Botrylloides saccus, KOTT, 2003

KOTT, PATRICIA, 2003, New syntheses and new species in the Australian Ascidiacea, Journal of Natural History 37 (13), pp. 1611-1653 : 1645-1647

Botrylloides saccus n. sp.

(figure 11A–D; plate 1E)

Distribution. Type locality: South Australia (Kangaroo I., Peneshaw jetty, 4–6 m, coll. K. Gowlett Holmes, 5 May 1999, syntypes SAM E2868). There are no other records.

Description. The syntypes are numerous, small (to 3 cm long) tear-drop to discshaped or almost sperical, sandy colony lobes attached to one another, if at all, by a long, flexible, thin sometimes branching strand of test. These strands may be attached, or adhering along their length, to the stalks of a hydroid. The test is thin and delicate, and the sand particles on the colony lobe and the connecting strands adhere to the surface and obscure the rather irregular terminal common cloacal aperture and the small sessile smooth-rimmed branchial apertures which open to the surface in a circle around the colony about half-way down, well removed from the terminal excurrent aperture. Each lobe is a single system of about 12 zooids.

Zooids are delicate with a thin body wall with fine longitudinal muscles. A distinct branchial sphincter surrounds the sessile incurrent aperture. The atrial opening is wide, exposing much of the dorsal half of the branchial sac to the common cloacal cavity. Stigmata are in six rows and three or four are between the three internal longitudinal vessels on each side of the branchial sac. The tight primary gut loop lies along the posterior end of the branchial sac, almost posterior to it on the left side and the rectum forms an angle with it, and the pole of the loop sometimes is anterior to the anus. The stomach is short, about half the length of the ascending limb of the primary loop and tapers toward its distal end. A curved gastric caecum arises about two-thirds of the way along the stomach from a suture line near its postero-lateral margin, extends at right angles to the longitudinal axis of the stomach to cross its outer (left) wall and bends ventrally into the gut loop, where it expands into a terminal ampulla in the pole of the gut loop. Gonads were not detected in these specimens.

Remarks. The specimens have been assigned to Botrylloides on the basis of the wide open atrial aperture which suggests that embryos would be unlikely to develop in the atrial cavity. The form of the small, sandy lobules, each a separate system, is unique in the Botryllinae . Macroscopically they appear similar to small, sandy Perophora or polyzoinid zooids, but the presence of a circle of zooids in each sandy lobule readily distinguishes them.

The unusual gastric caecum, crossing the outside of the stomach at right angles to its longitudinal axis from the postero-lateral margin, is also different from other taxa—the gastric caecum usually arising from the inner margin of the stomach and extending parallel to the long axis of the stomach into the pole of the gut loop (see Kott, 1985: Botryllus schlosseri , B. tuberatus , Botrylloides violaceum ).