Family HIANTOPORIDAE Gregory, 1893

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Family HIANTOPORIDAE Gregory, 1893

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

The family name is usually attributed to MacGillivray (1895), but Gordon (1984) noted that the name Hiantoporinae had already been used for this family grouping by Gregory (1893). The family includes two genera, Hiantopora MacGillivray (1887) and Tremopora Ortmann (1890). Both have very similar suites of characters, and Harmer (1926) regarded them as synonymous, but T.dendrocantha from Japan was figured by Ortmann with porous ovicells quite unlike those of Hiantopora. Hiantopora is represented by three Recent species, all from Australia, two of which have also been reported from the Tertiary of Victoria by MacGillivray (1895). The genus is characterised by its semi-encrusting habit, the colonies being anchored or supported above the substratum by numerous rhizoids, which originate from septular pores in the basal zooid walls, and may be partially calcified. The zooids are variously discrete, even within one colony. In some cases, each zooid is separated from its neighbours by calcified tubular extensions arising from septular pores, in the same manner as in colonies of Beania. In others, zooids are completely adherent laterally, the small lacunae among them being only visible on the basal side of the colony.

Autozooids have a smooth marginal gymnocyst and narrow cryptocyst surrounding an extensive frontal membrane and operculum. Unbranched oral spines are present, and at least one pair of disto-lateral spines occur. One of these has a greatly enlarged base, arising from the gymnocyst, and in contact with the visceral coelom. This spine enlarges further, and develops an avicularium on its distal face. On the proximal face the spine may bifurcate only once, or branch profusely, the branches fusing with each other and other spines to form a variously extensive frontal shield above the frontal membrane. Ovicells are hyperstomial, and may be prominent or immersed and hardly visible; the frontal has a large entooecial area. The avicularia are prominent, with a swollen subrostral chamber and acute mandible. The basal rhizoids may be very numerous, or virtually absent. When present they enable colonies to grow over otherwise unsuitable substrata, such as mud, as described by Hincks (1881) in H.radicifera.

The three species exhibit an increasing extent of fusion. of spinous processes above the frontal membrane forming a shield. In H.radicifera, the spines do not fuse and the membrane is exposed. Ovicells are large and prominent. In H.intermedia the spines cover much more of the membrane and the ovicells are immersed although the brooding zooids do not develop any spines at all. In H.ferox, the type species, the frontal shield is most extensive, with small foramina among the fused spine branches. The ovicells are immersed, but part of the entooecial area is visible among the spines. All three species were illustrated by Kirkpatrick (1890), and Harmer (1926) figured H.intermedia and H.radicifera, as did Wass (1994) in a series of detailed scanning electron micrographs. Wass & Yoo (1983) had already figured H.ferox in a similar manner.

All the species are widely distributed from shallow water to over 140m depth. H.ferox occurs from south of Perth, Western Australia throughout the Great Australian Bight and Bass Strait to New South Wales. H.radicifera has been reported from Perth to Bass Strait. H.intermedia occurs from Torres Strait and Western Australia, off Geraldton.

Hiantopora has been associated with both Membranipora and Beania in the past, but Gordon (1984) has suggested that its closest affinities are with the Chaperiidae.

Diagnosis

Colonies forming flat sheets growing over substrate surfaces, or as hollow curved sheets. All have numerous basal rhizoids arising from septular pores. Zooid arrangement varies from sheets of contiguous zooids to loosely joined zooids connected by calcareous tubes. Zooids deep, with practically no gymnocyst and no cryptocyst. Large oval area of frontal membrane is overarched by a complex branching spine system developed from one margin of the zooid. Usually a single lateral avicularium on each zooid, often forming part of the spine complex. Ovicells prominent to immersed, independent.