Didemnum plebeium, Kott, 2005

Kott, Patricia, 2005, New and little-known species of Didemnidae (Ascidiacea, Tunicata) from Australia (Part 3), Journal of Natural History 39 (26), pp. 2409-2479 : 2451

Didemnum plebeium sp. nov.

( Figures 9B,C , 16D )

Distribution

Type locality: Western Australia (N of Rosemary I., Dampier Archipelago, 19 ° 459S, 116 ° 459E 60–64 m, coll. S. SlackSmith and L. Marsh on Soela, Otter Trawl stn 0001/0029, 4 December 1979, syntypes WAM 155.93 ) .

Description

Colonies are small (never more than 0.5 cm in maximum dimension). Each branchial opening has a single small pointed papilla associated with it. Spicules are crowded throughout the colony. They are stellate, to 0.06 mm diameter with 9–11 regular conical rays in optical transverse section. Zooids are small, with four rows of only about six stigmata per row and a conspicuous lateral organ with a ventrally facing concavity on each side of the thorax. A robust tapering retractor muscle projects from about halfway down the stout oesophageal neck. Branchial apertures are on short siphons and six sharply pointed lobes are on the rim of the openings. Sessile atrial apertures cross the dorsum of the thorax. Thoraces are covered with conspicuous pointed projecting columnar ectodermal cells (see Kott 2001). Gonads were not detected in the syntype colonies, although tailed larvae are in the basal test. The larval trunk is 0.7 mm long and the tail winds about three-quarters of the way around it. Four pairs of rounded larval ampullae surround the three antero-median adhesive organs.

Remarks

The regularly stellate spicules of these colonies and the larvae lack any unique characteristic that contributes to their identification. Didemnum incanum Herdman, 1899 has similar but smaller spicules. Didemnum granulatum Tokioka, 1954 has larvae that resemble the present species in size and form, and its spicules are not dissimilar, but they have only seven to nine and occasionally only five rays in optical transverse section. The number of spicule rays appears to constitute the principal character separating these species. Further, although spicules up to 0.04 mm diameter often are recorded for D. granulatum (see Kott 2002), some up to 0.06 mm have been recorded (see Kott 2001, Figure 171F) and some of the specimens assigned to this species, especially those with larger spicules, may have been wrongly assigned.

Figures

Figure 9. (A) Didemnum moseleyi (QM G308713): larva. (B, C) Didemnum plebeium sp. nov. (WAM 155.93, holotype): (B) thorax; (C) larva. Scale bars: 0.1 mm.

Figure 16. Scanning electron micrographs of the calcareous spicules of: (A) Didemnum incanum (SAM E3238); (B) Didemnum leopardus sp. nov. (QM G308719, holotype); (C) Didemnum moseleyi (WAM 175.93); (D) Didemnum plebeium sp. nov. (WAM 155.93, syntypes); (E) Didemnum stragulum (WAM 162.93); (F) Didemnum ternerratum (SAM E3240); (G) Trididemnum amiculum (SAM E3263); (H) Trididemnum areolatum (QM G308746).