Symplegma zebra, Monniot, 2002

Monniot, Claude, 2002, Stolidobranch ascidians from the tropical western Indian Ocean, Zoological Journal of the Linnean Society 135 (1), pp. 65-120 : 68-73

publication ID	https://doi.org/ 10.1046/j.1096-3642.2002.00017.x
persistent identifier	https://treatment.plazi.org/id/436F3F3C-FFD3-FFA6-FC82-F97BFE881AE1
treatment provided by	Carolina
scientific name	Symplegma zebra
status	sp. nov.

Treatment

SYMPLEGMA ZEBRA View in CoL SP. NOV. ( Figs 3 View Figure 3 , 4A,B View Figure 4 )

Material

Holotype: MNHN S1 SYM 56 About MNHN , paratype MNHN S1 SYM 75 About MNHN : Madagascar: Nosy-Be, Sakatia Bay , 18 m, coll. P. Laboute, 1993.

Other material examined: Maldive Islands, 04°11.43¢N–73°25.05¢E, 7 m, coll. CRRF , 1997.

Description

The two colonies from Madagascar were attached to dead corals; the Maldive colonies incrusted an alga. The holotype colony has a very dark general colour, violet to black with radiating gold-yellow lines ( Fig. 4A View Figure 4 ). Others colonies have a violet colour with patches of cream colour ( Fig. 4B View Figure 4 ) .

Between the zooids the pigment is contained in vascular ampullae. The vascular ampullae fringing the colonies are elongated, parallel, and particularly visible. When fixed in formalin, the tunic becomes transparent and the pigmented lines disappear. The yellowish zooids have an opaque network of blood lacunae full of brown pigment cells in the body wall, in the branchial vessels, and in the bottoms of the stomach grooves.

In the holotype the zooids ( Fig. 3A View Figure 3 ) are each 3 mm in length with 12 rows of stigmata, while in the other Malagasy colony the zooids are 2 mm long with 10 rows of stigmata ( Fig. 3D View Figure 3 ).

The body wall is particularly thin and fragile. The muscles are poorly developed. There are 12 oral tentacles in two orders of size. The prepharyngeal band makes a dorsal V. The dorsal tubercle opens in a simple hole. The dorsal lamina is clearly displaced on the left side. There are 12 rows of stigmata on the right side and 11 on the left. The first left longitudinal vessel joins the dorsal lamina at the eighth stigmata row and the second vessel at the tenth row.

The gut is small and in a posterior position ( Figs 3A,D View Figure 3 ), the top of the loop reaching the eighth row of stigmata. The globular stomach ( Fig. 3B View Figure 3 ) has eight grooves. The curved caecum is linked to the intestine by two strips of tissue. The anus has a smooth edge.

The gonads are developed differently in the three colonies. They have either a reduced testis made of two undivided lobes with a very short sperm duct and an ovary containing several oocytes ( Fig. 3C View Figure 3 ), or a large testis with two lobes and an incompletely developed ovary ( Fig. 3E View Figure 3 ). The largest oocytes do not reach the common size of the eggs in other Symplegma species.

The base of the cloacal siphon is encircled by a ring of very thin tentacles.

Etymology

The specific name reflects the pigment design.

Remarks

The two-lobed testis shape with its round outline is characteristic of this species. In species having divided testis lobes, this becomes visible early in gonadogenesis. But S. zebra is in other ways close to S. viride Herdman (1886) (s.s.) from the tropical Atlantic, in which the testis lobes are not divided. The main differences between these two species are a larger gut in S. viride , its long sperm duct, and a difference of at least one stigmata row in the location of the gonad in this species.

Another species, S. reptans ( Oka, 1927b) , has a testis with two simple lobes. Nishikawa (1991) redescribed this species after examining numerous specimens. This species seems highly variable in the number of stigmata rows (8–11) and stomach folds (9–13). Nishikawa figured several different shapes of the stomach, all having incomplete folds near the caecum, but this is not the case in S. zebra . Kott’s description (1985) of S. reptans , as noted by Nishikawa (1991), corresponds to a very different species that has two unique characters in the Symplegma genus: zooids arranged in systems, and longitudinal branchial vessels on the left side reaching the bottom of the branchial sac.

Remarks on the colonial genera of Styelidae

Kott (1985) modified several diagnoses of Styelidae genera, among them three genera of colonial Styelidae that we report about here: Metandrocarpa Michaelsen, 1904 ; Amphicarpa Michaelsen, 1922 ; and Stolonica Lacaze-Duthiers & Delage, 1892 . The earlier diagnoses ( Michaelsen, 1922) were very precise and are founded on clear-cut characters: absence of branchial folds and separated male and female gonads in Metandrocarpa ; branchial folds and hermaphroditic gonads only on the right side in Stolonica ; branchial folds and female or hermaphroditic gonads on both body sides in Amphicarpa .

Kott (1985) placed the species M. agitata Kott, 1985 , which has branchial folds, in the genus Metandrocarpa . The species would normally be included in the genus Amphicarpa . Kott (1990: 277) gathered the genera Amphicarpa and Stolonica in a single genus Stolonica , excluding as a criterion the gonad distribution which she considers variable. We do not agree with her position and keep both genera, based on the presence of ovaries on one or both sides of the body wall.

METANDROCARPA MANINA MONNIOT C. & MONNIOT F., 1987 ( Figs 4C View Figure 4 , 5 View Figure 5 )

Metandrocarpa manina Monniot C. & Monniot F., 1987: 109 View in CoL , fig. 41 – Polynesia; Monniot C., 1988: 180 – New Caledonia.

Material

Mozambique: Ibo Island, external reef slope, coll. C. Monniot, 1995.

Description

This species is commonly found on the underside of flat corals. The colonies are made of red half-spheres 3 mm in diameter, 2–5 mm apart; they are linked by thin stolons ( Fig. 4C View Figure 4 ). The colour persists in formalin. The body wall is thin with a network of blood vessels and irregularly arranged muscle fibres. We counted eight thick oral tentacles in two orders. The prepharyngeal band has a single blade and lacks a dorsal curve; in fact, dorsally it lies closer to the tentacle ring. The dorsal tubercle, urn-shaped, opens by a round hole posterior to the prepharyngeal band. The neural ganglion lies somewhat posteriorly.

The dorsal lamina is smooth and high. The branchial tissue is flat. There are eight regularlyspaced longitudinal vessels on each side. These longitudinal vessels are high. The eight rows of long stigmata are crossed by parastigmatic vessels. There is an average of three to four stigmata per mesh.

The gut ( Fig. 5A,B View Figure 5 ) forms a short closed loop that is folded on itself. The stomach is conical with 12 short folds. The caecum is large, curved, with a terminal ampulla ( Fig. 5B View Figure 5 ). The pyloric duct opens in the middle of the caecum. The rectum is short and the smooth-edged anus does not surpass the level of the top of the intestinal loop.

Generally, there is on each side a testis made of two deep lobes with a large sperm duct and an ovary anterior to the testis. Small endocarps are scattered over the whole internal side of the body wall.

The larvae are brooded in the cloacal cavity ( Fig. 5C View Figure 5 ). They have a single sensory organ, three small adhesive papillae, and a crown of anterior ampullae ( Fig. 5D View Figure 5 ).

Remarks

The testes in Mozambican specimens are more deeply lobed than those from Polynesia and New Caledonia. The others characters – colour, colony structure,

branchial sac, gut and gonad arrangement – are similar.

METANDROCARPA REDUCTA MONNIOT C., 1988 ( Fig. 6 View Figure 6 )

Metandrocarpa manina reducta Monniot C., 1988: 180 View in CoL , fig. 7F,I – New Caledonia.

Metandrocarpa reducta Monniot C., 1991: 8 View in CoL , Fig. 1C View Figure 1 – New Caledonia.

Material

Mozambique: Ibo Island, external reef slope, a colony on Polycarpa mytiligera , coll. C. Monniot, 1995.

Description

The zooids are spheres 2–3 mm in diameter, covered with sediment and linked by thin stolons. Seen through the sand grains, the tunic is clear, a charac- ter that helps to differentiate this species from species of Amphicarpa .

The body wall is thin and in its transparency reveals the gut, the gonads and even the branchial tissue. The muscles form a network of thin fibres. Both siphons four-lobed are well apart. Sixteen oral tentacles were counted in three orders of size, the smallest sometimes reduced to mere buttons. The prepharyngeal band has a single crest and makes a dorsal V. The dorsal tubercle, oval and flat, has a simple hole. The neural gland extends along the sides of the ganglion.

The dorsal lamina is long, with a plain margin increasing in height down to the oesophagus.

The branchial sac has eight rows of stigmata and four longitudinal vessels on each side. The last row of stigmata is not complete: it does not reach to the dorsal lamina.

We counted this arrangment of stigmata in a mid-pharynx row: