The family Tonnidae is represented in NSW by the genera Tonna and Eudolium. Species of Tonna are large, globular lightweight shells, mainly tropical in distribution, that live from the shallow subtidal down to several hundred metres. They live mainly in sandy areas, where they can burrow beneath the sand and leave just the tip of their siphon exposed. They feed on sea cucumbers (holothurians). The two large tuns that occur in NSW belong to this genus; they are taken by fishing trawlers, Tonna cerevisina quite commonly and T. tetracotula less so.
The genus Eudolium contains smaller shells, up to about 80 mm in length, that occur in deeper water than Tonna - specimens have been taken down to 1829 m. Two species occur in NSW; both are widely distributed, one occurring in the Atlantic Ocean as well as the Indo-West Pacific. They are both rare, most specimens available for study locally having been trawled during fish surveys by the fisheries research vessel "Kapala".
Although a relatively small family, the Tonnidae has not been revised for many years, and there are taxonomic problems with many species. Uncertainties of relationships are noted below for the two most common Tonna species found in NSW.
In addition to the illustrated species, the following tropical species reach into northern NSW, with specimens very rarely being recorded as far south as Sydney: