Didemnum

Monniot, Françoise, 2009, Some ascidians from Indonesian marine lakes (Raja Ampat Islands, West Papua), Zootaxa 2106, pp. 13-40 : 18

Didemnum sp. 1

( Figures 3 A,B,C – 4 A,B)

Material. Indonesia. West Papua, Gam Island, Danau A Gam marine lake, 00°26.518’S – 130°41.134’E, 0.5m, 05/XII/2007, coll. L.J. Bell and L.E. Martin, DAG 0 95 ( MNHN A2 DID C 589).

Marbled white and brown in life ( Fig 3 A), the colonies form sheets 2mm thick, investing rocks and sponges. Brown zooids are suspended between a thin layer of tunic filled with dense spicules and a compact basal layer. Dense brown cells ( Fig. 4 A) are mixed with the spicules in the upper part of the colony. The abdomens are included into the basal tunic with dense spicules but sparse pigmented cells. The zooids are brown. The oral siphon is short ( Fig. 3 B) with 6 short lobes. The wide atrial aperture, without languet, uncovers the most part of the branchial sac in relaxed zooids. Six stigmata are in the first row. Small oval lateral thoracic organs are placed above the fourth stigmata row. The retractor muscle extends from the top of the oesophageal neck. The digestive loop is well open, folded on itself, with well marked compartments. The single testis vesicle ( Fig. 3 C) is spherical, protruding on the side and partly below the gut loop; it is completely encircled by 8–9 turns of the sperm duct. No ovary and no larvae were present. The spicules ( Fig. 4 B) range in size to 40µm, the largest are stellate, while smaller ones have few very short rays, all with a fibrous structure.

This species has characters in common with many Didemnum species, thus in the absence of larvae it cannot be identified at the species level. The presence of particularly large brown cells in the tunic and zooids is distinctive, they may be symbionts.

Figures

FIGURE 3. Didemnum sp. 1, A, colony; B, thorax; C, abdomen. B, C stained with hemalum, scale bar: 0.2 mm.

FIGURE 4. Didemnum sp. 1, A, pigment cells mixed with spicules in the tunic without stain; B, spicules. Scale bars: A = 100 µm, B = 10 µm.