Family ARACHNOPUSIIDAE Jullien, 1888

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Family ARACHNOPUSIIDAE Jullien, 1888

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

The family Arachnopusiidae was introduced by Jullien (1888, as Arachnopusidae) for Arachnopusia monoceros (Busk 1854), a magellanic species. Harmer (1957) attempted to modify the name to Arachnopodiidae, but authors have generally used the earlier form. The limits of the family have been expanded. It now includes Arachnopusia Jullien, with numerous species, all from cold-temperate to polar waters in the southern Hemisphere, together with Poricella (Tremogasterina auctt.), a pantropical to subtropical genus. Gordon (1989) also included eastern Pacific Anexechona Osburn, and two new genera from deep water off New Zealand, Brendella Gordon and Briarachnia Gordon (1984). He also included the Antarctic genus Trilaminopora Moyano.

The family thus includes genera with a wide range of morphologies; the only unifying feature being frontal shield with umbonuloid ontogeny and frontal foramina, together with the presence of oral spines and ovicells. The last two characters distinguish the Arachnopusiidae from the Exechonellidae. Some seven genera may belong to this family, whose boundaries are reasonably well circumscribed. The two commonest genera are the only arachnopusiid genera found in Australian waters, viz. Arachnopusia Jullien (Hayward & Thorpe 1988) and Poricella Canu (Cook 1977). Trilaminopora is known from the Antarctic only.

Species of Arachnopusia (A. acanthoceros, A. ajax, A. perforata, A. unicornis in Australia) have frontal perforations more or less evenly distributed in the frontal shield. Arachnopusia species characteristically have a long spine emerging from the orifice on one side, though there are exceptions. Curiously, A. acanthoceros and A. ajax have a long spine attached on or subjacent to the proximal rim of the orifice, described as 'chitinous' by Livingstone (1924). This feature, plus the straightness of the proximal rim and confining of adventitious avicularia to the zooidal margins suggests that these two species could be segregated as a separate genus. In Arachnopusia sensu stricto, an avicularium may sometimes replace the long spine.

Colonies can be encrusting and adherent to the substratum or raised above it by short basal calcareous stilt-like processes. Later development may result in a honeycomb-like growth of anastomosing, lobed and folded bilaminar sheets. Arachnopusia is characterised by a coarsely perforated umbonuloid frontal shield with a thin, cuticular epithelium above a hypostegal coelom and basal calcified exterior wall, overarching a membranous frontal wall with parietal muscles and a contiguous distal operculum. During ontogeny, marginal septular pores develop into areolae as calcification advances towards the zooidal centre. The associated hypostegal coelom spreads from the areolae and advances with the calcification. Each side of the shield may advance as a series of lobes (Hayward & Thorpe 1988), giving a superficial impression of costal processes like those of cribrilinids. The lobes leave uncalcified lacunae as they advance, which become the perforations. The perforations range from relatively small and numerous to quite large and few, and may be evenly distributed or confined to the centre of the shield. In one species, the shield is essentially lacking centrally, but traces of it along the lateral margins, plus other correlated characters, confirm its arachnopusiid affinities. The ancestrula is tatiform, with a circlet of spines.

The calcified orifice is quadrangular, with an umbonate proximal border which may bear avicularia. Characteristically but not exclusively, there is a distal pair of oral spines and one very large lateral spine, which remains prominent throughout ontogeny. The distal rim of the orifice is modified to form a descending lamina, the apertural plate, beneath the distal part of the operculum. This forms a concave shelf with a crenulate edge, and is a useful taxonomic feature. The ovicell, which can become partly concealed by secondary calcification, has a closing apparatus, like an operculum, separate from the zooidal operculum. Hayward & Thorpe (1988) remarked that 'the ovicell ... appears to be of a very unusual kind'. Both entooecium and ectooecium develop from septular pores on the proximal frontal shield of the zooid distal to the maternal zooid, which does not appear to contribute to the ovicell capsule in any way. In addition, the ovicell is closed by its own operculum. An aviculiferous ooecial cover, also derived from the distal zooid, immerses the ovicell in most species later in ontogeny.

Hayward & Thorpe (1988) reviewed previous records of magellanic Antarctic and subantarctic Arachnopusia and concluded that there were 'relatively few undescribed species'. They then illustrated and described eighteen species, eleven of which were new to science. Hayward (1995) included descriptions of ten species, of which most are from the Scotia Sea - South Atlantic region. Arachnopusia gigantea is recorded from the Australian Antarctic Territory, with records of A. decipiens and A. latiavicularis from the Ross Sea.

The characteristic features of species of Poricella (which have generally been described as Tremogasterina)(Tilbrooket al., 2001) are the small number of frontal foramina, which may even be occluded by later calcification, and the presence of oral spines. The foramina may become subdivided and have spinulate edges. The orifice and operculum may become much more discrete than in Arachnopusia, with the proximal peristomial rim describing a pseudosinus and pseudocondyles. Actual condyles may occur, however. A subopercular oral shelf is slight or absent. Since the operculum is capable of opening and the zooid feeding prior to completion of the frontal shield, the indentations and condyles presumably ensure that the operculum remains closely juxtaposed beneath the thickening calcification of the peristome. In some species a mucronate process develops proximal to the orifice, and buttresses may occur between the areolae. The avicularia are interzooidal and frequently large, with elongated mandibles. Brooding is in hyperstomial ovicells which are prominent at first and not closed by the zooidal operculum. Ovicells are similar to those of Arachnopusia and tend likewise to become concealed somewhat beneath secondary calcification. With the exception of the small number of frontal foramina, Poricella has several characters similar to those of Arachnopusia, and some species have been assigned both to Tremogasterina and to Arachnopusia in the past (Cook, 1977).

Poricella, like Exechonella, is an ancient genus with a world-wide range, probably extending from the Paleocene of Argentina and certainly from the Eocene of Tunisia to the Miocene of Europe meso-America and Australia. Some of the earliest species have several frontal foramina, thus resembling species of Exechonella. Recent species of Poricella occur in tropical to subtropical water of shelf depths, from all seas. Colonies may be encrusting, erect, or even bilaminar and branched. In Australia P. celleporoides forms large erect, unilaminar to tubular colonies originating from a base encrusting corals and shells. It was first collected from Torres Strait by the 'Challenger' and described by Busk in 1884. Its zooids have only one central foramen, and the avicularia are large and spathulate. P. celleporoides has also been reported from Western Australia, and from the Philippines from deeper water (Harmer 1957). A second species occurring from Australia, P. spathulata, greatly resembles some Tertiary European, and Recent West African species, having small zooids with numerous frontal foramina, and minute, truncate avicularia. It is most frequently found on coral rubble in very shallow water, and occurs from the Great Barrier Reef, particularly from Heron Island (Ryland & Hayward 1992). P. spathulata is very common on Mauritian reefs (Hayward 1988), and has a wide Indo-west Pacific distribution including the Philippines, the Red Sea (Cook 1977). and Vanuatu (Tilbrook et al. 2001). Like Anarthropora and Triporula, Poricella is present in the Australian Tertiary, and a species closely similar to T. spathulata occurs, albeit rarely, from the Miocene deposits of Bairnsdale, Victoria, and has only recently been described (Bock & Cook, 2000).

The genus Trilaminopora was included in the Arachnopusiidae by Gordon (1989) and by Hayward (1995). The umbonuloid shield develops as three plates which fuse incompletely, leaving a small, irregular central foramen, marked by suture lines. The marginal areolae are sunken and outline the zooids. There are no oral spines and the ovicells are immersed and imperforate. T. trinervis occurs from South Georgia and the Bellingshausen Sea to the Ross Sea and the coasts of the Australian Antarctic Territory. It frequently encrusts other bryozoans, and its deep brown colonies are conspicuous.

The Arachnopusiidae is the oldest-known family of umbonuloid-shielded Ascophorina. The earliest genus, Staurosteginopora, resembling Tremogasterina, occurs in the Santonian, a little before the middle Late Cretaceous. It evidently originated from cribriline ancestors that developed an umbonuloid shield over costal spines that subsequently became lost (Gordon & Voigt , 1996).

Diagnosis

Colony encrusting, with unilaminar and bilaminar expansions, fragile, but moderately well calcified, or erect, branched, well calcified. Zooids with umbonuloid frontal shield with uncalcified foramina, sometimes extensive or reduced to a single pore. Primary orifice uncalcified, calcified orifice sometimes with a large, unilateral spine. Other spines and sessile, adventitious, interzooidal and vicarious avicularia common. Ovicells hyperstomial, with an endooecial frontal area.