Index Home About Blog
Subject: Re: Red Tide
From: mgk@darwin.clas.Virginia.EDU (Mahlon G. Kelly)
Date: Sep 01 1995

> >What is a red tide?
> >
> >                   Trista Claxon
> Red tides are caused by outbreaks or blooms of dinoflagellates (Kingdom
> Protista, Division Pyrrhophyta), which color the sea red.  These algae can
> poison fish, mussles, clams, and anyone that eats them.  Gonyaulax excavata
> caused shellfish poisoning in Massachusetts.  Gonyaulax catenella has very
> powerful nerve toxins.  Any book describing algae will give you more
> information.
> John Browning
> Univ. of Maryland

Please be careful not to be TOO alarmist about the preceding
message. Dinoflagellates are found in an cubic millimeter of
seawater, anywhere in the world. There are only a very few
species that can cause toxic poisoning, and they are rare (and
the taxonomy of dinoflagellates is very confused). Apparently
all are in the genus Gonyaulax.

However, to produce a red tide, or fish kills, the population
density has to be huge. So large that it produces a color in
the water. Under those conditions fish may be killed, but for
reasons unknown. Some say a toxin, some say simple loss of
oxygen in the water.

Toxicity to humans is a different matter. You cannot be hurt by
swimming in a red tide, or even by drinking the water (except
for the damage done by a full glass of seawater). For human
harm the toxin must be concentrated. That normally occurs by
filter-feeders (usually bivalves) dining on the
dinoflagellates. The toxin then becomes concentrated in the
critters tissues, and only then is it VERY RARELY toxic to
humans. Yes, there have been deaths from shellfish poisoning.
Usually those are due to some sort of pathogenic bacteria the
bugs ate. But, yes, there have been deaths due to shellfish
poisoning associatied with dinoflagellate blooms.

However, one must be careful. Several years ago the Maine
seafood industry was severely hurt because of a single case of
dinoflagellate induced shellfish poisoning. Folks panicked.
They wouldn't eat fish from Maine, or Maine lobsters. They
didn't understand that the danger was from a very small
population of mussels in a single small bay. Lobsters, fish,
etc. CANNOT accumulate the toxin so as to hurt people.

Finally, red tides can be cause by other organisms. There is a
recurrent red tide in Lac de Tunis, in Tunisia, caused by red
sulphur bacteria. Simple red color of the water is NOT a reason
to suspect a dangerous red tide. Further the Fish and Wildlife
Service regularly monitors shellfish for those toxins. If you
feed your clam to a mouse and the mouse dies, leave the clam
alone. Otherwise, don't worry.
Associate Professor (Emeritus)
University of Virginia

Index Home About Blog