Monandrocarpa humilis Monniot, 2009, n. sp.

Monandrocarpa humilis n. sp.

Type: Vanuatu, Espiritu Santo, Aore Island, 15°31.92’S – 167°11.61’E, 30m, 30.I.2000, coll. CRRF n° CRCHO 463. (Registered in the Muséum National d’Histoire Naturelle Paris n° S1 MON 2)

Several specimens, some of them solitary but others in groups of 2 or 3 individuals, were found attached to a colony of Amphicarpa agnata (Kott, 1985). The body is oval, 1cm in maximum length, covered with sparse adhering sand (Fig.1). Some individuals are sessile, others are erect on a common base making a short and thick stolon. The siphons are slightly protruding and well apart. The tunic is leathery, but the body remains soft.

The internal body wall is opaque. In formalin it has a dark-pigmented superficial reticulate design. Internally, on each side of the body wall, a longitudinal darker pigmented band extends alongside the polycarp line. The siphons have strong sphincters. The body musculature forms a continuous network of thin fibres. The number of oral tentacles is variable, in 3 orders of size. The peripharyngeal band is dorsally curved in a low V; it has a single crest. The dorsal tubercle is button-like. The branchial tissue (Fig. 2) is thin and contains less pigment than other tissues. Among the 4 branchial folds, the first and the third, counted from the dorsal lamina, are higher. Two to 3 longitudinal vessels can be counted anteriorly between the folds, they progressively join the folds as the sac narrows posteriorly. A representative formula in a large specimen, near the peripharyngeal band is:

R-E; 5 -2- 8 -3- 6 -2- 8 -1-DL- 8 -3- 5 -2- 6 -3- 6 -1-E-L

There are 4 to 5 stigmata in a mesh between the folds. The number of stigmatal rows is difficult to count, but about18–20, basally grouped two by two due to a more or less complete division by parastigmatic vessels. The last rows are narrow with only a few stigmata. The dorsal lamina forms a high blade.

The digestive loop (Fig. 3 B) occupies a reduced posterior part of the left side (Fig. 3A). The oesophagus is narrow and straight, well delimited from the long cylindrical stomach. There are 8 to 9 longitudinal stomach folds. The pyloric caecum is long. The intestine curves in a single closed loop. The anus with a plain edge opens close to the oesophagus entrance. A pigmented network of vessels covers the intestine. An endocarp lies in the loop (Fig. 3 B). Round endocarps are scattered on the body wall (Fig. 3A), some of them intercalated between the polycarps. A ring of short thread-like papillae encircles the cloacal siphon.

An average of 10 polycarps are arranged in a line on each body side along and close to the ventral line (Fig. 3A). The left row curves to become parallel to the rectum. Each polycarp comprises a single testis follicle covered by the ovary which contains many oocytes (Fig. 3 C). The gonadal papillae are short and close together.

Remarks. Only a few species of the genus Monandrocarpa have been recorded: they belong to different oceans. They may be solitary or colonial, with a single testis lobe in each polycarp, a structure characteristic of the genus. Monandrocarpa abyssa Sanamyan & Sanamyan, 1999 is a deep sea solitary ascidian from the Antarctic. The 3 cm individuals are in balls on a peduncle. They possess only 2 branchial folds on each side, the stigmata are small and not numerous. The stomach is short and the rectum is long.

Monandrocarpa monotestis (Tokioka, 1953) originally as Polyandrocarpa (Eusynstyela) monotestis, from Japan, and found again in China (Tokioka 1967), forms massive colonies with zooids arranged in a single layer with elliptical systems. It has 4 branchial folds, an elongate stomach and a long rectum.

Monandrocarpa tritonis Michaelsen, 1904, type of the genus, comes from 10m depth in the Cape region of South Africa. It is solitary, globular and incrusted with sand. The branchial sac has 3 to 4 folds, and no intermediate vessels. The stomach has 11 folds and the intestine has transverse plications. According to Michaelsen, its stomach diverticulum may be due to a parasite.

Monandrocarpa tarona Monniot & Monniot, 1987 from Polynesia has bare zooids, isolated or grouped, flattened on the substrate, 5mm in size. Stolons have not been detected. There are no vessels between the 4 branchial folds and only 7 to 8 stigmata rows. The rectum is short and curved.

Monandrocarpa stolonifera (Monniot C. 1970) as Polyandrocarpa (Monandrocarpa) stolonifera Monniot C., 1970, from Brazil has 5 to 7 mm zooids erect above a meshwork of thin creeping stolons encrusted with sediment. There are 4 branchial folds on each side and several intermediate longitudinal vessels. The stomach is globular with 15 folds, the gut loop is short.

Monandrocarpa simplicigona (Millar, 1975), under the name Polycarpa, from the Philippines at 457m depth has solitary individuals, dorso-ventrally flattened, covered with sand with an equatorial ring of rhizooids. The branchial sac has 4 folds per side with occasionally one vessel between them. The gut is in a double loop with a short stomach with numerous folds and a lobed anus. Few endocarps are present on the body wall and between the gonads. This anatomy was confirmed by Monniot & Monniot (2003).

Monandrocarpa plana Kott, 1972 from New South Wales is solitary with long test hairs and adhering sand. No intermediate vessels lie between the 4 branchial folds, there is a sharp curve of the rectum and up to 16 crowded gonads per side. Kott (1985) suggested M. simplicigona to be a junior synonym, but this was not accepted (Monniot & Monniot 2003).

Kott (1985: 211) erroneously assigned Styela incubita Sluiter,1904, later on named Cnemidocarpa incubita by Tokioka (1967:186), to Monandrocarpa, a genus defined by a single male follicle in each polycarp. C. incubita has two male follicles per gonad and cannot be placed in Monandrocarpa..