Eudistoma convocatum, Kott, 2008

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 : 1116

Eudistoma convocatum sp. nov.

( Figures 2A, B )

Distribution

Type locality: Western Australia CSIRO SS10 / 05 (Albany, Stn 22, 118.2940E 35.3350S, 100 m, 22 November 2005, holotype WAM Z27515 , QM G328114 ) .

Description

The holotype is a vertical, sessile cone. The basal half is a thick, cylindrical stalk without zooid openings. Zooids open independently on the upper half of the lobe, which is a greenish tinge, possibly a result of ciliates mixed with the sand which forms a thin, hard layer in the surface test which forms a rigid framework around shallow, evenly distributed spaces in which the expanded thoraces could be accommodated. Sand becomes less crowded toward the centre of the colony, where the test is seen to be transparent and relatively soft. Sand is moderately crowded in the stalk.

Zooids are of the usual eudistomid form with separately opening branchial and atrial siphons. In the present specimen they are very contracted and withdrawn into the base of the colony. On the thorax, longitudinal muscle bands overlie the conspicuous layer of transverse muscles and then continue along the abdomen. Longitudinal furrows that are artefacts of contraction are in the otherwise smooth stomach at the posterior end of the abdomen. The post-pyloric part of the gut loop is kinked. Two large larvae are in the peribranchial cavity, although larval structures were not detected. The larval trunk is 0.55 mm long and the tail is wound all the way around.

Remarks

Sand is not uncommon in the test of colonies of Eudistoma spp. , and there is little intraspecific variation in its distribution. Also its zooids are conservative and few differences have been detected. However, some significant interspecific variation exists in the arrangement of zooids and the distribution of embedded sand. In the present specimen the colony contains embedded sand throughout. The sand is particularly crowded on the upper surface although it becomes less crowded towards the centre. The species differs from E. aureum Kott, 1990 (a southern Australian species with a vertical undivided colony) which lacks sand in or on its upper half. Of the species with sand throughout the colony, Eudistoma constrictum Kott, 1990a resembles the present species in the absence of rudimentary cloacal systems. In E. constrictum a naked area, free from sand, surrounds each pair of apertures on the upper surface of the cushion-like colonies (which also are different from the vertical colony of the present species). Eudistoma pyriforme ( Herdman, 1886) also has a sandy colony but its zooids are arranged in circles forming rudimentary systems and its colonies consist of shallow lobes on a common base. Eudistoma ovatum ( Herdman, 1886) , another species with sand throughout the test, has zooids arranged in circles and the distal part of the descending limb of the gut loop forms a stiff, yellow spiral rather than being kinked (see Kott 2004b). The present species appears to be a new species, characterized by its vertical sandy colony, zooids opening independently rather than forming rudimentary systems and with two large embryos being incubated in the atrial cavity.

Figures

Figure 2. Eudistoma convocatum sp. nov. (QM G328114): A, colony; B, zooid. Pseudodiazona longigona:(QM G328066) C, colony, with two small accessory lobes and sandfilled pharynges evident in some parts of the surface; D, thorax and abdomen; E, posterior abdomen with gonads. Scales: A, 5 cm; B, D, E, 1.0 mm; C, 5 cm.