Didemnum grande ( Herdman, 1886 )

Kott, Patricia, 2004, New and little-known species of Didemnidae (Ascidiacea, Tunicata) from Australia (part I), Journal of Natural History 38 (19), pp. 731-774 : 752

Didemnum grande ( Herdman, 1886)

( figure 11E )

Leptoclinum albidum var. grande Herdman, 1886: 291 .

Didemnum grande: Kott, 2001: 186 and synonymy.

Distribution. New record: Western Australia (Port Hedland, NTM E36, WAM 945.88). Previously recorded (see Kott, 2001): Queensland (Hervey Bay, Capricorn Group, Swain Reefs, Whitsunday Is), Indonesia, Philippines.

Description. Newly recorded colonies are extensive, irregular sheets, with a cartilaginous (rather than brittle) consistency. They are white or pinkish beige on the surface but white internally. Spicules are crowded throughout. They fill (rather than line) the lobes of the stellate branchial apertures. Ripple marks are on the base. Zooids are along each side of deep primary common cloacal canals which extend the full depth of the zooids, the abdomina being embedded in the lower part of the test connectives between the thin surface and the basal test. Sometimes thoraces are separated from one another by secondary thoracic common cloacal cavities. The colony has a quilted appearance, the surface test being depressed slightly over the deep primary canals. Spicules are to 0.09 mm diameter with 9–11 and occasionally seven conical rays in optical transverse section. The ray length / spicule diameter ratio is 0.35.

Zooids are relatively long, and the branchial sac is almost completely exposed to the large common cloacal cavity by the open sessile atrial aperture. Branchial siphons are relatively long and have either tulip-shaped or cylindrical branchial siphons. A long retractor muscle from the top of the oesophageal neck extends down through the connectives into the basal test. Oesophageal buds and stolonic vessels are present but neither gonads nor larvae are present in these newly recorded colonies.

Remarks. Kott (2001: 143, Key Character 49) relied on the presence of a superficial, conspicuous bladder cell layer with pigment cells to distinguish this species from others ( D. inveteratum Kott, 2001 , D. microthoracicum Kott, 2001 and D. stragulum Kott, 2001 ) with uniform, large (to 0.09 mm diameter) stellate spicules throughout and lacking a posterior abdominal component of the common cloacal cavity. The bladder cell layer is not conspicuous in the newly recorded specimens and appears to be rubbed off in one of them (WAM 945.88). The character does not appear to be an appropriate distinguishing character, and the species can be more reliably separated from the species listed above by various other characters, including the quilted surface resulting from the form of its cloacal system.

The long, strong, retractor muscle and the long pointed rays of the large stellate spicules are characteristic of this species, which has been recorded previously from Port Hedland. The spicules resemble (but are larger than) those of Didemnum madeleinae Monniot and Monniot, 2001 and D. perplexum Kott, 2001 , but D. grande has four pairs of larval ectodermal ampullae, D. perplexum has six pairs and D. madeleinae has about 12 pairs. Didemnum via Kott, 2001 , from Heron Island, has similar (but smaller) spicules with the same number of rays as the present species

Figures

FIG. 11. Didemnum fibriae (WAM 175.91): (A) thorax; (B) abdomen; (C) larva; (D) spicules. Didemnum grande (NTM E36): (E) spicules. Scales: (A–C) 0.2 mm.