Synoicum chrysanthemum Kott, 1992

Kott, Patricia, 2008, Ascidiacea (Tunicata) from deep waters of the continental shelf of Western Australia, Journal of Natural History 42 (15 - 16), pp. 1103-1217 : 1134-1136

Synoicum chrysanthemum Kott, 1992

( Figures 4B–D )

Synoicum chrysanthemum Kott 1992a, p. 485 .

Distribution

Previously reported (see Kott 1992a): Western Australia (Bluff Point, Dongara ). New records: Western Australia CSIRO SS10 /05 ( Albany , Stn 22, 100 m, 22.11.05, QM G328067 ; Albany , Stn 26, 212 m, 23.11.05, QM G328429 six specimens; Bald I., Stn 31, 106 m, 23.11.05, QM G328428 ; Pt. Hillier, Stn 57, 196 m, 27.11.05, QM G328097 eight specimens; Jurien Bay , Stn 82, 02.12.05, 85–92 m, QM G328149 ; Kalbarri, Stn 102, 96– 98 m, 05.12.05, QM G328060 G328061 ) .

Although previously only three specimens of this species were known, the new records suggest that this is a common component of the continental shelf benthic fauna from a relatively extensive depth range of 6–212 m around the south-western corner of the continent from Shark Bay to Albany. However, it appears to be more common at deeper stations ( QM G328097 eight specimens 196 m, QM G328429 six specimens 212 m) only one or two being taken at the other stations where depths ranged from 64 to 100 m, suggesting that the species’ optimal depth is at about 200 m near the edge of the continental shelf .

Description

The colonies always have a toadstool-shaped head, rounded on the upper surface with a cylindrical stalk of varying diameter (to 1 cm) and length (to 8 cm) arising from the centre of the under surface. The surface of the head is covered with a mosaic of oval elevations separated by a network of surface depressions over the circular common cloacal canals. The circular canals are lined on each side by zooids with their branchial openings visible along the margins of the common cloacal canals. Internally, the test contains sand, which is crowded in a layer around the outside of the stalk but less crowded in the centre of the stalk. Thread-like posterior abdomina project down into the stalk amongst the sand grains. Large sessile common cloacal apertures are randomly placed at the junctions of some of the canals. Stalks sometimes divide into two toward the base forming a sort of secondary prop that helps to support the colony. In the newly recorded preserved material the surface test of parts of the stalk is transversely wrinkled.

Branchial apertures are lined with six pointed lobes. The atrial aperture is a wide opening surrounded by a strong sphincter with two to four pointed lobes projecting from the anterior rim of the opening into the surface test over the common cloacal canals. Stigmata are in 12–16 rows of about 12 per row. The stomach wall is smooth and the short stomach in the relatively short, rounded abdomen is small and spherical. A marked constriction separates the thorax and abdomen. About 12 testis follicles are bunched together in an otherwise long, thread-like posterior abdomen. Larvae, present in the atrial cavity of colonies from Kalbarri (collected in December), have a trunk 1.0 mm long with the tail wound half-way around it. Three rows of about 12 stigmata in each half-row are in the larval pharynx. The usual antero-dorsal and postero-ventral strands of vesicles project posteriorly, and short, rounded lateral ampullae are on each side of the base of the median ampullae that alternate with the three antero-median adhesive organs.

Remarks

These robust colonies that appear to abound in the deeper waters of the continental shelf are readily identified by their long cylindrical stalks, circular common cloacal canals lined with zooids which have long atrial tongues and large sessile atrial openings, and small and smooth stomach walls. The larvae, reported for the first time, are similar to others in this genus, especially the tropical Synoicum buccinum Kott, 1992a which has larvae with similar median and lateral ampullae and larger and fewer vesicles than usual projecting from the postero-ventral and postero-dorsal test strands. However, the colonies of the latter species have branched stalks with smaller heads than the present deeper-water, temperate species.

Figures

Figure 4. Sigillina grandissima (QM G328124): A, larva. Synoicum chrysanthemum (B2,3, C, QM G328097; B1, D, (QM G328060)): B1–3, colonies; C, thorax, abdomen and posterior abdomen (showing male follicles); D, larva. Scales: A, C, 0.5 mm; B, 2 cm; D, 0.2 mm.