Marine species have been distributed as hull fouling organisms since shipping began. Other accidental introductions occur due to translocation of aquaculture products, for example oysters and mussels, which usually harbour encrusting or boring polychaetes (and other marine taxa). The advent of large vessels which use ballast water tanks filled with seawater represent a new vector for marine larvae, and it is apparent that the rate of accidental marine introductions has accelerated in recent decades. (Australia and many other countries are now implementing protocols to minimise this risk.)
Although tides, currents and flotsam might be expected to result in wide distributions, most polychaete species have been found to have fairly discrete distributions (eg for well-studied families, a high proportion of southern Australian polychaetes are endemic). Species which appear to have almost "cosmopolitan" distributions are often found on closer examination to represent a suite of sibling species, the Australian specimens having been misidentified. "Cosmopolitan species" of polychaetes are thus probably mostly spurious and reflect people applying a foreign species name to similar species in other places. Nearly every polychaete family has a few of these - it is remotely possible that some truly are introductions, but recent revisionary studies in families such as Terebellidae, Spionidae and Nereididae have shown truly cosmopolitan species to be very uncommon.
It is against this background that potentially introduced polychaete species must be assessed - any "overseas" polychaete identified from Australian waters, particularly in the pre-1980 literature, should first be assumed to be a misidentification. However, some Australian records of polychaete species endemic to other regions have survived careful scrutiny and are classified as certain or probable introductions to Australia, and we have listed them here.
List of probable introductions to Australia of polychaete species.
