Phylum Annelida
Class Polychaeta
Family Histriobdellidae
Common names: symbiotic or commensal polychaetes
Overview
Histriobdellidae is a small family of polychaetes that are ectosymbionts living commensally on marine and freshwater isopod and decapod crustaceans. The tiny (0.5�1.5 mm long), delicate body of histriobdellids is highly modified with little or no body segmentation or parapodia (paired unjointed lateral body outgrowths characteristic of polychates). They also have no setae (hairs). The head does possess a pair of short palps, and three short antennae, but there is no clear indication of prostomium (first segment) or peristomium (area around the mouth). However, members of the genus Stratiodrilus possess lateral appendages called cirri, and all histriobdellids possess a posterior pair of foot-like locomotory appendages that are used to crawl along. Histriobdellids have dark, chitinous, specialised mouthparts composed of numerous small jaw pieces. Their small size, transparent colouring and their symbiosis with crustaceans means these polychaetes are very rarely encountered.
Distribution and diversity
Histriobdellidae is a small family of three genera and 11 described species worldwide. Two Australian species are recognised from a single genus, Stratiodrilus tasmanicus (Tasmania) and S. navaehollandiae (eastern New South Wales), both of which are known only from type localities. However, given their small size and their specialised commensal habitat, histriobdellids may be more common than their current distribution records suggest. There are ten Stratiodrilus species described in total, with a Gondwanan distribution suggested for the genus as most live on freshwater crayfish in southern Australia, southern South America and Madagascar.
Life cycle
Little detail is known about the life cycle of histriobdellids. Sexes are separate and males possess a sclerotised spinous copulatory organ that is used to inject the sperm directly into the female body. Depending on the species, eggs are laid in clusters among the branchial filaments (gills), pleopods (swimming legs) or egg masses of their crustacean hosts. In Stratiodrilus eggs are typically laid amongst the branchial filaments. Development is direct, with no free-living larval stage, and miniatures of immature adults hatch from the eggs.
Feeding
Histriobdellids are omnivorous raptorial browsers that use their jaws to tear off and feed on the micro-organisms that grow on their host crustaceans.
Ecology
Histriobdellids are symbiotic occurring among the branchial filaments, pleopods or egg masses of their crustacean hosts with their precise location being species dependant. Stratiodrilus are associated with freshwater crayfish and Australian species are found the branchial chambers of the fresh-water crayfish Chaerops bicarinatus. The benefit received by the crayfish in the relationship is uncertain, but it is considered that histriobdellids receive a relatively stable environment in which to live and regular food source.
Stratiodrilus tasmanicus
Image credit: Illustration by Anna Murray.
From: Beesley, P.L., Ross, G.J.B. & Glasby, C.J. (eds). 2000.
Polychaetes & Allies: The Southern Synthesis. Fauna of Australia.
Vol. 4A. Polychaeta, Myzostomida, Pogonophora, Echiurida, Sipuncula.
Melbourne: CSIRO Publishing, p. 106.
� ABRS, used with permission.
