Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Annelida
Class Polychaeta
Family Nerillidae

Common names: Nerillids

Overview

Nerillidae are a small family of minute, mostly marine polychaetes. They are grub-shaped with seven to 14 segments. The prostomium (first segment) is fused to the peristomium (area around the mouth) and bears three slender smooth or articulated antennae and a pair of clavate palps (however, both antennae and palps may be absent). Typically, all segments bear setae (hairs), but may be absent from the first segment. Setae may be compound (tuft-like) or single and smooth (capillaries). Nerillids are transparent when alive, with a coloured gut and bright eye spots. They are tiny worms, measuring about 0.3-2 mm in length and move by creeping or swimming. The phylogenetic placement within the polychaetes of Nerillidae is unclear; they are currently placed as provisional Polychaeta incertae sedis.

Distribution and diversity

Nerillidae is almost totally a marine family with no freshwater Australasian species described. Just over 40 species of Nerillidae are known worldwide with only one described freshwater species: Troglochaetus beranecki, a widespread and common species in groundwater and caves across North America and Europe. Nerilla australis is the only published record from Australia and is known only from its type locality in coastal salt marshes in Western Port in Victoria. A second undescribed species of Nerilla has been collected from anchialine caves on Christmas Island. It considered likely that, with further work, more Australian species will be discovered.

Life cycle

Species of Nerillidae may either have separate sexes or be protandrous hermaphrodites, first functioning as a male producing sperm and then as a female for egg production. Eggs are produced in small numbers and are fertilised externally during pseudocopulation or via spermatophores attached to the eggs or deposited in their vicinity. Eggs are attached to the substrate individually or in groups (in cocoons), or in a number of species, attached to the female�s body until juveniles reach an advanced developmental stage. Development of larvae is direct.

Feeding

Nerillidae are omnivorous, browsing on diatoms, algae and bacteria attached to sediment particles.

Ecology

The vast majority of Nerillidae occur in shallow coastal and intertidal habitats. They mostly inhabit the interstitial spaces of sediments such as sand, shell or gravel but have been recorded from the sediments of subterranean waters also. The undescribed Nerilla sp. from Christmas Island was collected from amongst tree roots in an anchialine cave (fresh groundwater overlying seawater with a subterranean connection to the sea).