Clitella nutricula Kott, 2001

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 : 2466-2468

Clitella nutricula Kott, 2001

( Figure 13D,E )

Clitella nutricula Kott 2001, p 333 .

Distribution

Previously recorded (see Kott 2001): South Australia (Sir Joseph Banks Group, type locality). New record: South Australia (Edithburgh, Yorke Peninsula, SAM E3275 ) .

Description

The thick, gelatinous, firm but not tough encrusting colony has brownish, translucent test through which zooids can be seen in circles or in long rows along each side of long relatively shallow thoracic common cloacal canals. The zooids are surrounded by spherical bladder cells which are crowded throughout the test and create its firm yet soft and slippery consistency. Minute spherical pigment cells are crowded around the zooids in the interstices of the bladder cells. Spicules were not detected in the newly recorded specimen, although there are small clusters of white granular material in the siphons.

Zooids are robust with wide dorsal pharyngeal muscles, numerous longitudinal muscles in the pallial thoracic wall, conspicuous transverse muscles in the interstigmatal bars and a strong retractor muscle which, in these contracted zooids, is short, thick and pointed. Four rows of up to 14 stigmata are in the branchial sac. The gut loop is long and also is robust. Oesophageal buds are forming from the long oesophageal neck. The stomach is large and almost spherical and there is a wide duodenal region. An oval posterior stomach narrows at each end, proximally where it joins the wide duodenum and distally where it enters the proximal part of the rectum (or mid-intestine), which is separated from the distal part by a distinct rectal valve. The distal part of the rectum (or true rectum) is a translucent brownish colour and has a cuticle-like consistency that is different from the remainder of the gut. A large teardrop-shaped vesicle is in the loop of the gut, fitting snugly in the space between the stomach and the gut just distal to the rectal valve, and sometimes it curves over the gut. An opening into the tapered side of this vesicle from the stomach was not detected, although it possibly does exist; and branches or tubules from the vesicle were not detected surrounding this part of the gut. Nevertheless, it is possible that it is a reservoir of the gastro-intestinal (pyloric) gland. Gonads are in the gut loop. The large spherical testis is undivided and the thick vas deferens issues from its postero-lateral aspect and extends anteriorly in a straight line over its outer surface. Yellow eggs are in the basal test.

Remarks

In most genera of the Didemnidae the gut is divided into oesophagus, stomach, duodenum, posterior stomach, and rectum. In Polysyncraton , Lissoclinum , and the present genus, the proximal part of the rectum appears to be differentiated into a distinct additional chamber or mid-intestine which, in Lissoclinum and the present genus, is separated from the distal part of the rectum by a conspicuous rectal valve. This rectal valve is in a similar position to the circle of tubules of the gastro-intestinal gland in other genera (see Polysyncraton: Kott, 2001 ). The robust zooids, their strong thoracic musculature (unlike the relatively delicate retractor and pharyngeal muscles in Lissoclinum ) and the conspicuous differentiation of the gut, the unique larval adhesive apparatus (see Kott 2001), and the turgid thick gelatinous test filled with large bladder cells distinguish this genus from Lissoclinum . Polysyncraton has numerous male follicles and a narrow tubular distal part of the rectum (rather than a firm capacious chamber) that distinguish it from the present genus. The large thorax of the present genus resembles that of some Diplosoma spp. such as D. velatum and D. translucidum but the genera are distinguished by the strong muscles, conspicuous rectal valve, large rectal chamber, and unique larval adhesive equipment (see Kott 2001).

Although the pyloric (or gastro-intestinal) gland occurs throughout the Ascidiacea (see Goodbody 1974), a vesicle similar to the one detected in the present genus was formerly known only in several genera ( Distaplia , Hypsistozoa , Neodistoma , and a few species in Sycozoa : see Kott 1990a) of the Holozoidae and in two species of the Didemnidae , namely Didemnum lahillei Hartmeyer, 1909 and Diplosoma gelatinosum Milne Edwards, 1842 (see Fouque 1948, 1954; Goodbody 1974). Although an opening from the stomach has not yet been detected it probably will be found to occur in Clitella , as it is present in Didemnum lahillei and Diplosoma gelatinosum (see Fouque 1948, 1954). Further, in the two latter species, as in the present one, there are relatively few tubules of the pyloric gland encircling the duct. Nevertheless, despite the similarity of the gastric vesicle in these three species, all of which have relatively large zooids, a direct phylogenetic relationship is not implied, the present genus being distinguished from both Diplosoma gelatinosum and Didemnum lahillei by generic characters (see above).

The newly recorded specimen is only the second time the species has been collected. The zooids closely resemble those of the holotype, the only difference being the absence of spicules in the present colony.

Figures

Figure 13. (A–C) Lissoclinum coactum (WAM 120.90): (A) colony surface; (B) zooid; (C) larva. (D, E) Clitella nutricula (SAM E 3275): (D) thorax; (E) abdomen. Scale bars: 1.0 mm (A); 0.1 mm (B–E).