Synoicum citrum Kott, 1992

(Figures 4A–C, 9H)

Synoicum citrum Kott 1992a, p 486.

Distribution

Previously recorded (see Kott 1992a): South Australia (Port MacDonnell); Victoria (Wilson’s Promontory); Tasmania (D’Entrecasteaux Channel, Bruny I.). New records: South Australia (Kangaroo I., 10–12 m, SAM E3277; 10–14 m, E3287); Tasmanian Canyons (Big Horseshoe, 159 m).

Description

Colonies are soft, rounded translucent cushions with whitish zooids in small circular systems showing through the translucent test and a slightly lobed surface. Zooids are crowded and although the circular systems that Kott (1992a) thought would be present can be seen in the in situ photographs, they were not always detected in the preserved specimens. Zooids have atrial apertures produced on short siphons, each with a large forked lip from the anterior rim of the opening. Fine longitudinal muscles are in the pallial wall of the thorax and continue in an inconspicuous band along each side of the abdomen and the posterior abdomen. The branchial sac has four pairs of rows of about 18 stigmata per half row, apparently formed by the transverse subdivision of four primary rows, the stigmata at the ventral end of each pair of rows being reduced in length to leave a triangular imperforate area of pharyngeal wall between successive pairs of rows. The large smoothwalled barrel-shaped stomach is halfway down the relatively short abdomen which contains the vertical gut loop. The anal opening, between the third and fourth secondary rows of stigmata, is bilabiate. Gonads were not detected in this specimen. However, there is a stout, contracted posterior abdomen containing a conspicuous tightly undulating (presumably epicardial) tube. A V-shaped tubular heart is at the posterior end of the posterior abdominal extension.

Remarks

The colony resembles Distaplia pallida and the zooids (especially the paired rows of stigmata and the presence of the epicardial tube in the posterior abdomen) are not unlike those of Distaplia spp. However, in other characters, the present species appears to belong to the genus Synoicum in the family Poyclinidae (see Kott 1992a). In the latter family and related families, as in the present species (but not in the Holozoidae), muscles continue on to the posterior abdomen which has the heart at its posterior extremity, the gut loop is vertical, the atrial aperture is small and usually does not expose the branchial sac directly to the common cloacal cavity. Sigillina also has an epicardial tube in the posterior abdomen and has body muscles on it, but it has only three rows of stigmata and its apertures open separately to the exterior. Further, although the larva of the present species is not known, Synoicum is distinguished from both Holozoidae and Sigillina by its larva which has small adhesive organs in the antero-median line and characteristic polyclinid epidermal vesicles.

Synoicum bowerbanki Millar, 1963 has only four rows of stigmata, but these are not divided as they are in the present species.