Trididemnum discrepans ( Sluiter, 1909 )

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 : 761-763

Trididemnum discrepans ( Sluiter, 1909)

( figure 17A )

Leptoclinum discrepans Sluiter, 1909: 77 .

Trididemnum discrepans Kott, 2001: 267 and synonymy; not Monniot and Monniot, 2001: 262 =? T. dispersum ( Sluiter, 1909) , see Kott, 2001.

Distribution. New record: Western Australia (Rowley Shoals, WAM 183: 93). Previously recorded (see Kott, 2001): Australia (NW, NE, S Australia, Torres Strait), Indonesia, New Caledonia, Philippines, Fiji, Palau Is, Kiribati, Tonga, Thailand, Cocos Keeling Is.

Description. Colonies are soft, fleshy and form irregular, gelatinous sheets to about 3 cm in maximum diameter. Large bladder cells are packed throughout the test, with flecks of brown pigment crowded between them, varying in intensity from place to place. Sparse pigment also is scattered through the remainder of the test. A common cloacal cavity was not detected in the newly recorded colonies, which have embryos and abdomina in the basal test but thoraces disintegrating in the upper part of the colony. Only a few minute spicules are scattered through the test.

Zooids have 12 stigmata in the anterior row of the branchinal sac, a robust retractor muscle is present, dark squamous epithelium is on the abdomen and eight coils of the vas deferens surround the undivided testis. The deep and almost spherical larval trunk is 1.1 mm long. Large balloon-like bladder cells are in the larval test. A large, spherical yolk mass occupies the middle third of the trunk, anterior to the oozooid, which is at the posterior end of the trunk. The oozooid has a long vertical gut loop and a protruding cerebral vesicle. Five or six pairs of ectodermal ampullae form a corona around the three or four narrow antero-median adhesive organs. The tail winds only halfway around the trunk and its tip is expanded into a rounded terminal fin.

Remarks. The colony, zooids and larvae are as previously described. The dark, largely aspicular test with large bladder cells throughout, zooids with dark squamous epithelum and wide thoraces, eight coils of the vas deferens, large larval trunk with a large yolk mass, the posterior position of the oozooid and the five or six pairs of ectodermal ampullae are all characteristic. The aspiculate colony and large number of ectodermal ampullae are unusual in this genus (although they do occasionally occur). However, although the yolk mass is particularly large and conspicuous, its spherical shape and position, anterior to the oozooid, is characteristic of Trididemnum (see Kott, 2001).

Monniot and Monniot (2001) described this species as having small (5–6 mm), black, lobulating colonies, spicules mixed with faecal pellets in the basal test and zooids with only five or six coils of the vas deferens. Those characters suggest an affinity with T. dispersum rather than the present species although the larvae and the numbers of stigmata per row are the same as the present species. Possibly Monniot and Monniot (2001) had more than one species before them. Monniot (1991) correctly assigned (from New Caledonia) characteristic irregular colonies (to 4 cm) without spicules, with black test and body wall (probably squamous epithelium) and eight coils of the vas deferens.

Figures

FIG. 17. Trididemnum discrepans (WAM 183.93): (A) larva. Trididemnum nobile (SAM E2902): (B) thorax; (C) larva; (D) spicules. Scales: (A–C) 0.1 mm.