and distributive shock. Allergic reactions can result from ingesting seafood, or by breathing in vapours from preparing or cooking seafood. The most severe allergic reaction is anaphylaxis, a medical emergency requiring immediate attention and is treated urgently with epinephrine. Biotoxins Some species of fish, notably the fugu pufferfish used for sushi, can result in serious food poisoning if not prepared properly. These fish always contain toxins as a natural defense against predators; it is not present due to environmental circumstances. Particularly, fugu has a lethal dose of tetrodotoxin in its internal organs and must be prepared by a licensed fugu chef who has passed the national examination in Japan. Ciguatera poisoning can occur from eating larger fish from warm tropical waters, such as sea bass, grouper, barracuda and red snapper. Scombroid poisoning can result from eating large oily fish which have sat around for too long before being refrigerated or frozen. This includes scombroids such as tuna and mackerel, but can also include non-scombroids such as mahi-mahi and amberjack. The poison is often odourless and tasteless. Many fish eat algae and other organisms that contain biotoxins, which are defensive substances against predators. Biotoxins accumulated in fish/shellfish include brevetoxins, okadaic acid, saxitoxins, ciguatoxin and domoic acid. Except for ciguatoxine, high levels of these toxins are only found in shellfish. Both domoic acid and ciguatoxine can be deadly to humans; the others will only cause diarrhea, dizziness and a (temporary) feeling of claustrophobia. Shellfish are filter feeders and, therefore, accumulate toxins produced by microscopic algae, such as dinoflagellates and diatoms, and cyanobacteria. There are four syndromes called shellfish poisoning which can result in humans, sea mammals and seabirds from the ingestion of toxic shellfish. These are primarily associated with bivalve molluscs, such as mussels, clams, oysters and scallops. Fish like anchovies can also concentrate toxins such as domoic acid. If suspected, medical attention should be sought. The toxins responsible for most shellfish and fish poisonings, including ciguatera and scombroid poisoning, are heat-resistant to the point where conventional cooking methods do not eliminate them. Mercury and other toxic metals Fish products, especially those from apex and higher-order consumers up the food chain, have been shown to contain varying amounts of heavy or toxic metals due to biomagnification. Toxicity is a function of solubility, and insoluble compounds often exhibit negligible toxicity. Organometallic forms such as dimethyl mercury and tetraethyl lead can be extremely toxic.