First detected in the plankton in Australia in Tasmania in the Derwent River in 1980, Huon River in 1985/86 (Hallegraeff et al. 1988), Triabunna (east coast) in 1993, in Victoria (Lorne) in 1993 (Sonneman & Hill 1997), Port Lincoln SA in 1996, and Hawkesbury NSW in 1996 (Bolch & de Salas 2007). Sediment cyst studies have traced the organism back to 1971 in the Huon River, Tasmania (McMinn et al. 1997); 1975 in Port Lincoln, SA,(McMinn et al. 2001) and 1937 in New Zealand (Irwin et al. 2003). Molecular tracing (sediment ancient DNA) in Maria Island deep sediment cores is in progress (Armbrecht et al. 2021). Bolch and de Salas, 2007 detected a single nucleotide polymorphism among 63 rDNA-ITS sequences examined, separating G. catenatum strains into two rDNA-ITS ribotypes. Australian and New Zealand strains clustered together with a strain isolated from the Seto Inland Sea, Japan, (a confirmed source of woodchip carriers travelling to Tasmania), separate to strains from other global populations. A re-examination of population genetics using a larger number of strains and broader and more variable marker regions such as mSATs, restriction site–associated DNA markers (RADseq) would be advisable to fully resolve this question of source of introduction.

Gymnodinium catenatum is a toxic, bloom forming species of microalgae with a green-brown colour. It is usually seen in long, swimming chains of tiny cells, with up to 32 cells in a chain (occasionally 64). It is more difficult to identify when seen as solitary cells. The size of these cells ranges from 38-53 µm long and 33-45 µm wide. The cells are circular to squarish in shape, with many rounded organelles, a large central nucleus and horse-shoe shaped apical groove (Blackburn et al. 1989). Cysts of G. catenatum are brown, spherical and range in size from 45-50 µm in diameter with a distinctive microreticulate surface ornamentation (Anderson et al. 1988). G. catenatum is the only known unarmoured dinoflagellate that produces toxins responsible for PSP (Paralytic Shellfish Poisoning) (Oshima et al. 1987).