Family PETALOSTEGIDAE Gordon, 1984

Museums

Museum list

Regional Maps

Family PETALOSTEGIDAE Gordon, 1984

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

The family Petalostegidae, introduced by Gordon (1984), comprises two deep-sea genera that are characterised by erect, delicate, jointed colonies of feeding and kenozooids. In Petalostegus Levinsen (1909) the main axes of the colony are made up of feeding zooids and all branches end in kenozooids. These are spine-like or club-shaped, and the latter can be replaced by similar-shaped avicularia. In all species the autozooid consistently has five flattened, petal-shaped spines (costae) that make up the frontal shield. These only partially fuse where they abut, leaving small spaces between them. The orifice is semicircular in shape, with one or a pair of avicularia at the distolateral corners or the corners are merely pointed projections. Each zooid has a long tapering portion, and new zooids are budded either mid-distally and/or from a knee-like projection on one side of the zooid. The ovicelled zooid can be quite striking. It is relatively large and conspicuous, because the ovicell is at least the same size as the female chamber bearing it or it is larger, and sometimes with flaring processes. The number of petaloid costae making up the frontal shield of the female may be five or six, depending on the species, and the suboral pair of costae may have frontal projections. Specimens of Petalostegus bicornis have been collected from depths of 520 to 1000 m, off the east coast of southern Australia. This material is in the collection of the Museum of Victoria, but has not yet been documented.

In Chelidozoum Stach (1935) the colony axes are, by contrast, composed mostly of kenozooids, rarely with autozooids interpolated, with autozooids borne in series along the kenozooidal branches. Autozooids are mostly bilaterally symmetrical, although one of the distolateral avicularia or non-aviculiferous prolongations may be lacking. In contrast to Petalostegus, the frontal shield has a relatively small costal component, ranging from five (three large, two small) to three, to a vestigial suboral pair, depending on the species. Female zooids have been described in only one species - the frontal shield has the same number of costae as the autozooid, with small processes and an adjacent avicularium, and the ovicell is very similar to that of Petalostegus. Small avicularia may also be produced from the kenozooidal segments in this genus. Chelidozoum pararium is described from depths of 800 - 1000 metres off Point Hicks, Victoria (Gordon & d'Hondt 1991).

This family has an interesting history. Confined to the Indo-Pacific of the southern hemisphere, it was for a long time represented only by single species, Petalostegus bicornis Busk (1884), collected by the Challenger Expedition near Tahiti and classified either in the Alysidiidae or Bicellariellidae ('Anasca'). A second species was discovered off New Zealand (Powell 1967) and subsequent scanning electron-microscopic examination of additional material of putative P. bicornis led to the establishment of a new family of Buguloidea (Gordon 1984). Then, in the late 1980s, a significant collection of New Caledonian Bryozoa yielded five new species of Petalostegus and three species of a curious new form that was not immediately recognised. Subsequent investigation showed that these belonged to a little-known bryozoan genus, Chelidozoum, previously recorded from the Miocene of Victoria (MacGillivray 1895). At the same time, a new living species was recognised from off Victoria, bringing the number to four (Gordon & d'Hondt 1991). Interestingly, the earliest record of a species of Petalostegus is also from the Victorian Miocene. The finding of so many new species, and especially the recognition that Chelidozoum, with its extensive gymnocystal frontal shield, led to the realisation that the family has its closest relatives in the ascophorine superfamily Catenicelloidea.

Little is known of the ecology of petalostegids. The family ranges from 64-3548 m depth, and colonies may attach to small rock and shell fragments and even to foraminiferal ooze. Judging from the presence of bore-holes in ovicells, small unknown predators feed selectively on developing embryos.

Diagnosis

Colony erect, uniserial or branching, delicate. Zooids elongate, with long, tubular, proximal portions, the opesia distal. Frontal membrane overarched by flattened spines, which are intermittently fused. Avicularia sessile. Ovicell terminal hyperstomial on brooding zooid. Elongate and spine-like kenozooids occur, former with tiny opesiae.