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There are about 300 species in the genus Conus. The shells of these animals are very beautiful, especially when the periostracum (outer covering) is removed. Collectors pay high prices for nice and rare specimens. They are common animals in the intertidal zone in the tropics, although some occur at large depths in the ocean. The deeper the animal lives, the less likely it is to have contact with humans. About 90% of the species occur in the Pacific ocean, 70% live in the Indian ocean and only 15% are found in the Atlantic ocean. Cone snails ("cone shells") occasionally provoke severe problems in divers. At least 18 species have been implicated in human envenomations. Conus textile and C. geographus have killed humans.
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During the day cones burrow in the sandy bottom. They emerge at night to feed. One can divide the species in three main groups, according to their feeding behaviour. Some are mainly fish-eating species (piscivorous), some are worm-eaters (vermivorous) and some eat other mollusks (molluscivorous). Species which are piscivorous need to have a very strong and fast-acting neurotoxic venom in order to be able to paralyse and eat their fast swimming prey. If the venom would be too slow in acting, the fish would swim out of reach of the slow moving mollusk. Cones that feed on fish or mollusks are the most dangerous.
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Cone snails have a venom gland which is surrounded by a hollow muscular pump. During the passage of the milky venom through the exit duct, the composition of the venom changes (it becomes clear). If this processing represents activation of toxins is not known yet. These predatory snails inject a rapid-acting venom by means of a unique apparatus. The animals have a special organ, a storage sack, where superbly crafted dartlike mini-harpoons (radular teeth) are synthetised. The sculpted barbed teeth are marvels of cellular engineering and design. They can become 10 mm long. Each species has its own characteristic shape of harpoon. A radular tooth is released from the sheath into the pharynx, where it is coated with venom. It is then transferred to the tip of the proboscis. The animal will quickly sting a nearby prey, and will wait for the effects of the venom.
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The venom is a complex mixture containing small peptides (12-20, max 35 amino acids). The small size promotes fast diffusion in the tissues and a quick onset of symptoms. The toxins are known as conotoxins. There are many different types of conotoxins (more than 100 have been identified). Several of these are used in research. All conotoxins seem to target the neuromuscular junction and ion-channels.
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Symptoms include local pain. Local ischemia is followed by cyanosis and numbness. Soon afterwards, paresthesias are noticed. They will spread and regional paralysis follows. Systemic paralysis is characterized by diplopia with blurred vision, dysphagia with absent gag reflex, weakness and areflexia (compare with the effects of curare). Paralysis of the diaphragm will lead to death. As treatment, the same pressure-immobilisation technique as for neurotoxic snakes is used. Soaking in hot water (45°C) can destroy heat-labile toxins. Artifical ventilation might be necessary. It is not clear if neostigmine is useful, but an edrophonium test makes sense. There is no antivenom. A sting wound has to be checked for a retained radular tooth (foreign body). If the patient survives, the symptoms resolve in 2-3 weeks.
