Cone Snails (Conus)

Of the phylum Mollusca and class Gastropoda, cone snails 15cm long are beautiful, seemingly innocent, yet potentially deadly animals. Found typically in shallow Indo-Pacific waters, most of these predatory marine snails have a highly developed venom apparatus. There are over 500 species, and 18 have been known to be dangerous to humans. The cone snails that eat fish (as opposed to other mollusks or worms) are most dangerous to people, as we are also vertebrates. Fortunately, the snails are nocturnal, burrowing in the sand and coral during the day and coming out at night to feed. The snail has four protrusions: a siphon for respiring, two eyestalks to sense prey, and a proboscis to inject venom. The toxins are produced in the venom duct, which can be over seven times the length of the snail itself.

The duct is attached to the venom bulb that contracts to push the venom through the harpoonlike ‘tooth’. These teeth that may reach up to 1cm in length are modified hollow radular teeth, made in the radular sac. The proboscis still attached to the snail by a thread impales the fish, immediately paralyzing it. The snail proceeds to retract the thread and engulf the prey through its radular opening into its stomach.

The active components of the venom are small peptides of 12-35 amino acids in length. Due to the high density of disulfide bonds, these conotoxins are highly constrained. Different toxins block different ion channels in the nervous system, stopping chemical signals from traveling. This halt in communication causes paralysis in the fish victims. Three paralytic toxins seem to be the main focus: alpha-, omega-, and mu-conotoxins. The A-superfamily, including the -conotoxins and the A-conotoxins, binds to and inhibits the nicotinic acetylcholine receptor. The O-superfamily, including -conotoxins, the -conotoxins, the O-conotoxins, and the -conotoxins, decimates the release of acetylcholine through the prevention of voltage activated entry of calcium into the nerve terminal. The M-superfamily (-conotoxins) directly inhibits muscle action by binding to the postsynaptic sodium channels. Cone snails are able to produce hundreds of conotoxins, creating a lethal combination of one of two kinds of paralysis: excitotoxic shock (all muscles contracted at the same time: rigid) or flaccid paralysis (no muscles contract: limp). The venom of these snails can cause a variety of symptoms on humans. Mild stings usually on the hand initially resemble a bee sting, followed by numbness around the sting. More serious stings can cause partial paralysis and respiratory and cardiac failure. Weakness, nausea, and loss of coordination, hearing, vision, and/or speech are also common. At least two species (Conus textile and Conus marmoreus) have been known to kill humans. Unfortunately there is no antitoxin for cone snail venom; researchers have, however, begun to find that the venom could be used for forms of painkillers such as morphine, but would not have the side effects of addiction.