Family HOLOZOIDAE

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Family HOLOZOIDAE

Compiler and date details

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

Introduction

Holozoidae Berrill, 1950 have zooids embedded in soft fleshy test (without sand or other inclusions). Colonies are either sessile cushions or sheets, or they have relatively thick fleshy stalks with rounded to long or fan-shaped heads containing the zooids. Branchial openings are 6-lobed. In certain genera, atrial openings are on 6-lobed siphons either to the exterior (Polydistoma Kott, 1990 and Sigillina Savigny, 1816), or into a cloacal cavity (Hypodistoma Tokioka, 1967). In other genera, the atrial openings are sessile and wide, with an anterior lip, and they expose a great part of the branchial sac directly to a common cloacal cavity. In this family, the gut loop is relatively short and the stomach is about half to two-thirds of the distance down the descending limb. The gonads are in the loop of the gut or project behind it into the top of the vascular stolon, or in a sac separated from the abdomen by a narrow constriction. They are smaller and more contained than those of Clavelinidae, testis follicles being fewer in number and often arranged in a circle, and the ovary small and sac-like. Fertilisation occurs in a brood pouch (formed by a loop of the oviduct) that projects out from the posterior end of the thorax. Body musculature is mostly longitudinal, the fine bands on the thorax continuing along each side of the abdomen and often onto a posterior abdominal vascular stolon (in Sigillina, Hypodistoma, Polydistoma and Hypsistozoa Brewin, 1953). Muscles are confined to the thorax in Neodistoma Kott, 1990, Distaplia Della Vale, 1881 and Sycozoa Lesson, 1832. Occasionally some fine circular bands are also in the thorax (in a few species of Sigillina). Often a large gastric vesicle is halfway along the gastric duct that extends between stomach and intestine.

Replication in Sycozoa and Distaplia is known to occur by strobilation of an isolated vegetative stolon (which contains vestiges of the left epicardial sac). In the other genera it may occur by strobilation of the posterior abdominal stolon (which also contains the epicardial sacs), although this has not been investigated specifically. The vegetative and vascular stolons are in the stalk or the base of the colony, where rows of developing vegetative zooids often are found.

Although diverse, this family is thought to be monophyletic because of the thoracic brood pouches, long vascular stolons, replication by division of a posterior abdominal stolon, relatively small zooids with short abdomina, and a limited number of rows of stigmata. The highly organised cloacal systems of Distaplia, Sycozoa and Neodistoma, and their zooids with musculature confined to the thorax, are different from the other genera in the family. There are, however, intermediate forms, such as Hypsistozoa which do have muscles on a vascular stolon like those of Sigillina although their zooids and larvae otherwise resemble those of Distaplia; and Polydistoma and Hypodistoma which have cloacal systems and short almost horizontal gut loops like Distaplia, but 6-lobed apertures and muscles on the vascular stolon like Sigillina. Further diversity in the zooids is displayed in Distaplia which has gonads either projecting posterior to the gut loop in a stalked pouch or enclosed in the gut loop.

Larvae are relatively large in all genera and display generic differences. Larvae of Sigillina and Hypodistoma are some of the largest known in the Ascidiacea with trunks up to 4.0 mm long and particularly large median adhesive organs. In Hypsistozoa, Distaplia and Sycozoa larvae are smaller, with triradially arranged adhesive organs and sometimes blastozooids developing from a stolon of the oozooid.

The family is well represented in Australian waters. Sigillina, Sycozoa and Distaplia in particular are well represented. Sigillina is a genus of the tropical to temperate Australian and African waters; Sycozoa is not known north of Japan but is common in the Antarctic and Subantarctic; Hypsistozoa is known only from temperate Australia and New Zealand; Hypodistoma is known only from South Africa and temperate and tropical Australian waters; and Polydistoma and Neodistoma are known only from southern Australia. Distaplia is the only genus with a cosmopolitan distribution.

The family was proposed first as a subfamily, Holozoinae, of the family Clavelinidae Forbes & Hanley, 1848. It was revised and elevated to family status by Kott (1990).