Family EUTHYRISELLIDAE Bassler, 1953

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Family EUTHYRISELLIDAE Bassler, 1953

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

The family Euthyrisellidae was introduced by Bassler (1953) for three, monospecific, purely Australian genera, Euthyrisella Bassler (1936), Pleurotoichus Levinsen (1909), and Neoeuthyris Bretnall (1921). The three type species had all been originally assigned to a genus with a preoccupied name, Euthyris. Bassler also included another Australian genus, Urceolipora, which is now the type genus of the family Urceoliporidae. Cook & Chimonides (1981) discussed the Euthyrisellidae, and also excluded Urceolipora, but included the genus Tropidozoum Harmer (1957), species of which occur from the Celebes (Sulawesi) and southeastern Africa.

The Euthyrisellidae have colonies in which all the calcified walls are interior, and are only in contact intermittently with an investing, extrazooidal cuticle. Neoeuthyris is encrusting; all the other genera are erect. The calcified walls of zooids contact the cuticle round the margin of the orifice and through small spinous processes frontally and basally. In erect colonies the extrazooidal hypostegal and basal coeloms are continuous round the margins of the branches, and the zooidal visceral coeloms communicate with them both, through septular pores or foramina in the frontal and basal wall respectively. In addition, the visceral coelom of each zooid is in direct contact, through septular pores, with the hypostegal coelom of the next distal zooid in a series. The hypostegal coelom also extends, at least partially, below the frontal calcified shield.

Neoeuthyris differs from the other genera in possessing adventitious oral avicularia, and large ovicells. The other genera have brooding zooids with enlarged orifices; but the embryos often fill the zooid cavity within an interior ovisac.

Colonies of Euthyrisella and Pleurotoichus are flustrine and flexible, growing up to 185 mm high, and branching dichotomously in one plane. Colonies of Tropidozoum are cellariform, a basal coelom occupying the concave surface of each internode. The marginal coeloms of the flustrine genera form tubular, strengthening structures similar to the marginal kenozooids of several unrelated genera with a similar colony form, including Onchoporella and Euthyroides.

The cuticle is ontogenetically thickened, and in Euthyrisella only, calcified spicules, which grow to form plates, extend from the marginal zooid wall into the marginal coelom. Similar plates occur in Onchoporella. The marginal tubes extend proximally to form rhizoids which anchor the colonies.

Euthyrisella obtecta has been reported from sandy sea-bottoms, at shelf depths in northwest Australia, Queensland and New South Wales. Pleurotoichus clathratus occurs in very shallow water, and under rock ledges from South Australia, New South Wales, and southern Queensland at least as far north as Heron Island (Hayward & Ryland 1995). Although the original record of Neoeuthyris woosteri was also from Queensland, all subsequent reports have been from Western Australia. Colonies encrust one surface of the fronds of the alga Metamastophora plana, covering them so closely that the alga is difficult to detect (Hastings, 1964).

Diagnosis

Colony encrusting or erect, weakly calcified, flexible and often flustrine. All calcified walls of zooids are interior walls, and extrazooidal coeloms are present basally or marginally. The frontal shield is lepralioid, and the hypostegal coelom extends partially below the calcification. Avicularia and ovicells are rare. Brooding zooids with enlarged orifices and interior ovisacs are usually present.