Gonyaulax is a genus of Alveolata. They undergo rapid multiplication and they make the sea appear red

Red tide

Red tide is a common name for a phenomenon also known as an algal bloom , an event in which estuarine, marine, or fresh water algae accumulate rapidly in the water column and results in discoloration of the surface water. It is usually found in coastal areas...

. They produce a potent toxin known as Saxitoxin

Saxitoxin

Saxitoxin is a neurotoxin naturally produced by certain species of marine dinoflagellates and cyanobacteria Saxitoxin (STX) is a neurotoxin naturally produced by certain species of marine dinoflagellates (Alexandrium sp., Gymnodinium sp., Pyrodinium sp.) and cyanobacteria Saxitoxin (STX) is a...

 which affects nerve impulses and can lead to paralysis of muscles and may eventually end in asphyxiation.

Members are covered by closely fitting cellulose plates and have two flagella: one extends backward from a longitudinal groove in the armour, and the other, in an encircling groove, may help to keep the animal afloat. There is no eyespot (stigma), and the pigment-containing chromatophores are yellow to dark brown. G. catenella, a toxic planktonic form, is sometimes abundant enough to colour water and cause the phenomenon called red tide, which may kill many fish and other animals. Human beings may be poisoned by eating mussels that have ingested large quantities of G. catenella.

It includes the species Gonyaulax polyedra.