Family PETRALIELLIDAE Harmer, 1957

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Family PETRALIELLIDAE Harmer, 1957

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

Harmer (1957) introduced the family Petraliellidae for four genera with pantropical and subtropical species, which had frequently been referred to 'Lepralia' or 'Mucronella' in the older literature. Harmer also revised some of the generic attributions of Stach (1936), who made a very detailed comparative study of the morphology of a large number of species, many occurring from Australia. Stach referred the species to six genera or subgenera of the family Petraliidae, but Harmer separated all but the genus Petralia from that family and reduced the number of Stach's genera he included in the Petraliellidae to four. The question of the separation of the Petraliellidae from the Petraliidae remains open; the unusual colony form of the genus Petralia is the main criteria for distinction of the two families.

The family is characterised by large unilaminar colonies which may be loosely encrusting or erect, and are often anchored or supported by extensive basal rhizoid systems. Colonies may also be recognised by the opaque red, purple or orange tissues covering the frontal calcification (Cook 1985). Genera tend to be distinguished by orificial characters, but the resultant groups are not always consistently correlated with other features (Cook & Chimonides 1981). Autozooids have a well-calcified, cryptocystidean frontal shield which has numerous pseudopores and some marginal septular pores. The orifice is large and rounded, sometimes with a border of oral spines, as well as a proximal mucrone. The edge of the primary orifice bears one or more spine-like processes, called lateral denticles. These may define lateral sinuses and proximal lyrulae (Harmer 1957). The tentacle crowns are large and bell-shaped (Cook, 1985).

Ovicells are typically large, hyperstomial and very prominent. The ectooecium is principally cuticular; the calcified entooecium is minutely porous and finely tuberculate (Cook & Chimonides 1981). Adventitious avicularia are usually present, often occurring in pairs associated with the orifice. Rhizoids arise from large septular pore complexes in the basal walls (Harmer 1957). Rhizoids develop rapidly and enable colonies to overgrow competitors, even sponges, which usually are inimical to bryozoans (Cook, 1985). They also attach colonies to algae and erect bryozoans, and anchor the small, lunulitiform colonies of the genus Riscodopa to particulate sea-bottoms (Cook & Chimonides 1981; Gordon 1989). The early astogeny is known for very few species. Two have a tatiform ancestrula which is not closely adherent to the substratum, but raised above it by cuticular expansions of the body wall (Cook & Bock, 2002?in press).

In Australian waters, the large, purple and orange, anastomosing tubular colonies of Hippopetraliella magna may reach a height of 170 mm and are often washed up on the Queensland shore (Hall 1984). Colonies are also the substratum and refuge for other animals, as illustrated by Marcus (1921) in colonies from Western Australia (Cook & Bock, 2002?). Hippopetraliella magna is the only species in which the ovicells are not prominent, and remain cuticular long after the surrounding autozooids have completed their ontogeny.

Several species occur on coral from the Great Barrier Reef including the large, loosely encrusting, dark brown colonies of Sinupetraliella litoralis (Ryland & Hayward 1992).

Riscodopa has only been reported from deep water, from New South Wales and New Zealand (Cook & Chimonides 1981; Gordon 1989), but has close relatives in the Victorian Tertiary deposits (Cook & Bock 2001?).

Numerous species of Mucropetraliella have been described from Victoria and South Australia (Stach 1936). One of the common species is M. ellerii, which has brilliant red, frilled colonies growing on Laminaria , the bryozoan Amathia and on rock surfaces in shallow waters. The similarly coloured nudibranch Madrella sanguinea is a common associate of M. ellerii and is believed to be a predator on the bryozoan (Klemke & Keough 1991).

Fossil petraliellids are known from the Tertiary of southern Australia. The earliest record for the family is from the Late Eocene of the Indian Ocean (Taylor 1993).

Diagnosis

Colony well calcified, often brilliantly pigmented, encrusting a wide variety of substrata, or erect, often tubular, even lunulitiform and free-living, anchored or supported by rhizoids arising from pore chambers in the basal walls. Frontal shield lepralioid with numerous pseudopores, orifice with denticles and sometimes a lyrula and suboral mucro. Avicularia numerous, adventitious, oral or sutural. Ovicells hyperstomial, prominent, finely porous.