Key to Australian Freshwater and Terrestrial Invertebrates
Phylum Annelida
Class Polychaeta
Family Spionidae
Common names: freshwater polychates, mud worms, palm worms
Overview
Spionidae are a diverse family of mostly marine polychaetes with a handful of freshwater representatives. Their body is moderately long and worm-like with more than 15 segments with the first segment (prostomium) prolonged posteriorly. Eyes may be present or absent. Spionids are distinguished by a pair of highly mobile, grooved, tentacle-like feeding palps that arise from the side of the head (these are often lost in preserved specimens). A median antenna may also be present and most have gills. The parapodia are biramous (branched) with leaf-like notopodia (dorsal branches) and neuropodia (ventral branches). Setae (hairs) may be smooth and slender (capillaries), spine-like or hooked. In many spionids, all segments are similar but in polydorids (a group of distinctive genera), the fifth segment is modified and carries one or several additional types of setae including simple spines, stout brush-tipped and terminally cuspate spines, and more slender accessory setae. Adult size is up to 150 mm long and 10 mm diameter, but most species are 10-30 mm in length.
Distribution and diversity
Spionidae occur worldwide in most marine habitats (450+ species). Of the over 120 described species in the region, only five freshwater species from four genera are known from Australasian waters, mostly from coastal freshwater habitats with fluctuating salinities.
Life cycle
Asexual reproduction has been recorded in several spionid genera via autotomy in which both new head and tail regions may be regenerated from damaged body fragments. Spionids also reproduce sexually by either gametes being freely released into the water where fertilisation occurs or via copulation and internal fertilisation. Some produce egg capsules that are either directly released into the water and all development takes place in the plankton or are deposited in the sediment inside which the larvae develop, many of which consume unfertilised eggs are they grow before emerging into a planktonic stage.
Feeding
Spionids are selective detritus feeders that use their two long, grooved palps to locate prey that are lined with cilia to transport food particles to the mouth. In some species, the palps may also be used to catch plankton and drifting particles from the water.
Ecology
Spionidae are among the most ubiquitous of all polycaetes occurring in soft sediments throughout most marine habitats. Some spionids are burrowers in substrates such as limestone, dead coral and living mollusc shells. Others build permanent tubes in soft substrata or are free living in sand and mud. Only a few species of this large family occur in freshwater and are mostly found close to the sea in habitats with fluctuating salinities such as coastal rivers and lakes or lagoons and estuaries. Some marine species may also be found upstream in rivers in near freshwater conditions. Boccardia limnicola inhabits coastal freshwater lakes in southern Australia. Western Australian species have been found in fresh or brackish underground springs or limestone caves, along the seashore and in anchialine waters (those with a subterranean connection to the ocean).
