Symplegma brakenhielmi ( Michaelsen, 1904 )

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 315-317

Symplegma brakenhielmi ( Michaelsen, 1904)

Diandrocarpa brakenhielmi Michaelsen, 1904: 50 . Type locality: Mexico gulf.

Symplegma brakenhielmi – Monniot C. & Monniot F. 1997: 1636, fig. 7A, C, all oceans.

MATERIAL EXAMINED. — Mariana Islands. Guam, Apra Harbour, off Buoys, 0-3 m, 12. VI.1998 ( MNHN S1 SYM 73).

Symplegma rubra Monniot C., 1972 ( Figs 87 ; 127B, C)

Symplegma rubra Monniot C., 1972: 622 , figs 2E, F; 3. Type locality: Bermuda. Synonymy and distribution: see Monniot F. & Monniot C. 1997: 26, fig. 15, pl. 3B, C, Zanzibar, Mozambique .

MATERIAL EXAMINED. — Federated States of Micronesia. Yap, Colonia, main wharf, 9°30.91’N, 138°07.57’E, 10 m, 7.VIII.1995.

Palau. KB Channel, Airai side South of KB Bridge, 7°21.03’N, 134°30.95’E, 33 m, 12.VII.1993 ( MNHN S1 SYM 66). — Koror, Malakal Harbour, Buoy #5, 7°19.16’N, 134°27.18’E. — Koror, Malakal Harbour, on buoy chain, 8 m, 7.VIII.1995. — Koror, West Malakal Harbour mouth, 7°20.37’N, 134°25.78’E, 34 m, 25.VIII.1997 (Samples: CRRF).

Indonesia. Banta Island, 22 m, 1998, coll. Erhardt ( MNHN S1 SYM 71).

DESCRIPTION

The distribution of this species now includes the three oceans. There is a large colour diversity in the Indo-Pacific region (Fig. 127B, C), while in the Atlantic it is pink or red. The disposition of the pigment ring linking both siphons is characteristic. The colonies collected at Yap Island are encrusting and 5 mm thick. The zooids lie oblique to the surface and not flattened as they are in most other species of Symplegma . The zooids are in contact with the basal layer of the colony only by the posterior part of their ventral side. In life the zooids are opaque; at Yap this is due to a white pigment uniformly distributed in the body wall, while at Palau they are pink. All colours disappear after formalin fixation.

There are 12 oral tentacles in two orders, well forward in the siphon, and sometimes small buttons between them. The prepharyngeal band has a single circular crest without a dorsal curve ( Fig. 87C ). The dorsal tubercle is very small with a round or oval opening. The rim of the hole does not stain as it usually does. The dorsal lamina has two low parallel blades.

The branchial sac has four longitudinal vessels on each side. The first two on the left join the dorsal lamina at the level of the tenth and twelfth rows of stigmata ( Fig. 87A ). The longitudinal vessels are prolonged anteriorly beyond the branchial perforations. We counted 14 rows of large stigmata; the second row does not reach the dorsal lamina. At the level of the sixth row, the formula is:

R.E. 8 v 7 v 7 v 7 v 8 D.L. 5 v 7 v 6 v 7 v 7 E.L. On the right side the first vessel parts from the dorsal lamina posteriorly and about 12 stigmata lie between them.

Some differences between the Atlantic, Mozambican, and Pacific samples concern the gut. In the present collection the gut ( Fig. 87A, B ) is limited to the posterior third of the left side, while in Africa and North America it is larger, reaching half or even three fifth of the left side. The stomach has 14 folds, decreasing in length on its internal side toward the caecum. The pyloric caecum is long and curved ( Fig. 87D, E ); it is linked to the intestine by two strips of tissue variably positioned according to the colonies. The long rectum ends in a gaping anus without lobes, or one with four low lobes when contracted. The gonads lie at the level of the sixth and seventh rows of stigmata on the left side and the seventh and eighth rows on the right ( Fig. 87A ). This near-symmetry may reflect a short gut; in African specimens there is a difference of two rows of stigmata, but on the left side in them the gonad may well be pushed anteriorly by the larg- er gut. The male and female parts of the gonads are not simultaneously mature. When the oocytes are developed the male part is rudimentary ( Fig. 87F, G ) and during the male phase only the female papilla persists ( Fig. 87H ). It appears that the zooids are first female, as no oocyte remains in the traces of ovary during the male phase. In Caribbean and African populations, as in Symplegma alterna Monniot C., 1988 from New Caledonia, which also shows sequential hermaphroditism, the rudimentary ovary contains numerous small oocytes during the male phase, and this may indicate a protandric activity.

REMARKS

Taking into account the characteristic aspect of the living colonies, we believe that there is a single cosmopolitan species. The diversity of colour in the Indo-Pacific populations compared to the uniformity of colour in the Atlantic Ocean may mean that the latter are immigrant populations, as they are mostly found in harbours.

Genus Stolonica Lacaze-Duthiers & Delage, 1892

Figures

FIG. 87. — Symplegma rubra Monniot C., 1972; A, female zooid ventrally opened; B, ventral side of a male zooid; C, anterior dorsal area; D, detail of the gut; E, external side of the stomach; F-H, successive development stages of the gonads. Scale bars: A, B, 2 mm; C, F-H, 0.5 mm; D, E, 1 mm.