GONYAULAX

genus of dinoflagellates (single-celled aquatic organisms) that inhabit fresh, salt, or brackish water. Members are covered by closely fitting cellulose plates and have two flagella: one extends downward from a longitudinal groove in the armour, and the other, in an encircling groove, appears to keep the animal afloat. There is no eyespot (stigma), and the pigment-containing chromatophores are yellow to dark brown. Gonyaulax may colour the sea red during the day and make it luminescent at night. G. polyedra, a toxic planktonic form, is sometimes abundant enough to colour water and cause the phenomenon called red tide (q.v.), which may kill many fish and other animals. Human beings may be poisoned by eating mussels that have ingested large quantities of G. catenella.