Microcosmus bitunicatus F. Monniot & C. Monniot,, 2001

Mondal, Jhimli & Raghunathan, C., 2022, Addition of three Ascidian species to Indian waters from Andaman and Nicobar Islands, Records of the Zoological Survey of India 122 (3), pp. 275-282 : 278-281

3. Microcosmus bitunicatus F. Monniot & C. Monniot,

2001

Material examined: 3 ex.: India, Andaman and Nicobar Islands, District South Andaman, North Bay , 11°43.006ʹN, 92°45.465ʹE, Depth : 10 m, 05. viii.2016, Coll.: Tamal Mondal (Reg. No.: ZSI/ ANRC – 15345) .

Description: Collected specimens are 3.5 cm in length and 4.3 cm in width ( Figure 4a ). The siphons are divergent from each other. The test is fully covered with muddy and sandy particles with few epibionts. The specimens were located under the boulder at the site of the shipwreck. The species was also searched in the reef area near the shipwreck, but it was not found. The specimen retains its muddy colour after fixation also. All the collected specimens have two tunics ( Figure 4b ). The internal tunic was also covered by muddy and sandy sedimentation. It also has been found that the internal tunic is attached to the external one at the ventral side with a thick thread-like structure. The internal tunic is very thin and is of muddy colour.

Body musculature is strong and pyurid type. Both siphons are with strong longitudinal muscles. The prepharyngeal area is papillated. 20 tentacles are arranged in three-sized order. All the tentacles follow 3 rd order branching pattern. The dorsal lamina is a smooth and thin membranous structure. The dorsal tubercle is horseshoe shaped with in-rolled coiled horns ( Figure 4c ). The branchial sac has six folds on each side of the body ( Figure 4d ). 10 stigmata are present per mesh. Parastigmatic vessels are present.

Branchial formula: DL 3 (26) 6 (27) 4 (22) 4 (21) 3 (21) 4 (16) 1 E (left side)

Gut forms a loop ( Figure 4e ). The hepatic lobe is large with parallel glandular ridges which consist of small papillae. The anus is bilobed and smooth bordered. Gonad consists of three blocks-like structures on both side of the body, on left the first lobe is enclosed inside the gut loop. Gonoduct is short. A gonoduct opening is a simple hole, with a membrane-like structure, that opens at the base of the siphon ( Figure 4f ).

Remarks: The closest species to Microcosmus bitunicatus is Microcosmus manaarensis , which also has two layers of tunics. But the tunics of Microcosmus mannarensis contain one normal internal tunic and the external one is made up of a network of ramified hairs shielded by sediment agglomeration (Monniot F. and Monniot C., 2001). The internal tunic is linked to the external one with the tunic vessels as stated by Monniot and Monniot (1996). In 1996, Monniot F. and Monniot C. examined the species from the Philippines but identified it as Microcosmus mannarensis . However, in 2001 they confirmed that the specimen was wrongly identified after obtaining the second specimen, and they described both specimens as new species, which they named Microcosmus bitunicatus (Monniot F. and Monniot C., 2001) .

Distribution: India: Andaman and Nicobar Islands ( Figure 1 ); Elsewhere: Philippines (Monniot F. and Monniot C., 1996, 2001).

Figures

Figure 1. Study areas of Andaman and Nicobar Islands.

Figure 4. Microcosmus bitunicatus Monniot & Monniot, 2001. a. Preserved specimen. b. Double test. c. Dorsal tubercle. d. Branchial sac with folds. e. Gut loop and gonad in the body wall. f. Opening of gonad. (Abbreviation: DL: Dorsal Lamina, DT: Dorsal Tubercle, E: Endostyle, G: Gonad, GL: Gut loop, GO: Opening of Gonad, L: Liver, S: Stomach, T1: First Tunic/Outer tunic, T2: Second Tunic, each number is denoting each branchial fold), (Scale: c– 1mm; f– 5mm, 7.80x).