Family MEMBRANIPORIDAE Busk, 1854

Museums

Museum list

Regional Maps

Family MEMBRANIPORIDAE Busk, 1854

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

Busk (1884) introduced the family Membraniporidae for Membranipora and the now obsolete genus 'Lepralia'. Only one of the taxa he included in Membranipora, the type species, M. membranacea, is now assigned to that genus. The family includes four genera which have representatives in Australia, Membranipora, Biflustra, Jellyella, and Conopeum. All are characterised by large, simple autozooids with extensive opesiae. Ovicells are absent and all species produce quantities of small eggs which are shed into the water through an intertentacular organ and which develop into planktotrophic 'cyphonautes' larvae (Ryland & Hayward 1977). Autozooids are generally monomorphic, although they may have modified tentacle crowns, and minute kenozooids sometimes occur. Genera differ from those of the closely related family Electridae in the virtual absence of gymnocyst and large spines. There are a number of species which remain contentious in their family placement. Conopeum, for example, lacks the twinned ancestrula characteristic of other Membraniporidae, yet is apparently not closely related to Electra. Many species found in Australia have been assigned to either Membranipora or Biflustra, but are more likely to belong to distinct genera or families. As the skeletal characters of these forms have few distinctive sharacters, the better understanding of their relationships may require genetic analysis.

Species of Membranipora have twinned ancestrular complexes. Where known, the larvae are large, bivalved, and have an extensive free-living life of several months. Species of Membranipora are usually associated with algae. Although many belong to species-complexes which have not been fully investigated, some do seem to have a genuinely cosmopolitan, or at least very extensive distribution. A setigerous collar, similar to that found in the Ctenostomata, has been reported in one species of Membranipora by Banta et al. (1995). Large dimorphic zooids, interpreted as avicularia, have been illustrated in species variously assigned to Membranipora and Biflustra (Cook 1968; Bock 1982).

Membranipora membranacea occurs in southern Australia, forming, wide, lace-like expanses on Macrocystis (Bock 1982). In spite of its apparently simple structure, M. membranacea and other similar species, possess quite sophisticated incurrent and excurrent water patterns. This is achieved by developing groups of zooids with polymorphic tentacle crowns in regularly spaced patches across the colony (Cook & Chimonides 1980; Dakin 1987). Similar patterns have been described in M. villosa by Lidgard (1981). M. membranacea also develops groups of 'tower cells', which are polymorphic zooids with cylindrical expansions of frontal membrane up to 1 mm in height (Cook & Chimonides 1980).

The genus Biflustra has been discussed by Bishop (1987). Species resemble those of Membranipora closely, but usually have heavier calcification and may have an erect colony form. All the records of Australian species require examination, but three forms appear to be referable, at least provisionally, to Biflustra. B. savartii is a species-complex with a world-wide distribution. The Australian form has been figured, as Membranipora delicatula, by Hincks (1880), and as Biflustra delicatula by MacGillivray (1881). Colonies may be large, bilaminar and anastomosing, or encrusting. Autozooids usually have a distinct proximal denticle arising from the cryptocyst, although this is a variable feature (Cook 1968). B. savartii, as illustrated by MacGillivray (1891) appears to be a species of Antropora. Biflustra pura (Hincks 1880) was also described, as B. uncinata, by MacGillivray (1890) from Victoria. It has autozooids with a pair of spinous processes distally.

B. perfragilis was figured by MacGillivray (1881) from Bass Strait and Queensland, and was described from South Australia and Victoria by Bock (1982) as Membranipora perfragilis. The bilaminar, anastomosing plates of the yellow-orange colonies form large, fragile, honeycomb-like masses at least 150 mm in diameter. B. perfragilis was also recorded from Heard Island by Busk (1884), as Membranipora crassimarginata var. a erecta. Large, spatulate, polymorphic zooids, interpreted as avicularia, were figured by Bock (1982). This species is unusually rich in sulphur-containing organic compounds, and is foul-smelling when collected (Blackman et al. 1992). The area of Bass Strait where this bryozoan is encountered by scallop fishermen is colloquially known as 'shit ground'. Blackman et al. (1992) identified dimethyl disulphide, dimethyl sulphide, methanethiol, methanol, dichloromethane, and chloromethane in the extracts, and suggested that the compounds were produced by associated micro-organisms. A subsequent study by Blackman et al. (1993) identified and described two new sulphur-containing isoquinoline alkaloids from the same species.

The genus Jellyella was introduced by Taylor & Monks (1997) for Membranipora eburnea Hincks 1891, and M. tuberculata (Bosc 1802). J. eburnea, like Membranipora, has minimal calcification, forming the vertical walls only. These walls are produced as a series of short spike-like processes. The species is found in the tropical and subtropical Pacific and Indian oceans, encrusting floating objects, such as the gastropod Janthina, shells of the cephalopod Spirula, fragments of pumice and floating seeds (Taylor & Monks 1997). J. tuberculata has large, white gymnocystal tubercles and encrusts Sargassum, especially the 'floats', in all seas. It has recently been reported for the first time in Australia, from the Great Barrier Reef (Hayward & Ryland 1995).

Colonies of Conopeum are encrusting, often on rock and shell, and have a single ancestrula. They are frequently found in conditions of reduced or variable salinity. The most remarkable records from Australia are of C. aciculatum (MacGillivray 1891), originally described from New South Wales. In the Coorong Lagoon, South Australia, this species occurs in large masses in association with the tubes of serpulid worms. The masses may be up to 300 mm high and 400 mm wide, and extend over several hundred metres of rocky substratum along the eastern coast of the lagoon. The association has been estimated to have a 700-year-old history (Bone & Wass 1990). The bryozoan growth is seasonal, with colonisation on hard substrata taking place in spring, and with colony mortality during late summer due to the increasing salinity as a result of evaporative concentration in the lagoon (Sprigg & Bone 1993). The cyphonautes larvae tolerate the higher salinities and survive until conditions are again favourable for settlement. Sprigg & Bone (1993) also identified C. aciculatum in the hyposaline waters of Lake Clifton, Western Australia, growing among calcareous thrombolites.

The fossil record of the family appears extensive from the published identifications. However, there is considerable uncertainty over the relationship between the many species with an extensive opesia and no ovicells, which have been placed in Membranipora, Biflustra or Acanthodesia.

Diagnosis

Colonies encrusting or erect; unilaminar or bilaminar; weakly to well-calcified. Zooids with vertical and basal calcified walls, but virtually no frontal calcified wall: most of the frontal surface is occupied by frontal membrane. Intertentacular organ present. Spines or tubercles may be present at corners of zooid.Spines at the margin of the opesia in few species. Larvae not brooded; planktotrophic, of bivalved (cyphonautes) form where known. Ancestrulae generally twinned. Kenozooids may be present in a few species; modified zooids analogous to avicularia are rare.