U”, inverted “U”, or other shapes). The branchial sac has a smooth dorsal lamina and 4 folds. The second and fourth on each side (6–8 vessels on each) are less developed than the first and third (10–14 vessels). There are up to 10 stigmata per mesh between folds, crossed by parastigmatic vessels. The atrial area has a field of filiform papillae.

The digestive system has an elongated stomach with folds (over 15). The primary loop is closed (the descending intestine is adjacent to the stomach) and the secondary loop is very open. The final part of the intestine has a characteristic twist. It is bent towards the atrial opening, and at this level there is a narrowing of the intestine at which end lies the anus with a fringed margin. The gonads are characteristic, two at each side. They consist of a branched ovary and male follicles at the ends of the branches. The genital apertures (both male and female close together) are at the extreme of the gonads, and they generally point anteriorly, not towards the atrial siphon. Even in immature specimens, incipient branching gonads and their apertures are visible. In large individuals, accessory apertures exist at other parts of the branching gonad. There are plenty of endocarps at both sides of the mantle. In some specimens, embryos and larvae have been observed in the peribranchial cavity. The larvae have ca. 150 µm of trunk length and bear a single pigmented spot.

Remarks. this species, characterized by its gonad morphology, is limited to Falkland Islands and mainland Patagonia, down to 121 m (Kott 1969). The fact that it is able to brood larvae had not been previously described and is extraordinary for a species of Styela . Brooding has been described only in a few solitary styelids, belonging to the genera Dendrodoa and Polycarpa (Pérez-Portela et al. 2009) .

Figures

FIGURE 13. Styela changa. A, whole individual; B, C, dissected digestive system as seen from the branchial side (B) and from the mantle side (C); D, Specimens of Styela paessleri partially overgrown by Didemnum studeri; E, dissected mantle of an immature individual, branchial sac removed (stained material); F, anterior part of a larva. Scale bars: A, 2 mm, B, C, 0.5 mm, D, E, 1 cm, F, 50 µm.