Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Annelida
Class Polychaeta
Family Parerogodrilidae

Common names: terrestrial polychaetes

Overview

Parergodrilidae are a small family of minute, grub-shaped polychaetes that mostly inhabit damp terrestrial habitats. They lack eyes, head appendages and parapodial lobes (paired unjointed lateral body outgrowths characteristic of polychates). The prostomium (first body segment) is rounded and conspicuous. The body has between eight and 13 segments, all of which lack parapodia. Numerous sensory papillae are scattered across the strong body cuticle. Setae (hairs) occur ventrolaterally in bundles of two to eight and may be smooth and slender (capillaries), spine-like or forked. Parergodrilids are tiny worms only reaching 1-2.5 mm in length. They are very uncharacteristic polychaetes and their phylogenetic placement within Polychaeta is uncertain; they are currently placed as provisional Polychaeta incertae sedis.

Distribution and diversity

Worldwide, only two species of parergodrilids are known, each from a single genus. Parergodrilus heideri occurs throughout Europe in wet leaf litter. Stygocapitella subterranea is a widespread species reported from beaches and caves in the North-East Pacific, North Atlantic, North Sea, Mediterranean, Black Sea and in Western Australia (Augusta and Margaret River regions) and from coarse sand beaches in New Zealand. There is molecular evidence that this cosmopolitan species is actually a cryptic species complex containing several species.

Life cycle

Sexes are separate and reproduction generally occurs throughout the year. After sperm transfer from the male, females produce transparent, leathery egg capsules that are deposited between sand grains. Each egg capsule contains a single fertilised egg. Development is direct and larval stages occur within the egg.

Feeding

Little is known about the feeding biology of parergodrilids, but it is assumed they are detritus feeders that use their eversible tongue-like organ to graze food particles from the substrate.

Ecology

Parergodrilidae are unusual in being semi-terrestrial or terrestrial polychates. The cosmopolitan Stygocapitella subterranea is interstitial in sand from caves, and at or above the high-tide line on beaches. The European Parergodrilus heideri occurs in wet, decomposing leaf litter and rotten stumps, and less frequently in and beside freshwater streams and pools.