Octacnemus bythius Moseley, 1876

Monniot, Françoise & López-Legentil, Susanna, 2017, Deep-sea ascidians from Papua New Guinea, Zootaxa 4276 (4), pp. 529-538 : 534-536

Octacnemus bythius Moseley, 1876

Moseley, 1876, 2°33’S–144°4’E Herdman, 1888

Millar, 1959 and synonymy

Monniot C & Monniot F, 1991 b and synonymy

Station CP 4435, 1 specimen (MNHN P 6 OCT 19) The single specimen collected during the KAVIENG expedition has the same star-like morphology ( Fig. 4 ) as the specimen-type collected from the same region. The distance between opposite oral lobes measures 6 cm. The vitreous tunic is very thin and smooth. The oral lobes are triangular with a pointed tip and no lateral denticles. At the base of each lobe on the oral side there are round vesicles ( Fig. 4 B). These vesicles appear to be prolongations of the tunic since no trace of them remains when the tunic is removed ( Fig. 4 B’). The musculature is typical of the genus, with circular fibres around the base of the oral lobes gathered by radial ribbons at regular intervals. These ribbons spread laterally and internally drawing arcs that connect with each other ( Fig.4 A’, B’). A strong, fanshaped muscle lies at the posterior side of the mouth and some muscular bundles are also present at the dorsal side of the aperture, behind the neural ganglion. A sphincter closes the mouth opening. The neural ganglion is close to the oral sphincter, giving two lateral nerves on each side. A small neural gland is associated with the ganglion. There is no branchial sac. The pharyngeal pouch is lined by a wrinkled tissue pierced in the bottom right by a simple small hole. The pharyngeal tissue is raised forming a ridge close to the ganglion and may either be a dorsal lamina or a contraction artefact. The oesophagus opens at the left side of the pharynx pouch. The shape of the gut is indistinct. Male and female gonads are tightly attached to the gut. The body wall around the abdomen is particularly thin and apparently devoid of muscles. A narrow atrial cavity along the rectum opens into a hole close to the posterior part of the gut.

Octacnemus bythius differs from the Atlantic O. zarcoi Monniot C & Monniot F, 1984 in that O. zarcoi has unequal oral lobes and a muscular oral siphon. O. alatus Monniot C & Monniot F, 1985 from the southern Indian Ocean has two large oral lobes that look like wings. O. vinogradovae Sanamyan & Sanamyan, 1999 from Macquarie Island has the same albeit smaller oral lobes than O. bythius . However, O. bythius has a pharynx with longitudinal vessels and branchial perforations. O. kottae Sanamyan & Sanamyan, 2002 has pinnate oral lobes, a pedunculate abdomen and a pharynx that is entirely perforated. O. ingolfi Madsen, 1947 from the northern Atlantic has pinnate oral lobes and a long posterior appendage. The branchial sac of this species is also entirely perforated. All members of the genus Octacnemus are deep-sea species.

Figures

FIGURE 4. Octacnemus bythius. A, A’ body with and without tunic, dorsal side. B, B’ oral side with and without tunic. Scale bar 1 cm.