Family ELECTRIDAE D'Orbigny, 1851

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Family ELECTRIDAE D'Orbigny, 1851

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

The family name Electridae was introduced by Stach (1937) to replace the name Electrinidae (d'Orbigny 1952) for the genus Electra Lamouroux (1816). The principal characteristics of the genera included are the presence of a well developed proximal gymnocyst wall, expanding distally to surround a frontal membrane often bordered by spines. The cryptocyst is marginal, and avicularia and ovicells are absent; an intertentacular organ is developed, and the larvae, where known, are planktotrophic cyphonautes. Most species are encrusting, and many are associated with algal substrata. In Australia, the family is represented by five species of the widespread genus Electra, and by two exclusively indigenous genera, Mychoplectra and Bathypora. In the Antarctic, the family is represented by both Electra and the genus Harpecia Gordon (1982).

Colonies of the Electra pilosa species-complex are typically part of fouling faunas throughout the world. Gordon & Mawatari (1992) noted the occurrence on algae and ships' hulls, and Bock (1982) reported E. pilosa from southern Australia, mostly on brown algae. The gymnocyst has large pores; there are 7-12 marginal spines with cuticular tips, of which the most proximal is greatly enlarged, characteristics which occur in European E. pilosa (Prenant & Bobin 1966). These authors reported a range of tentacle numbers from 11-15, and the cyphonautes larva was described by Ryland & Hayward (1977). The species illustrated from Port Phillip, Victoria, by MacGillivray (1882) as Membranipora flagellum is certainly a form of Electra and may be part of the E. pilosa complex. Its zooids are more regular in size and encrust stems of the seagrass Amphibolis antarctica with a verticillate pattern. The proximal spine is even more developed than in 'typical' E.pilosa and is 1-2 mm long. Of the three other species of Electra found in Australia, one also has a world-wide distribution in warm waters. E. bellula forms delicate, semi-transparent strap-shaped encrustations on algae, although it is capable of erect growth. Unlike E. pilosa, the gymnocyst has no pores, but 1-3 fine spines. There are paired oral spines, and three proximal marginal spines which are variously branched and extend over the frontal membrane. In Australian specimens on red algae, these branched spines are profusely developed and completely hide the membrane. Slightly less branched spines were figured from specimens on Sargassum by Hayward & Ryland (1995). Hincks (1881) first described the variations in spines from Australian specimens. Cook (1985) noted that west African colonies had 10-11 tentacles and that the ancestrula of Indo-Pacific and Australian specimens were very large, thinly calcified and more than twice the size of subsequent primary autozooids. E. multispinata was first described on algae from Western Australia, as a form of Membranipora pilosa, by Hincks (1882). It differs in having numerous fine, marginal spines which cover the frontal membrane and interdigitate terminally. The gymnocyst has no pores but develops three large proximal spines. E. multispinata was later recorded by Whitelegge (1889) from New South Wales but has not been reported from Victoria. The remaining species, E. bengalensis (Stoliczka) was recorded from Australia for the first time by Hall (1984), and is common at Townsville, Queensland. This species also has spines arching over the frontal membrane, which may bifurcate terminally; in addition, there are two short bifurcate spines on the surface of the operculum.

The genus Bathypora was introduced by MacGillivray (1885) for a species which he had earlier described as Membranipora porcellana, from Portland, Victoria. M. porcellana is a junior synonym of M. nitens Hincks (1880), also from Australia. It is characterised by its oblong, Membranipora - like autozooids, and elongated gymnocysts. The proximal gymnocyst is smooth, and there is a tooth on the edge of the cryptocyst. Gordon & Parker (1991) referred Bathypora to the Electridae and noted the occurrence of a comb-like structure on the basal surface of the gymnocyst which they called a 'plectriform apparatus'.

A similar, paired structure is present in the other, purely Australian genus, Mychoplectra, but is much more developed. Mychoplectra was introduced by Gordon & Parker (1991) for Lepralia pocula Hutton (1878) from the shores of St. Vincents Gulf, South Australia. M. pocula is a senior synonym of Membranipora polita Hincks (1880), who also described it from Glenelg, South Australia. The species was recorded from southern Australia by Bock (1982) as Pyripora polita, and he remarked 'this species is mainly found in shallow water, with the colonies wrapping around the stems of the seagrass Amphibolis antarctica'. The second species of Mychoplectra, M. crassa, was first assigned to the ascophoran genus Hippothoa by MacGillivray (1869), who described it from Victoria, later (1885) assigning it to Pyripora. Both species 'occur along the coast of southern Australia' according to Gordon & Parker (1991), and both are characterised by autozooids with massive, porcellanous raised gymnocysts, which have lateral frontal pores but no other pores. Gordon & Parker (1991) suggested that the presence of some form of plectriform apparatus was characteristic of many members of the suborder Malacostegina.

Hayward (1995) recorded Electra longispina (Calvet) from South Georgia, and added records of Harpecia spinosissima (Calvet) from Adelie Land and the Ross Sea. This species had been reported by Livingstone (1928), and is 'perhaps quite widely distributed in Antarctic waters' on small algae, rocks and shells in shallow water habitats.

Diagnosis

Colonies encrusting, forming branching chains of discrete zooids, or irregular laminar expansions; often growing on algae or seagrass. Zooids often thinly calcified, less often heavily calcified, with extensive gymnocyst and a large opesia and frontal membrane. Intertentacular organ present. Flexible cuticular or partly calcified spinous processes may extend from the margin of the opesia; in some cases these are branched and spread over the frontal membrane. Larvae free-swimming, planktotrophic, of cyphonautes type. Ovicells and avicularia absent.