Bathyoncus lanatus Monniot C.

Kott, Patricia, 2009, Taxonomic revision of Ascidiacea (Tunicata) from the upper continental slope off north-western Australia, Journal of Natural History 43 (31 - 32), pp. 1947-1986 : 1978-1979

Bathyoncus lanatus Monniot C. and F., 1991

( Figure 3D–E )

Bathyoncus lanatus Monniot C. and F. 1991, p. 406

Distribution

Previously recorded ( Monniot C and Monniot F 1991): New Caledonia. New record: CSIRO 05 / 07 western Australia ( Station 047-002, Port Hedland, 19.14 S 117.77 E, 115 m Sediment grab, 4.6.07, QM G328559 ) .

Previously recorded specimens are from about 400 m to 1000 m.

Description

The newly recorded specimen is sandy, more or less spherical and about 1.5 cm diameter. The apertures are close together in a narrow depression in the centre of the upper surface, each on a small wart-like protuberance. The test is particularly thin and flexible in the depressed area around the apertures and lacks the hair-like extensions of the test with their enmeshed sand that cover the remainder of the body, especially the basal half where there is more sand and the test extensions are longer, thicker and more root-like.

The body wall adheres closely to the thin test. Strong longitudinal muscles radiate from the apertures external to a layer of fine transverse fibres. A ring of simple, narrow, pointed tentacles is at the base of the incurrent opening. A simple U-shaped slit is on the dorsal tubercle and the dorsal lamina is a plain-edged membrane rolled over to the right. The delicate branchial sac consists of an open network of four broad but delicate internal longitudinal vessels held in place by six transverse sinuses and lacking the perforated pharyngeal wall of the majority of ascidian taxa. The gut loop, open at the pole, lies across the postero-ventral part of the body and the rectum extends anteriorly to form a secondary loop. The stomach is pear-shaped with about 25 regular, conspicuous folds in its wall. Small rounded lobes are around the margin of the anal opening. The gonads consist of slightly flattened, oval testis follicles along each side of a central ovary containing large eggs. Thin vasa efferentia join to form a vas deferens on the surface of the ovary. Male and female gonoducts open together near the atrial aperture. Two sausage-shaped gonads converge across the right pallial wall toward the atrial aperture, the anterior one relatively long and slightly convoluted and the other shorter and thicker. On the left a single large, almost sausageshaped gonad extends across the pallial wall toward the atrial aperture anterior to the pole of the gut loop. Flat-topped and sometimes leaf-shaped endocarps are scattered on the internal pallial wall around the gonads and in the loop of the gut.

Remarks

This single specimen is only the second record for this species, although the first record (including the types) from around New Caledonia referred to a number of specimens. Despite the significant distance separating these populations, the newly recorded specimen agrees in all respects with those described from New Caledonia except for the absence of a stalk, which is a character usually found in species with an open network of branchial sinuses replacing the pharyngeal wall and individuals are held aloft by a relatively narrow stalk ensuring that their incurrent aperture faces the oncoming current, thus compensating for the absence of cilia-driven incurrent flow (see Biogeography, earlier). Possibly the small size of this specimen has removed the necessity to maximize the individual’s exposure to passing currents .

Six species are now assigned to this genus, but they can be divided into two distinct groups, one with short gonads resembling those of Polycarpa with a limited number of male follicles associated with each sac-like ovary and the other group with long sausage-like gonads that resemble those of Cnemidocarpa with male follicles surrounding the long tubular ovaries.

Key to the species presently assigned to Bathyoncus

1. Gonads long, sausage like....................................... 2 Gonads short, sac-like.......................................... 3

2. Internal longitudinal vessels grouped into vestigial folds....................................................... B. mirabilis Herdman, 1882 Internal longitudinal vessels not grouped into vestigial folds.............................................. B. lanatus Monniot C. and F., 1991

3. Gonads in a row each side of the endostyle........................................................ B. arafurensis Monniot F. and C., 2003 Gonads not in a row each side of the endostyle...................... 4

4. Internal longitudinal vessels grouped to form vestigial folds; gonads more than one on each side of the body............ B. discoides Herdman, 1886 Internal longitudinal vessels not grouped to form vestigial folds; one gonad on each side of the body......................................... 5

5. Stalk present............................. B. minutus Herdman, 1886 Stalk not present.................. B. tantulus Monniot C. and F, 1991

Figures

Figure 3. (Continued). (E) body opened along ventral midline showing gonads, endocarps and gut loop. (F) Molgula delicata (QM G328510), body seen from above showing muscles radiating from around the apertures and extending halfway down the body wall and the fine transverse muscle crossing the dorsal mid-line. Scales: A, 1.0 cm; B, D, 0.5 cm; C, 0.5 mm; E, 1.0 mm; F, 2.0 mm.