Order STOLIDOBRANCHIA

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Introduction

Like Phlebobranchia, the suborder Stolidobranchia (order Pleurogona) has the gut embedded in the parietal body wall outside the large branchial sac with its internal longitudinal branchial vessels. However, the branchial sac itself is folded (unlike the unfolded pharyngeal wall of Phlebobranchia), and the regenerative tissue in replication is ectodermal, from test vessels or the parietal body wall (unlike the endodermal-epicardial-regenerative tissue in most Aplousobranchia and the mesodermal tissue in Clavelinidae and Perophoridae).

The suborder appears to have evolved from a phlebobranch ancestor through selection for adaptations associated with relatively large solitary organisms. Thus, in comparison with Phlebobranchia, the muscles of the parietal body wall form thicker layers of crowded fibres; the filtering area of the branchial sac is amplified by the development of the deep folds; the glandular part of the stomach wall projects out into folds or pockets and sometimes an arborescent liver branches off the pyloric region; gonads are on both sides of the body, and their size or number is increased to the extent that they sometimes cover much of the body wall outside the gut loop, although sometimes the gonads on the left side are enclosed in the gut loop. The test has become more fibrous and tough (unlike the firm but gelatinous test of most phlebobranch ascidians) with strong adhesive properties. Stalks and root-like extensions contribute to firm adherence or stability in sandy substrates and sometimes hair-like extensions grow out all over the test and attach a thick layer of insulating and protective sand around it. Sand and other foreign particles are often included in the test, making it hard and brittle (as in some aplousobranch colonies and a few phlebobranch species). Decoration of the outer surface of the test with hairs, spines and scales also occurs. Usually both siphons are lined with crowded overlapping scales or spines directed outwards toward the external aperture and usually they continue onto the test around the outside of the opening, directed away from it. Probably also protective, possibly detecting and/or filtering larger particles or predators from the incurrent water, are the branched and bushy tentacles at the base of the branchial siphon that are characteristic of both Pyuridae and Molgulidae. In the Styelidae (as in other suborders) the tentacles are simple and unbranched, but crowded.

The genus Cynthia Savigny, 1816 contained sessile ascidians with coriaceous test, four-lobed apertures, branchial folds, simple or branched tentacles and gut and gonads at the side of the pharynx. In fact all stolidobranch taxa were contained in the four subgenera or divisions of Cynthia, namely, Simplices, Caesira, Pandocia and Dendrodoa. Savigny did not assign type species to either Cynthia or its subgenera and in due course the genus was recognised as a supra-generic grouping, new genera were erected, and some pre-existing ones were found to accommodate the various components of the hyper-genus Cynthia (Heller 1877).

Colonial forms occur in subfamilies of the Styelidae, but not in Pyuridae or Molgulidae. Changes associated with the evolution of this habit are the reverse of those that occurred in the evolution of Stolidobranchia: they parallel the evolutionary changes in Aplousobranchia, namely size reduction and simplification of zooids, development of colonial systems, viviparity, and the development of larval organs.

The characters relevant at familial level are the body muscles; The numbers of branchial folds and form of the stigmata; the form of digestive diverticula; the numbers and position of gonads, the form of branchial tentacles; and the colonial status of the taxa. Most families, with the exception only of some abyssal ones, occur in Australian waters (Kott 1985).

General References

Heller, C. 1877. Untersuchungen über die Tunicaten des Adriatischen und Mittlemeeres (3). Denkschriften der Kaiserlichen Akademie der Wissenschaften zu Wien 37(1): 241-275

Kott, P. 1985. The Australian Ascidiacea Pt 1, Phlebobranchia and Stolidobranchia. Memoirs of the Queensland Museum 23: 1-440

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