Botryllus gregalis, (SLUITER, 1898)

Monniot, Claude, 2002, Stolidobranch ascidians from the tropical western Indian Ocean, Zoological Journal of the Linnean Society 135 (1), pp. 65-120 : 66-67

BOTRYLLUS GREGALIS ( SLUITER, 1898)

Botrylloides gregalis Sluiter, 1898: 46 , pl. 2, figs 6–9 – Mozambique.

Botryllus gregalis: Monniot C. & Monniot F., 1997: 1623–1643 , fig. 5, pl. 1C-D – Gulf of Arabia ( Bahrain); Monniot F. & Monniot C., 1999: 1–35, pl. 3E – Tanzania; Monniot C. et al., 2001: 74, figs 4A,37 – South Africa, and synonymy.

Material

Madagascar: Nosy-Be, Baie de l’Entrée, coll. P. Laboute, 1992.

Mozambique: Ibo, intertidal, sea grass beds, mangroves and barrier reef, coll. C. Monniot, 1995.

Djibouti: coll. C. Monniot, 1996.

Remarks and distribution

Botryllus gregalis is the commonest Botryllus species along the west coast of the Indian Ocean. Its distribution extends from the Red Sea and Arabian Gulf down to South Africa. The colonies are very polymorphic and of varied colours. In Mozambique the prevailing colour is brown (from beige to black), but there are also green, orange, or black and orange colonies. The species is abundant in the intertidal zone and in sea grass beds. Some rare samples from mangrove roots were red. On the external slope of the coral reef, small colonies were settled on Halimeda . A single, uniformly orange, colony was collected at Madagascar at 20 m depth on a sedimentary bottom. Until now Botryllus niger ( Herdman, 1886) has been the only Botryllus species recorded in Madagascar, from Tulear. I have described colonies from Bahrain, Djibouti and Tanzania in which zooids are gathered in patches separated by wide channels constituting spacious, meandering common cloacal cavities. The zooids are joined by their ventral side, the cloacal siphons opening outward into channels surrounding the group of zooids. This pattern is the reverse of that in many other species, such as B. schlosseri ( Pallas, 1774) and B. leachi Savigny (1816) where the zooids are arranged in systems each having a central common cloacal cavity. The arrangement of groups of zooids separated by meandering channels is most often encountered in colonies growing on marine phanerogams, in this case ones 2 cm in width. The colonies living on more fragmented substrates have several common cloacal openings in less developed systems that are often circular. This is the case with the type specimen.

BOTRYLLUS MAEANDRIUS MONNIOT C. ET AL., 2001 ( Fig. 1 )

Botryllus maeandrius Monniot C. et al., 2001: 75 , figs 4B,38 – South Africa.

Material

Mozambique: Ibo Island, coral reef, 15 m, on Halimeda , coll. C. Monniot, 1995.

Description

The tunic is filled with sand. The colony is more than 1 cm thick, made of two circular systems around a slightly depressed centre. The zooids are 1.5–2 cm long lying perpendicular to the colony surface. The cloacal siphon has an elongated languet ( Fig. 1A ) which keeps a reddish tint in formalin. The cloacal opening may be widely open or reduced to a tube ( Fig. 1A ).

There are 12 rows of stigmata. The second row is incomplete and only ventral, interrupted at the level of the second longitudinal vessel. At the posterior end, the last row has only a few, round stigmata. We counted 16 stigmata in a half-row in the middle part of the branchial sac.

The gut forms a loop placed mainly under the branchial sac ( Figs 1B,C ). The oval stomach has 10–11 folds, its anterior part makes a crown of protrusions around the oesophagus. The folds are well individualized along their whole length and have rounded pyloric ends. There is a large, finger-like caecum, slightly widened at its tip, and parallel to the intestine ( Fig. 1A ). The top of the gut loop does not exceed the level of the last, incomplete stigmata row. The long rectum ends in a smooth anus between the seventh and eighth stigmata rows. The pyloric gland is made of small scarcely visible ampullae and does not narrow the intestine.

The male gonads lie at the same level on the left and right sides of the body ( Figs 1B,C ). A young ovary with two oocytes is located on the left side, posteriorly and ventrally to the testis ( Fig. 1B ), and bulges out from the external side of the body wall. The colony does not contain buds.

STOLIDOBRANCH ASCIDIANS FROM THE INDIAN OCEAN 67

Remarks

Botryllus maeandrius has only been recorded from South Africa (Monniot et al., 2001). Its occurrence in Mozambique considerably extends its distribution. It remains the only sandy Botryllus species of the east coast of Africa. Three other sandy species are known from other parts of the world: B. okai Monniot C., 1988 (a changed name for Psammobotrus purpureus Oka, 1932 :Monniot C., 1988: 172) from Japan and southern and western Australia; B. arenaceus Monniot C., 1988 from New Caledonia. These three species have several oocytes anterior to the testis.

Figures

Figure 1. Botryllus meandrius: A, left side of a zooid. B, gut and gonads seen from the left side. C, gonad on the right side. Scale bars: A = 0.5 mm, B, C = 0.25 mm.