Microgastra granosa ( Sluiter, 1904 )

Kott, Patricia, 2006, Observations on non-didemnid ascidians from Australian waters (1), Journal of Natural History 40 (3 - 4), pp. 169-234 : 212-213

Microgastra granosa ( Sluiter, 1904)

Ascidia granosa Sliuiter 1904, p 36 .

Microgastra granosa: Kott 1998, p 163 and synonymy.

Distribution

Previously recorded (see Kott 1998): Queensland ( Low Is , Bowen , Townsville , Innisfail , Princess Charlotte Bay , Gordonvale ); Indonesia, Japan. New records: Tasmanian Canyons (Ling Hole, 174 m); Queensland (14.685–18.755 ° S, to 66 m) .

Description

The largest newly recorded specimen is 10 cm long. As usual, the sandy test is thin, flaccid to brittle with some sandy, irregular tags from the posterior end of the body. The newly recorded specimens range from relatively short, cylindrical bodies to long, narrow ones about 1 cm diameter and 10 cm long. The branchial siphon is terminal and the atrial is two-thirds of the way down the body. About 50 single branchial tentacles are short and well-spaced and the dorsal membrane is double. That part of the body wall containing the dorsal tubercle and ganglion and ciliated funnel-like openings of the branches of the neural duct (opening along the body wall behind the branchial sac) is closely attached to the test. The body wall is thin, diaphanous, transparent, closely applied to the test. Parallel transverse muscles forming an almost continuous coat are along the right but muscles were not detected on the left side of the body. In the branchial sac are many rows of short stigmata with papillae on the transverse vessels about every two or three stigmata. Internal longitudinal vessels were not detected in the larger specimens. The prepharyngeal groove loops across to the right in front of the dorsal tubercle creating a dorsal fold in the branchial sac. The small gut loop lies embedded in an outgrowth of the body wall that is constricted off from the rest of the body. The stomach is small and oval with internal longitudinal folds. The post-pyloric part of the gut loop is flat and forms a tight primary loop, with the distal end of the gut folded back to the descending limb to form a secondary loop. The gonads are in external pouches of the body wall embedded in the test. The testis, consisting of small crowded follicles, is on the left about one-third of the body length from its posterior end. The ovarian sac associated with a seminal vesicle is embedded in the test of the posterior rim of the atrial opening. The vas deferens, embedded in the test, extends posteriorly from its origin halfway down the testis and loops around its posterior margin before extending anteriorly to the atrial aperture.

Remarks

This is a remarkable species, its body held in place by the parts of it (along the dorsal midline and around the gonads) that are embedded in the test. It is the thin, sand-strengthened test that protects both the gonads and the neural elements of the individual. The filtering area is large but the gut loop is small and entirely evaginated from the body wall in a tight nucleus, presumably held in place between the inflated pharynx and the test. The species has much in common with Ascidia scaevola , which has a similar sandy test, a branchial fold, compact, flattened gut loop and thin body wall closely adhering to the test. These two species also occupy similar sea-floor habitats and have a similar geographic range from temperate and tropical waters (although A. scaevola has not been recorded from Japan).

In a survey extending from north of Cooktown (14.5 ° S) to Broad Sound (about 22 ° S), the species was taken at 12 stations (of a total of 383). It was one of the best represented sea-floor species of ascidians in this region, 30 specimens having been taken at one of the stations. Polycarpa chinensis , P. decipiens , and P. obscura are the only species taken in similar numbers at any one station. The record from the temperate water of the Tasmanian Canyons (174 m) is unusual, being the only time the species is recorded outside the tropics, except for the Japanese record. Apparently it has a wide range both north and south of tropical waters.