Family STEGINOPORELLIDAE Hincks, 1884

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Family STEGINOPORELLIDAE Hincks, 1884

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

The family Steginoporellidae was introduced by Hincks (1884) (as Steganoporellidae) for the genus Steginoporella Smitt (1873) alone. Later, other genera were included, which are now assigned to the families Onychocellidae and Thalamoporellidae. The genus Steginoporella had a wide distribution extending from the Eocene, which continues today, species occurring in shallow shelf waters to temperate waters throughout the world (Pouyet & David 1979; Cook 1964, 1985).

The first Australian species to be described were assigned to the genus Membranipora, and several were from the Victorian Tertiary deposits (MacGillivray 1895), the names being later given to Recent specimens from northern Australia (Harmer 1900, 1926).

The genus Steginoporella is characterised by large colonies, which may form sheets, or erect, anastomosing plates, more than 0.5 m. wide and high, which are often brightly coloured, red, orange or yellow (Bock 1982; Wass 1984; Cook 1985). Colonies are composed of two types of zooids, both of which possess functional tentacle crowns and viscera. Generally the A-zooids, which are the smaller and more frequent, are regarded as the equivalent of autozooids. The larger, less frequent B-zooids appear to be a specialized kind of avicularium. This view is supported by the occurrence, in only one species, of interzooidal avicularia without tentacles or viscera, in colonies where no B-zooids are present (Banta 1973). All zooids have large membranous frontals, and well-developed cryptocysts which are porous, and descend distally to meet the distal or basal walls, leaving paired opesiular indentations laterally, and forming a polypide tube centrally. The tube may have lateral expansions. The distal part of the opesia is rimmed by an oral shelf, onto which the large operculum fits when closed. The oral shelf is very wide in the B-zooids of some species, to accommodate the large, hooked cuticular teeth which occur on the submarginal sclerite of the operculum. (Harmer 1900, Cook 1964). The opercula are visible with the naked eye (Wass 1984). The tentacle crowns are large and bell-shaped when protruded; the tentacle number ranges from 24 to 28.

Brooding of the single, large embryo takes place in an interior ovisac which occupies the distal half of the zooid. In species where the cryptocyst is inserted high on the distal wall (eg. S. buskii), there is no room for development of the embryo. The calcification of the cryptocyst and polypide tube is progressively resorbed, and the embryo can be seen to occupy the larger part of the zooid when fully developed. The cryptocyst does not appear to recalcify (Cook 1964).

More than a dozen nominal species of Steginoporella have been recorded from Australia. Two of these, S. magnilabris and S. buskii, have a wide distribution in other seas, but others, like S. alveolata, S. haddoni and S. lateralis, are known only from the southwest Pacific. All these species have a northern Australian distribution, from the Northern Territory, Torres Strait or Great Barrier Reef regions (Harmer 1900, Ryland & Hayward 1992). S. truncata and S. tubulosa have a southern range, from Bass Strait and Tasmania. S. truncata, particularly, is widely distributed, and the large colonies 'develop in exposed locations on vertical walls and under overhangs' (Bock 1982). The colonies are probably the largest of local bryozoans, exceeding 0.5 m in diameter, and provide a habitat for a number of other organisms. S. greavesi Livingstone (1926) was reported from near Perth. lt has large, loosely encrusting colonies, and B-zooids are rare or absent.

Diagnosis

Colonies erect or encrusting. Zooids subrectangular, with proximal perforate cryptocyst descending at the sides, forming opesiular indentations. Cryptocyst extending centrally to form a polypide tube which may have a terminal expansion. Cuticular operculum reinforced with sclerites closing on a distal oral shelf. Zooids of two types, the 'B-type' being larger, considered analogous to an avicularium. Larval brooding internal; brooding zooids not differentiated.