Family HIPPOTHOIDAE Busk, 1859

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Family HIPPOTHOIDAE Busk, 1859

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

The family Hippothoidae was introduced for Hippothoa by Busk (1859). It is a widespread and very distinctive group which is characterised by zooids with a gymnocystal frontal shield and associated compensation sac. The zooids lack a hypostegal coelom and are typically hyaline and colourless or opaque and white. As colonies are often small or runner-like, they are generally inconspicuous. Another distinctive feature is zooidal sexual polymorphism. Depending on the genus, zooids may be sterile, female and male. Some reduced, (or, rarely, enlarged) zooids may parallel avicularia. Certainly, adventitious avicularia are absent. The zooid orifice is distinctly sinusoid, and the operculum accordingly has a proximal tab or flap. Although tubercles may sometimes be associated with the orifice, basally jointed oral spines are lacking in all living species. Some Cretaceous species may have had them.

Ovicells are well developed, and female zooids frequently have enlarged orifices. Male zooids are small with diminutive orifices and reduced polypides. Currently, nearly 50 living species in 4 genera are recognised. These are Hippothoa, Celleporella, Haplota and Plesiothoa. Haplota is unknown from Australia and is unusual in producing erect, uniserial chains of zooids (Hayward & Ryland 1979). All the other genera are encrusting. Species have basal pore-chambers, by which zooids communicate with neighbours and from which new zooids are budded. Their shape, which is best seen in transmitted light in wet or slide-mounted portions of colony, varies among species. Those of Hippothoa and Plesiothoa are triangular and small or conical, and are accommodated within the lateral walls. Those of Celleporella tend to be tubular (Gordon & Hastings 1979). As hippothoids lack a hypostegal coelom, frontal budding and nodular growth would appear to be impossible. Some species of Celleporella have frontally-extended pore-chambers with a cuticular windows which can bud autozooids. Proximal budding is therefore possible and one Californian species is known to produce nodular and semi-arborescent colonies (Morris 1980).

The best-known hippothoid is Celleporella hyalina, a predominantly British and Western European algal encruster that has sporadically been reported from Australasia, including Port Phillip (Ryland & Gordon 1977; Morris 1980). This species, (and indeed others of the genus) is one of a growing number of bryozoans known to have placental brooding of the embryo in the ovicell, increasing in size from fertilised egg to larva. The position of the ovicell in the colony is variable in Celleporella: in most species the female zooid and ovicell occur regularly in the zooidal series and in the same zooidal layer. In C. hyalina and a few other species, male and female zooids are budded adventitiously from frontally uncalcified and extended pore-chambers and thus sit at a level above the feeding zooids. The polypides of the sexual polymorphs are reduced. Those of the male have a tiny lophophore, with four functional tentacles in C. tongima Ryland & Gordon (1977) from New Zealand, evidently for sperm release, but no digestive tract. Female zooids are able to undergo four successive reproductive cycles before senescence, and the egg increases 15-fold in volume after transfer to the ovicell. Details of reproduction, such as they are known, vary among species of Celleporella, and what obtains for C. hyalina is not the case for all the many species of the genus. In some species female zooids are almost as large as autozooids and replace them in a series; in others males are unknown (Hughes 1987; Hunter & Hughes 1993).

Differences occur not only in sexual polymorphism but also in the form of the ancestrula and the pattern of early astogeny. On the basis of these differences Moyano (1986) segregated Celleporella into four subgenera. Two of these occur in the Australian fauna: Celleporella sensu stricto has multiserial unilaminar or bilaminar colonies arising from an ancestrula with a sinusoid orifice, with a distolateral first zooid and spiral early astogeny, e.g., C. (C.) hyalina. Antarctothoa has multiserial unilaminar colonies with a similar ancestrula, but with two daughter zooids and radial astogeny. The five other Australian species of Celleporella probably all belong to this subgenus, viz C. (A.) aporosa, which was figured by Bock (1982), as Hippothoa, C. cancer, C. delta, C. pellucida and C. tuberculata. The female zooids are not adventitious as they are in C. hyalina. All these species encrust algae but C. (A.) pellucida also occurs on the dorsal surfaces of the erect Bugularia dissimilis. Hayward (1993; 1995) has described five species of Celleporella from Antarctica, three of them new.

Hippothoa, which has strictly uniserial colonies, with stolon-like prolongations, is represented by four species in Australia: H. calciophilia, H. distans, H. flagellum, and H. peristomata. Hippothoa distans, which has a tatiform ancestrula, occurs on algae and on shell, according to Bock (1982). The three other species, which in two cases have a kenozooidal ancestrula (unknown in H. peristomata), occur on shelly substrata. Male zooids have not been positively identified in Hippothoa; dwarf zooids (zooeciules) may occur, but their function is unknown. Specimens of H. distans, from Port Jackson, New South Wales, were reported by Hastings (1979) to have rare, enlarged zooids with huge mandible-like opercula and large muscles which she considered were vicarious avicularia. The New Zealand species H. calciophilia was described for the first time from the Great Barrier Reef by Ryland & Hayward (1992), as was H. flagellum by Hayward & Ryland (1995). Hayward (1995) also described H. flagellum from numerous Antarctic localities and remarked that it 'appears to be genuinely cosmopolitan in continental shelf seas'.

Plesiothoa, which uniquely among Cheilostomata has a stomach gizzard (Gordon 1975), has an oligoserial, ramifying mode of colony growth. Zooeciules, which appear to be avicularium-like, occur in some species, with their mandibles in reversed orientation. Although their function is unknown, it seems unlikely to be male, as abundant sperm is produced by the autozooids in at least one species. Colonies of Plesiothoa can superficially resemble some of Celleporella, but their female orifices are not dimorphic. The ancestrula has a smooth basal outline in some species, but is crenellated in others. One species, possibly P. trigemma, is known in Australia, and one, P. calculosa, in Antarctica (Hayward 1993, 1995).

The Hippothoidae is the earliest-occurring cheilostome family known to have had a compensation sac, in Dacryoporella typica and Boreasina nowicki, both from the Coniacian (Upper Cretaceous) ( Voigt 1991). Although adventitious sexual polymorphs would seem, to be a derived feature in Celleporella, they occur in the earliest-known species of the genus, Celleporella felderi from the uppermost Cretaceous (Maastrichtian) of Belgium, which has frontally extended pore-chambers and adventitious female zooids, although its early astogeny is unknown.

A range of fossil and Recent genera including Trypostega have been included in the Hippothoidae, but appear to be sufficiently distinct to justify defining a new family. The genus Trypostega was introduced by Levinsen (1902) for Lepralia venusta Norman (1864), from the Channel Islands. T. venusta forms a world-wide species-complex, the various populations of which have never been directly compared, but which share a suite of distinctive characters.

Trypostega has encrusting colonies of small zooids. The frontal shield is thin and gymnocystal, but perforated by numerous tiny pores, the orifice subterminal and sinuate. There are usually three kinds of zooids, the autozooids, the 'zooeciules' and the brooding zooid complexes. The zooeciules are small zooids, sometimes minute, but in other cases as much as half the size of an autozooid. The zooeciules are interzooidal, and often occur distally to many of the autozooids. Zooeciules have an operculum; their function is unknown, but those distal to a maternal zooid produce the large globular ovicell as a kenozooid by frontal budding (Cook, 1979).

T. venusta was reported from Victoria, as Gemellipora striatula, by MacGillivray (1887); from the Torres Strait, as Schizoporella, by Kirkpatrick (1890), and from the Great Barrier Reef by Hastings (1932). Harmer (1957) has reviewed the synonymy. It is considered probable that detailed study of the populations ascribed to this species will prove to be distinct.

Diagnosis

Colony thinly calcified, encrusting, uniserial and branched, or multiserial, occasionally erect. Zooids have a gymnocystal frontal and almost terminal orifice, a gizzard sometimes present. Complex pore-chambers with frontal windows often present, allowing growth of frontally-budded polymorphs. Colonies may include autozooids, small male zooids, dimorphic brooding zooids with large ovicells, minute zooeciules of unknown function, or rare vicarious avicularia.