Didemnum longigaster, Rocha, Rosana Moreira Da, Neves, Isabela Monteiro & Gamba, Gustavo Antunes, 2015

Rocha, Rosana Moreira Da, Neves, Isabela Monteiro & Gamba, Gustavo Antunes, 2015, New species of Didemnidae (Tunicata: Ascidiacea) from the tropical coast of Brazil, Zootaxa 3905 (3), pp. 381-396 : 390-391

Didemnum longigaster sp. nov. Rocha & Neves

( Figs 8 , 9 )

Examined material: Holotype: MZUSP 0 0 0 92 one colony, Quebramar Norte, Salvador, Bahia, 12°57’52” S, 38°30’57” W, 4 m, col. R. M. Rocha, 11.12.2007.

Etymology. The name refers to the elongated stomach that is uncommon in didemnids.

The colony is encrusting, around 10 cm long and 2 mm thick. Beige with orange zooids, the colony has large circular cloacae with lobed rim ( Fig. 8 A). When preserved in formalin the colony turns white. Spicules are very dense in the surface of the colony, but less so in the middle and basal tunic layers. Spicules are large and stellate, between 60 and 100 µm with 9–10 short conic rays in optical transverse section ( Fig. 8 B).

Zooids are 1.5 mm long. When relaxed, the thorax is larger than the abdomen. The oral siphon is tubular with shallow rounded lobes. The atrial aperture is wide exposing the pharynx, atrial languet is absent. The muscular process projects from the base of the thorax and is longer than the abdomen. Thoracic organs are elongated, protruding, located between the third and fourth rows of stigmata close to the endostyle. Each row has 5–6 stigmata on each side ( Figs. 8 C, 9A).

No stolonic vessels were observed in the abdomen. The esophagus is short. In general, all zooids have elongated stomach, but not all of them as long as in Figs. 8 D and 9B. The intestine does not form a secondary loop and the ascending portion passes beside, without overlaying, the stomach. The testis has a single and spherical follicle, surrounded by 8 coils of the sperm duct ( Figs. 8 E, 9B). Larvae are oval, 0.4 mm trunk length, around which the tail winds half way. Three adhesive papillae are present, with long and wide stalks plus six pairs of ectodermal ampullae. The sensorial vesicle (including ocellus and otolith) is in the mid-dorsal region of the larval trunk ( Figs. 8 F, 9C).

Remarks. No other Didemnum species has an elongated stomach as in D. longigaster . The combination of this elongated stomach, large spicules and larvae with six pairs of ectodermal ampullae is also unique. In the Atlantic, two other species are of the same colony color. Didemnum granulatum have a beige color morph, but the colony surface has small projections filled with spicules, that are much smaller than in D. longigaster ( Rocha et al. 2005) . Didemnum vexillum Kott, 2002 does not occur in Brazilian coastal waters, and it has smaller spicules and larger larvae ( Kott 2002).

Figures

FIGURE 8. Didemnum longigaster sp. nov. A, living colony (cloacal aperture in detail). B, spicules. C, thorax. D, abdomen showing the long stomach on the right. E, abdomen showing the sperm duct. F, larva. Scale bars: C – F = 0.5 mm.

FIGURE 9. Didemnum longigaster sp. nov. A, thorax. B, abdomen. C, larva. Scale bar: 0.5 mm.