Family BONELLIIDAE Lacaze-Duthers, 1858

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Family BONELLIIDAE Lacaze-Duthers, 1858

Introduction

Bonelliids show pronounced sexual dimorphism. The females have the trunk cigar, ovoid to sausage-shaped, 7-150 mm long and usually light to dark green in colour. The proboscis is often bifid and capable of great extension (1.5 m in Bonellia viridis). There are one or two nephridia (gonoducts) with either basal (proximal) or terminal (distal) nephrostomes. Usually two ventral chaetae are present but, in some deep-sea species chaetae are absent. Anal chaetae are lacking. The two anal vesicles usually branch frequently. The vascular system is closed, with dorsal and neurointestinal vessels usually connected through capillaries or lacunae in the walls of the intestine.

The male usually lives in a nephridium of the female, but may also reside on the proboscis and body wall or in a specialised tube within the female. The male is usually very small, 1-6 mm long, but may be up to 20 mm in Metabonellia. The body is flat, planariform or nematoform, and lacks pigment and a proboscis; it has the alimentary canal, vascular, excretory and nervous systems much reduced, but the reproductive system well developed. The body surface is covered densely with multiciliate cells. Ventral chaetae may be present or absent. The male is dependent on the female for food and protection. The type genus Bonellia Rolando, 1821.

The two most commonly collected intertidal and subtidal bonelliids in Australia are Metabonellia haswelli (Johnston & Tiegs, 1920) and Pseudobonellia biuterina (Johnston & Tiegs, 1919). Both are green in colour and have a bifid proboscis capable of great extension. The two are readily distinguished on dissection, the former having a single nephridium and the latter, two nephridia; in P. biuterina the very small male is situated in a small tube between the nephridia.

Bonelliids are tube dwellers and in Europe B. viridis is reported to inhabit burrows with multiple exits in rocky substrata (Schembri & Jaccarini 1978). There does not appear to be any record of a bonelliid making its own tube, the burrows inhabited by B. viridis often being made by crabs of the genus Upogebia. In South Australia, M. haswelli is often collected by divers from under large boulders that rest on sandy sediments, so that when the boulders are moved the animals float away from their homes. Pseudobonellia biuterina has been collected from both coral and limestone formations.

Bonelliids have also been dredged from the ocean floor at great depths, for example, Vitjazema ultraabyssalis at 9700 m and Torbenwolffia qalatheae at 8300 m. Four deep-water species, Zenkevitchiola brevirostris, Sluiterina alba, Vitjazema ultraabyssalis and Protobonellia papillosa were collected off the south coast of Australia by Murina (1978). Descriptions of the animals were given in English by Edmonds (1987). Zenkevitch (1966) considered that the Bonelliidae are '… a characteristic [component] of the abyssal and ultra-abyssal fauna and reach to the greatest depths of the oceans'. He also suggested that the sexual dimorphism of the family and the parasitism of the male are adaptations to life at great depths in a way '… similar to the conditions observed in angler fishes of the order Lophiiformes'.

The determination of sex in developing B. viridis has been studied by zoologists for more than 100 years and still engages their attention. The green pigment bonellin is now known to have lethal effects on a number of organisms and biological processes.

The family Bonelliidae is large, comprising about 30 genera, and over 70 species. Dattagupta (1976) divided the family into a number of sub-families.