Family ASCIDIIDAE

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Family ASCIDIIDAE

Compiler and date details

P. Kott, Queensland Museum, Brisbane, Queensland, Australia

Introduction

The family Ascidiidae Herdman, 1882 contains solitary forms, usually with a relatively firm, inflexible, gelatinous, translucent test. The branchial sac is more or less flat, the branchial wall sometimes having minute undulations but usually lacking internal folds. The large gonads are enclosed in the gut loop and often spread over the inside of the gut, which is bent up in the left parietal body wall. The testis consists of branching follicles joined to a common vas deferens. The tubular ovary also branches. The vas deferens and thick ovarian tube are between two limbs of the gut loop, opening with the anus near the base of the atrial aperture. Body muscles usually form an irregular network over the right side of the body and anterior to the gut on the left, although they are inconspicuous over the gut.

Occasionally the test is brittle with embedded sand, e.g. Ascidia thompsoni Kott, 1952 and Ascidia scaevola (Sluiter, 1904). In Ascidia scaevola, long stiff, cylindrical tubes project up from around each sessile aperture providing channels to and from the surface for the excurrent and incurrent streams of water of individuals submerged in bottom sediments-a habitat that appears to be available owing to the stiff sand-embedded test that forms the rigid tubes. Other aspects of the morphology of this species are associated with the rigid, brittle test which would prevent overall contraction of the body. Generally, the body wall is thin with little or no musculature. However, short strong muscle bands are present across the dorsum to close folds of test over the apertures, and others along the right side narrow and depress the body inside its rigid casing. In addition, an unusual fold of the pharyngeal wall compensates for the narrowing of the right side of the body which has drawn the dorsal mid-line onto the upper surface.

The unusual morphology of Ascidia scaevola is unique amongst the otherwise relatively uniform species of this family and demonstrates a dramatic response to environmental pressure that parallel adaptations in Plurella spp. which has similar thin, rigid, sandy test.

The family contains three closely related genera:

Ascidiella Mueller, 1776, which contains only one well known species (the type of the genus), is known mostly from Europe. However, it is now recorded from Australia and New Zealand. It may have been spread by ships (Kott 1985). It differs from the other genera in the absence of secondary papillae projecting into the pharynx from the internal longitudinal vessels.

Phallusia Savigny, 1816 species are large with thick, firm and translucent test. The ciliated pit, the primary opening of the neural duct (at the base of the branchial siphon), is one- to two-thirds of the body length distant from the neural gland (just anterior to the atrial siphon). This is an appreciable distance when individuals are up to 60 mm or more in length. The accessory openings of the neural duct into the atrial cavity (characteristic of this genus) may be associated with that long neural duct (see Ruppert 1990 for an account of the role of the neural gland in the regulation of blood volume). Although their species diversity is not high (only five species being known), at least one Phallusia species is relatively common around the Australian continent.

Ascidia Linnaeus, 1767, has 19 recorded species in Australian waters. It is possible that this number will increase when more reliable characters (possibly obvious only in living specimens) are demonstrated. Although some species become as large as Phallusia, others remain relatively small, often with a thin fragile test, especially on the left side of the body where they are often attached to the substrate. Accessory openings of the neural gland duct do not occur.
Kott (1985) reviewed the family in Australian waters.