These are a group of microscopic planktonic plants, some of which are delicate and difficult to examine and preserve, but the armoured forms are protected by cellulose cases, often of a bizarre shape with curved projections like horns. Usually the cells are solitary, but occasionally chains similar to those of the diatoms are found. One species of dinoflagellate, Gymnodinium, is reddish in colour and extremely poisonous to other living organisms. The presence of enormous numbers of this species is responsible for the “red tides” in the Gulf of Mexico and other parts of the world where they cause enormous mortality of fish. Fortunately this does not occur in New Zealand. The red waters sometimes seen in Wellington harbour are formed by a small protozoan which, so far as we know, is harmless.

by Richard Morrison Cassie, M.SC.(N.Z.), D.SC.(AUCK.), Senior Lecturer in Zoology, University of Auckland.