PRINCIPLES OF TOXICOLOGY

428 PROPERTIES AND EFFECTS OF NATURAL TOXINS AND VENOMS
dangerous to humans, and these mainly occur in the tropical Pacific. Inexperienced divers should avoid
handling cone shells.
The octopus envenomates its prey with a posterior salivary gland secretion. The only octopus that
is toxic to man is the tiny Australian blue-ringed octopus, which appeared in the James Bond movie
“Octopussy.” Bathers have been known to play with this pretty little animal, often found among beach
rocks, without realizing how dangerous it is! While all other octopus venoms contain protein toxins
that are not dangerous to humans, this species instead secretes tetrodotoxin, the same toxin used by
pufferfish.
The ability of bivalve molluscs to concentrate dangerous quantities of dinoflagellate toxins such
as saxitoxin and domoic acid has already been discussed above.
Coelenterate (Cnidarian) Venoms
Cnidaria is a more recent name for this phylum, which indicates that all species contain small stinging
capsules called cnidae (nematocysts). A wide variety of cnidae exist, even within a single animal. The
largest, most formidable cnidae, capable of discharging venom deep within the victim’s skin, are found
in the classes Scyphozoa (jellyfish) and Hydrozoa (man-o’-war, etc.), so it is not surprising that most
cnidarian human envenomations result from jellyfish (Figure 17.7) or Portuguese man-o’-war stings.
However, all species (10,000) belonging to this phylum are potentially toxic, if not venomous. The
world’s most dangerous species of jellyfish, Chironex fleckeri, is found along the Australian coast.
Swimmers have been know to collapse within seconds after multiple stings by this species, which
precludes swimming at certain times of the year. Barriers are used to keep these jellyfish out of
swimming areas, and lifeguards must undergo extensive training in order to assist the unfortunate
victims. Most other jellyfish can also cause very unpleasant stings, but these are rarely life-threatening.
The fire corals occurring in tropical waters, like the man-o’-war, are actually hydrozoans rather than
true corals. Their inflammatory sting is probably due to the presence of toxins similar to that of the
man-o’-war.
Nematocysts discharge when the nematocyte cell in which they are contained is mechanically and
chemically stimulated. The tubule within the nematocyst is explosively evaginated, causing a proteinaceous
venom to be injected into the skin of the victim. Only recently have a few of the major jellyfish
toxins been isolated, since they are large, unstable proteins that are difficult to purify. Most of the
limited data on these toxins suggest that they primarily act as pore-formers, causing the depolarization
of nerve, muscle, and inflammatory (basophil, etc.) cells.
Most symptoms observed in envenomated persons and experimental animals can be predicted
assuming massive release of numerous chemical mediators of inflammation and transient stimulation
of nerve terminals in various kinds of muscle including cardiac and vascular. While antihistamines
provide considerable relief for the purely inflammatory symptoms, they are not sufficient to counteract
all actions of the most active venoms, such as that of Chironex. Many treatments have been suggested
for limiting the further discharge of nematocysts on the victims skin, including alcohol, acetic acid,
and protease mixtures like meat tenderizer. Topically applied vinegar (acetic acid) is probably the best
common means of initial treatment. Development of a more rational therapy for these envenomations
awaits further analyses of the pharmacological actions of individual toxic components of jellyfish
venoms.
One of the most potent marine toxins, palytoxin, is found in zoanthids, which are small colonial
sea anemones found in tropical reefs. This toxin, which acts by converting the sodium-potassium pump
into an ion channel, actually is synthesized by a marine bacterium that lives in the zoanthid. Like
ciguatoxin, palytoxin occasionally causes human food-born intoxications because it can also be passed
up the food chain into edible fishes.
Sea anemones possess a variety of peptide and protein toxins that affect ion channels in electrically
excitable cells in a manner similar to scorpions. In fact, the anemone toxins bind to the same site on
sodium channels as the scorpion α-toxins, and slow down the process of sodium inactivation in
essentially the same fashion. Some anemones also contain smaller peptide toxins that selectively block