Stolonica vermiculata, Kott, 2005

Kott, Patricia, 2005, Novel Australian Polyzoinae (Styelidae, Tunicata), Journal of Natural History 39 (32), pp. 2997-3011 : 3006-3008

Stolonica vermiculata sp. nov.

( Figures 3 , 5C )

Distribution

Type locality. New South Wales (Lord Howe I., 13 m, coll. N. Coleman, 17 July 2002, syntypes QM G308501 ) .

Description

The sheet-like colonies containing small (about 4 mm diameter) dorso-ventrally flattened, circular zooids are on the test of a specimen of Herdmania sp. The zooids are flat on their undersurfaces and the thin test is hard to separate from the Herdmania that they are attached to. A continuous layer of test may be between the zooids attached firmly to the substrate but this was not clearly demonstrated. The inconspicuous, sessile apertures are on the upper surface, the atrial aperture more or less in the centre and the branchial aperture between the atrial aperture and the anterior end of the zooid. The delicate body wall, closely adherent to the thin test, has fine muscles radiating a short distance from the apertures. The meridian of the body, around which it is folded, is about halfway down the left and right sides, although the branchial sac is folded along its ventral and dorsal midlines. Six larger branchial tentacles alternate with six smaller ones inside the branchial aperture. The ciliated pit is a small, circular, anteriorly directed opening. The branchial sac is flat with up to 15 internal longitudinal vessels per side, seven transverse vessels and two long stigmata per mesh crossed by a parastigmatic vessel. The gut forms a double loop in the posterior half of the body on the left side, projecting out into the fold around the meridian. The short stomach, in the centre of the ascending limb of the loop, has 10 rounded folds and a long gastric caecum that curves around in the pole of the gut loop. The descending limb of the primary loop forms a deep secondary loop with the rectum, which curves around the dorsal edge of the folded branchial sac on to the upper surface of the zooid as it extends anteriorly to the atrial aperture. Only unisexual gonads are on the left side of the body and hermaphrodite gonads are on the right side. In these zooids a cluster of male follicles always is on the left side of the body anterior to the gut loop. In some zooids these are immature, small tear-shaped follicles each with a short duct directed toward the atrial aperture, but in other zooids with more mature male follicles they are large, lobed, overlapping pyriform masses, each with a relatively long duct that projects into the atrial cavity from near the centre of the mesial surface of the follicle and trails toward the atrial aperture. A row of five to six hermaphrodite gonads to the right of the endostyle (in the fold around the meridian) have their short, wide ducts irregularly orientated. Occasional female gonads are also present in this row along the right side of the endostyle. In the syntypes, larvae are crowded in the atrial cavity. They have triradially arranged adhesive organs, an otolith halfway along the trunk covered over by a circle of about 20 ectodermal ampullae. The larval trunk is 0.3 mm and the tail is 0.6 mm.

Remarks

The hermaphrodite gonads of the present species (with a small sac-like ovary and one or a pair of testis follicles) resemble those of many genera of the Polyzoinae , although the separate (dioecious) male and female gonads that also are present indicate an affinity with species assigned to Stolonica (see Kott 1990), which also have hermaphrodite and sometimes female gonads on the right side of the body and numbers of single follicle male gonads on the left. Some Stolonica spp. have separate male gonads on the right as well as on the left and sometimes separate female gonads as well as hermaphrodite gonads on both left and right. Kott (1990) found much of the variation in the number and disposition of the male, female and hermaphrodite gonads resulted from the ephemeral nature of the gonads and differences relating to age and sexual maturity of the zooids. This is confirmed in the present species, in which the female gonads were only occasionally detected on the left side of the body. Subsequent investigations could indicate further variation than that presently observed in the syntypes, although, at this stage, the species appears to be characterised by the presence of separate male gonads only on the left, hermaphrodite gonads only on the right and occasionally separate female gonads only on the left.

Although previously recorded Stolonica spp. have from two to four branchial folds, there are no folds in the branchial sac of the present species. Nevertheless, the number of internal longitudinal branchial vessels resembles that in other species (irrespective of the presence or the number of branchial folds). This is taken as yet another indication of variability in higher-level taxa of evolving colonial zooids. Other genera of the Polyzoinae with flat branchial sacs are Polyzoa (which has only hermaphrodite gonads in a row on each side of the endostyle), Metandrocarpa (which has only separate sex gonads on each side of the body) and Theodorella (an indigenous New Zealand genus which has its gonads in rows either each side of or beneath the endostyle, male gonads only on the left and hermaphrodite and sometimes some separate male gonads on the right). None of these genera have the clumps of male gonads present in Stolonica . In this context, the disposition of the gonads is considered to be a character of greater phylogenetic significance than whether or not the branchial sac is folded.

As well as the absence of branchial folds, the present species is distinguished from most other Stolonica spp. by its embedded zooids, a habit it shares with Stolonica carnosa Millar, 1963 from southern Australia. The latter species has a shorter stomach with a longer curved caecum, it lacks the patches of crowded male follicles and it has separate flask-shaped male follicles and sac-like ovaries on each side of the endostyle.

Figures

Figure 3. Stolonica vermiculata sp. nov. (QM G308756, holotype). (A) Colony surface; (B) ventral view of zooid showing rectum curving around branchial sac; (C) dorsal view of internal body wall of the zooid, branchial sac and the upper half of the zooid removed; (D) testis follicles; (E) larva. Scale bars: 0.2 mm (A); 0.4 mm (B, C); 0.1 mm (D, E).

Figure 5. In situ images. (A) Polyandrocarpa colemani (QM G308755) (photograph: Merrick Ekins); (B) Oculinaria australis (SAM E3267) (photograph: K. Gowlett Holmes); (C) Stolonica vermiculata (QM G308756) (photograph: Neville Coleman); (D) Botryllocarpa elongata (SAM E3232) (photograph: K. Gowlett Holmes).