Besides the two major groups (diatoms and dinoflagellates), 18 species were recorded to cause red tide in Hong Kong. These species belong to nine classes: Cyanophyceae, Raphidophyceae, Dictyochophyceae, Haptophyceae, Cryptophyceae, Euglenophyceae, Prasinophyceae, Chlorophyceae and Ciliatea.