Red tide is the name commonly given to an algal bloom of "Karenia brevis", a microscopic marine dinoflagellate which is common in Gulf of Mexico waters. In high concentrations it discolors the water which often appears reddish-brown in color. It produces a toxin which paralyses the central nervous system of fish so they cannot breathe. Dead fish wash up on beaches around Texas and Florida. Humans can also become seriously ill from eating oysters and other shellfish contaminated with the red tide toxin.