Poisonous or venomous?
Venomous fish cause illness or death by injecting a toxin. Poisonous fish have poisonous flesh and cause symptoms when they are eaten. There are species with a very unpleasant sting, like most Anemones and the Lionfish, and there are some whose venom can lead to death within a few minutes or hours (e. g. the Stone fish). Usually venom is delivered through a modified dorsal fin that contains a poison gland at the base. Some Rays have a sharp spine at the end of the tail and poison gland are spaced along the teeth of the spine. Most poisonous fish toxins are similar but vary in strength in different species. The most virulent toxin is the one of the puffer fish. It is particularly dangerous if the fish is eaten without the removal of the poison glands. Chefs in many countries are very responsible because death from puffer fish can result within several hours after a fish has been eaten in which the gland was improperly or not at all removed. The toxin, tetrodotoxin, is very strong. It acts on the nervous system causing a tingling feeling that spreads from the tongue and lips down the body. The touch sensation of the victim is reversed- hot feels cold and cold feels hot. A similar toxin is found in the South American Poison Arrow frog.

Ingested Toxins
Shellfish
Shellfish are sometimes contaminated by dinoflagellates, which produce the so-called saxotoxin. It is water-soluble and heat resistant, therefore it is no affected by steaming or cooking. It inhibits sodium channels of excitable membranes, thus blocking propagation of nerve and muscle action potentials.
The symptoms usually occur within 30 minutes and include parasthesias of the lips, tongue gums and face. The process proceeds down the body and may lead to paralysis and respiratory arrest. The gastrointestinal form may appear within hours or days after ingestion. It is characterized by nausea, vomiting, diarrhea and abdominal pain.
There is no specific treatment. If oral sensations are perceived, one should stop eating. Sometimes even respiratory support and monitoring may be needed.

Ciguatera
This is a form of food poisoning that sometimes occurs in certain coral reef fish. It is caused by a tiny organism (dinoflagellate) attached to algae growing usually on dead coral. Plant-eating fish eat it and in the end, it accumulates in large predatory fish such as mackerel, coral trout and cod. The toxin has no taste and odor and is not destroyed by cooking and freezing. That is why all reef fish weighting more than 10 kg should be treated with caution. The symptoms of this poisoning begin several hours after the fish has been eaten. They may include breathing difficulty requiring artificial respiration. If symptoms occur, vomiting should be induced.

Tetrodotoxin
Toad (pufferfish) contain this kind of toxin, which is also contained in the saliva of the blue-ringed octopus. These fish, though easily caught, must never be eaten. The toxin concentrates in the liver and gonads. It inhibits sodium transport, affects neuronal transmission in the central and peripheral nervous system and also cardiac nerve conduction and contraction.

The symptoms depend on the dose ingested- oral paresthesias (a pricking, tingling or creeping sensation), muscular fasciculations; a flaccid type of paralysis occurs. Treatment includes gastric lavage and respiratory support for 24 hours or more.