Vitrum mjoebergi (Hartmeyer, 1919)

Kott, Patricia, 2009, Taxonomic revision of Ascidiacea (Tunicata) from the upper continental slope off north-western Australia, Journal of Natural History 43 (31 - 32), pp. 1947-1986 : 1963-1964

Vitrum mjoebergi (Hartmeyer, 1919)

( Figure 1E–F )

Sigillina mjoebergi Hartmeyer 1919, p. 117 . Kott 1990, p. 96; 2003, p. 1619.

Distribution

Previously recorded ( Kott 1990, 2003): western Australia (Cockburn Sound, Jurien Bay, Dongara, Geraldton, Dampier Archipelago, Montebello Is., Port Hedland, Cape Jaubert). New record: western Australia CSIRO 05/07 (Station 082-071, Imperieuse, 120.1447 E 18.4602 S, 80 m, Sled tow, 19.6.07, QM G328563).

The species has been recorded only from the continental shelf of north-western Australia from Cockburn Sound to Broome at 30–140 m depth. It appears to be a frequent component of communities in this region .

Description

The newly recorded specimen has, as previously reported, a long, narrow, tough stalk with a tuft of short basal roots and an oval to tulip-shaped, glassy transparent head (about 3 cm long) one third of the length of the stalk. The small zooids are clearly visible through the firm glassy test of the head of the colony in which they are embedded. They are tightly attached and are difficult to remove. They are arranged regularly, just beneath the surface test, in what appear to be spiralling rows along the length of the head. These spirals may be apparent only, resulting from the positioning of the zooids which seem to alternate with adjacent zooids rather than being level with them. They increase in size from the top of the stalk to the top of the head, the smallest ones at the top of the stalk apparently separating from narrow vegetative stolons that extend in parallel down into the stalk. The thoraces lie perpendicular to the surface of the colony with the atrial aperture toward the free end of the head. They are wider than long, with three rows of stigmata and about 25 stigmata in each half row. The relatively short abdomen is never more than half the length of the thorax and extends behind the thorax into the central core of the oval head. The rounded, smooth-walled stomach is half to two-thirds of the distance down the abdomen and a small rounded posterior stomach is in the distal third of the descending limb of the gut loop. Gonads have up to 20 crowded testis follicles and a single-egg ovary and are crowded in the gut loop. In some colonies a large larva is in a brood pouch constricted off from the postero-dorsal corner of the thorax. The larvae have similar large antero-median adhesive organs consisting of clumps of adhesive cells in the base of long tubes recessed into the trunk of the larva. These tubes evert before settlement, bringing the adhesive cells to a terminal position.

Details of breeding season and of the larvae and their incubation in the incubatory pouch are given in Kott (1990). Larvae are not present in the newly recorded colony.

Remarks

Replication appears to occur through strobilation of the isolated posterior vegetative stolon and the regenerating zooids gradually move from the stalk up into the head of the colony where the largest zooids are near the free terminal end of the colony as in the genus Sycozoa (which also has colonies consisting of soft heads on long, narrow, tough stalks that are very similar to the present ones ( Caullery 1909)). The vegetative stolons appear to be stolonic vessels, possibly isolated either from the large zooids at the top of the colony or from more mature senescent zooids following a quiescent stage and subsequent resorption. It appears to be a process suited to species in which there is a seasonal regression of the head, which subsequently is replaced with a new one ( Shepherd 1983) into which replicates generated in the stalk progressively move up to form new common cloacal systems in the regenerating head of the colony.

The higher taxon differences, including the different larval adhesive organs, displayed in these otherwise similar genera ( Sycozoa and Vitrum ) suggest their polyphyletic origin and a degree of convergence in their replicative systems and colony form. It is the last two characters that informed Kott’s (1990) view of the holozoinid affinity of Sigillina to which the present species had long been assigned. However, the Holozoidae are distinguished from Sigillina and from the present species by more numerous rows of stigmata and the presence of common cloacal systems. Caullery (1909) also suggested that Sigillina was distinct from the Holozoidae and was a subfamily (Sigillininae) of the Distomidae . Like Sigillina the present species has three rows of stigmata and lacks a posterior abdomen. However, its large invaginated tubular adhesive organs suggest an earlier phylogenetic divergence between these taxa. Similar inverted tubular adhesive organs occur in the smaller larvae of Euherdmaniidae and Pycnoclavellidae ( Kott 1990, 2007), but these families have other characters that are significantly different from the present species.

The present species appears to be distinct from known families of the Ascidiacea

Figures

Figure 1. (A–B) Diazona fungia (QM G328570); (A) colony; (B) zooid. (C) Diazona textura (QM G328571) colony. (D) Polycitor translucidus (QM G328573) colony. (E–F) Sigillina mjöbergi (QM G328563); (E) colony; (F) zooid. Scales: A, C, D, 0.5 cm; B, 1.0 mm; E, 1.0 cm; F, 0.5 mm.