Family FLUSTRIDAE Fleming, 1828

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Family FLUSTRIDAE Fleming, 1828

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

The family Flustridae was introduced informally, as Flustrées, by Lamouroux (1821), further defined by Busk (1852) as Flustradae, and by Smitt (1867) as Flustridae. Levinsen (1909) gave a clear diagnosis, recognising six genera, and Silén (1941) reviewed the taxonomy of the family, stressing the importance of the brooding structures, and presenting a key to eight genera. Moyano (1972) introduced four additional genera, two of which are endemic to Antarctica. Most authorities continue to recognise the genera defined by Levinsen (1909) and Silén (1941). The latest review of the family is by Mawatari & Mawatari (1979).

The typically 'flustrine' colony is erect, usually bilaminar, with flexible, bifurcating, flattened fronds. Some genera may be wholly encrusting, or unilaminar as well as bilaminar, arising from an encrusting base. Fronds form clumps which may be more than 200 mm in height or width, and their edges are often strengthened by elongated kenozooids with thickened cuticle. Calcification is usually minimal, and autozooids are rectangular and membraniporine, with an extensive frontal membrane, little or no gymnocyst and a narrow cryptocyst. Marginal spines are often present, and may be branched or terminally flattened and bifurcating, extending across the frontal membrane but not fusing medially. Avicularia are usually present and are interzooidal or large enough to be vicarious and intercalated within zooidal rows, or at the bifurcation of rows. They are budded in regular patterns and often are associated with ovicells. The subrostral chamber often forms a distinct kenozooid. Brooding may be in interior ovisacs, or in reduced, vestigial ovicells formed by a small extension of distal wall. Some ovicells are distally extensive, but are always endoozooidal and closed by the operculum. They are not usually prominent but may bulge beneath the succeeding zooid or be covered by the subrostral kenozooid of an avicularium. Brooding zooids may occur in distinct astogenetic zones within fronds. Elongate kenozooids are also characteristic, forming the edges of the fronds and the axillary areas of branch dichotomies in most species; in others they may give rise to tubular rhizoids which anchor the colony.

The family is a large one, including more than a dozen nominal genera, some of which are commonly present in the shallow waters of most seas. Genera are usually distinguished by ovicell type and avicularian pattern. Harmer (1926) considered that colony form was an important generic character, and Gordon (1984) agreed that differences among genera were reflected in growth patterns. The Australian fauna may yet be found to include additional species described from the western Pacific by Silén (1941) and Mawatari and Mawatari (1979). At present it includes 11 species, several of which exhibit variations on the typical colony growth pattern. The genus Carbasea is represented in Australia by 4 species . Two further species are present in Antarctica. Carbasea was originally introduced by Gray (1848) for unilaminar species with neither ovicells nor avicularia. It has since been expanded to include some bilaminar species, although these have also been assigned to the genus Chartella, which is not recorded from Australia (Prenant & Bobin 1966; Ryland & Hayward 1977). Busk (1852) described Carbasea elegans and C. pisciformis from Tasmania, and these species were also recorded by MacGillivray (1880) from Victoria. C. cyathiformis MacGillivray is a junior synonym of C. indivisa (Busk 1852), originally recorded from New Zealand. In Australia, C. indivisa is usually associated with catenicellid bryozoans as a substratum (Stach 1938; Gordon 1986). Colonies may have short fronds, or be cup-shaped (the 'cyathiformis' growth form). C. indivisa is also unusual in its brooding. Instead of interior ovisacs, it develops several embryos at one time, each enclosed in an ovisac which is attached to the inner distal edge of the operculum, but is positioned outside the zooid (Stach 1938; Ström 1977). In the southern oceans Carbasea is represented by C. ovoidea (Busk 1879), and by C. desbruyeresi d'Hondt & Redier (1977), from Kerguelen. C. curva is an endemic Antarctic species which may possess a chemical defence system (Winston & Bernheimer 1986; Hayward 1995). Living colonies of C. ovoidea were illustrated by Winston (1990), who noted that they provided shelter for numerous invertebrates.

The genus Gregarinidra was introduced by Barroso (1948) for Membranipora gregaria, an entirely encrusting Mediterranean species. Three species are known from Australia. Autozooids usually have marginal spines, and there is an avicularium proximal to each autozooid; the small ovicells are immersed in the succeeding autozooid, or within a kenozooid surmounted by an avicularium. Two closely similar species from Victoria are principally encrusting. They were illustrated by MacGillivray (1886). G. serrata may have erect lobes (Gordon 1984), and G. inarmata has no avicularia. G. denticulata was described by Busk (1852) from Bass Strait, where it is common (Bock, 1982, who assigned it to Spiralaria). It has narrow fronds up to 50 mm in height. This species may also be assigned to the genus Hincksinoflustra Bobin & Prenant (1961; Gordon 1984, by implication). Specimens show considerable variation in zooid characters: it is possible that more than one species of similar colony form are represented. The occurrence of a novel alkaloid, hinckdentine-A, was first reported from this species by Blackman et al. (1987).

The genus Spiralaria was introduced for Spiralaria florea from Australia by Busk (1861) and redescribed from Victoria by MacGillivray (1880), who mentioned that it was 'not uncommon'. S. florea has colonies forming flexible tufts about 200 mm high. The branches are unilaminar and twisted into an Archimedes-like screw or spiral (Wass & Yoo 1983; Gordon 1984). The margins are formed by alternating kenozooids and large avicularia. A smaller avicularium occurs proximal to each autozooid; these have small marginal spines resembling those of G. denticulata. The complexities of the spiral branching pattern of Spiralaria have been analysed and described in detail by McKinney & Wass (1981) and by McKinney & Jackson (1989).

The genus Retiflustra also may have a complex spiral budding pattern and resulting colony structure. It was introduced by Levinsen (1909) for a species originally called Retepora cornea by Busk (1852), from Queensland. Realising that it did not belong to the genus 'Retepora' (Phidoloporidae), he later reassigned it to Carbasea, unnecessarily altering the specific name to 'cribriformis' in the 1852 Catalogue. The colony structure consists of a curved, unilaminar sheet of zooids, which form fenestrae by regular bifurcations and anastomoses of zooid series, the whole growing in spiral around a vertical axis (McKinney 1980; Arnold 1987). Unlike the long spiral fronds of Spiralaria florea, the vertical axis, and the subsequent spiral, is compressed in Retiflustra, so that the colony forms a low cone about 40 mm in diameter and 15 mm high, with the expanding growing edge forming one radius of the convex face. Stout rhizoids develop, each from a kenozooid occurring at the bifurcation of zooid series. Arnold (1987) observed colonies alive in situ and pointed out that they live, anchored by the rhizoids, with the top of the cone upwards, and the concave, zooidal face downward. Arnold also found that a second species, R. schoenaui, originally described by Levinsen (1909) from the China Sea, was also present in Queensland. It differs from S. cornea in possessing rare vicarious avicularia and vestigial ovicells, the embryos being brooded in interior ovisacs. A third species, R. reticulum, was first described by Hincks (1882) from Victoria, but also occurs from the East Indies and the Torres Strait, according to Harmer (1926). It does not have a conical colony. but its narrow fronds do form occasional fenestrae. Rhizoids are also produced at branch bifurcations, and large vicarious avicularia and ovicells are present (Harmer 1926). Colonies grow with the frontal surface facing towards the surface of the sea-floor, and may be found in areas of high wave or current activity. Retiflustra is associated with sandy and muddy conditions. Arnold (1987) described the behaviour of feeding lophophores of R. schoenaui and noted that the fenestrae acted as excurrent chimneys.

The genus Guilleia was introduced by d'Hondt & Redier (1977) for G. kerguelensis from Kerguelen. Colonies are flustrine and bilaminar, with large avicularia. Although prominent, the ovicells are actually immersed in a distal avicularian kenozooid (d'Hondt 1984). However, Hayward (1995) regards Guilleia as a junior synonym of Klugella.

In Antarctic and subantarctic waters, a large number of flustrine forms has been described (Kluge 1914; Moyano 1972). Nematoflustra Moyano was introduced for Flustra flagellata Waters (1904), a unilaminar species with an avicularium proximal to each autozooid, which has an elongated setiform mandible. Hayward (1995) has described three species of the genus Isosecuriflustra Liu & Hu (1991), and one species of Austroflustra Lopez Gappa (1982) from Antarctica. However, one group of flustrine species which also occurs in these waters differs fundamentally from the Flustridae. The group includes Klugeflustra Moyano (1972), introduced for F. vanhoffeni Kluge (1914) and F. drygalskii Kluge (1914). Both species have recently been redescribed by Rosso (1994) and Hayward (1995). These species have large, fenestrate, hyperstomial ovicells. The ancestrula of K. vanhoffeni is membraniporine, and the early colony growth is encrusting (Ristedt 1991). A similar genus Neoflustra Lopez Gappa (1982) has smooth hyperstomial ovicells and small adventitious avicularia on a distinct gymnocyst. The nature of the ovicells means that neither genus can be accommodated within the Flustridae, and they show interesting links with the 'flustrine' reproductive phase of the cribriomorph genus Corbulipora described by Bock & Cook (1994; 2001), which has almost identical ovicells and vicarious avicularia.

Diagnosis

Colony erect with broad or narrow flattened fronds arising from an encrusting base, or wholly encrusting. Fronds uni-or bilamellar. Zooids not heavily calcified, generally rectangular, with a wholly membranous frontal surface, or with a short gymnocyst. Spines present or absent. Avicularia vicarious or interzooidal or absent. Ovicell endozooidal or lacking.