Family INVERSIULIDAE Vigneaux, 1949

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Family INVERSIULIDAE Vigneaux, 1949

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

The small, but distinctive family Inversiulidae was introduced by Vigneaux (1949) for the then monospecific genus Inversiula Jullien (1888). Ironically, Jullien introduced the genus not on the basis of the reversed orientation of the operculum, which he failed to notice, but on the orientation of the ascopore in I. nutrix. The family name was introduced independently for the same genus and species by Harmer (1957). Inversiula seems to be entirely limited to the southern Hemisphere and is unique among cheilostomes in having a zooidal operculum which is hinged along the distal edge and opens in the reverse of the normal direction. Consequently the near-semicircular orifice has a straight distal rim. The frontal shield is thickly calcified and densely perforated, and there is a prominent suboral ascopore. Other characteristic features of the family include paired oral avicularia, which may be raised, and which have small mandibles; as well as numerous buttressed pore-chambers and a complete lack of articulated spines and of ovicells. Brooding in interior ovisacs is known in I. nutrix; and in I. fertilis, the brooding zooids are enlarged and may be regarded as gonozooids.

I. nutrix ranges from Tierra del Fuego to the Antarctic Peninsula, and has been reported from Adelie Land; I. patagonica (Hayward & Ryland 1991) is known from the Patagonian Shelf, I. fertilis Powell (1967) from New Zealand; and I. inversa (Waters 1887) is distributed from New South Wales to the Torres Strait, and northwards to the Philippines (Harmer 1957).

Inversiula colonies develop as thick unilaminar to multilaminar sheets, usually encrusting hard substrata. Autozooids are thickly calcified; the frontal shield is typically ridged or nodular, with closely spaced pores. In the Antarctic, I. nutrix the pores are all equally spaced and more or less regularly distributed; the subtropical to tropical species, I. inversa, has round, cribriform pores frontally, and deep areolar pores marginally. In all species the suboral ascopore is conspicuous, as are the paired avicularia, which are lateral and suboral, with stout, cylindrical cystids and minute, terminal mandibles. Nothing is known about the polypide, reproduction, or ecology of the Australian species.

The earliest records of the genus are those of I. airensis and I. quadricornis from the Late Eocene (or Early Oligocene) of Victoria (Maplestone 1911).

The presence of an ascopore in Inversiula has meant that the family has often been associated with the Microporellidae. The frontal shield and the gonozooid of I. fertilis suggest a closer relationship with the Adeonidae (Powell, 1967). Within the genus, however, there is considerable uncertainty as to species relationships and even if they are all congeneric. The Patagonian type species has a lepralioid frontal shield, wheras the Australasian species are umbonuloid-shielded.

Diagnosis

Colony forming, multilaminar sheet, thickly calcified. Frontal shield umbonuloid or lepralioid, with marginal pores and a central ascopore. Operculum hinged on distal margin. Avicularia adventitious and oral. Calcified brooding zooids dimorphic, interior ovisacs known in some species.