The Deadly Beauty
by Chuck Louch, PTMSC Docent
Snails of the genus Conus are famous for their beautifully colored and elegantly shaped shells which can be found in gift shops wherever tourists abound. There are even cone shell clubs with web sites where enthusiasts can exchange information about their favorite finds and other sites where you can buy shells at prices ranging from cheap to frightful. In fact, cone shells have become a commodity and most people never stop to consider that they are produced by living animals.
Cone snails live in a variety of marine habitats including coral reefs, mangrove swamps, and mud flats. They are highly specialized predators, some species going after marine worms, others preferring mollusks, and still others angling for fishes. Since they are sluggish creatures they can't run down their prey as cheetahs do but must use more ingenious methods.
Their weapons of choice are harpoons derived from modified radular teeth. As you can see in the accompanying diagram, each harpoon consists of a hollow, chitinous tube with a sharply barbed tip. A number of these are stored in the dart sac (a) from which they move, one at a time, into the proboscis (d) and become attached to the salivary gland (b) which has been modified to produce a venomous stew for subduing their prey. When a prospective victim has been located with the aid of the siphon (c), which is armed with large numbers of chemosensory cells, the harpoon is fired into it and the venom injected into its tissues. This venom is not just one chemical compound but a complex mixture of neurotoxins that quickly paralyze the prey so that it can be pulled back to the snail by means of the attached filament and engulfed. In some cases several harpoons are shot into the prey one after the other, each one injecting a different mix of toxins.
Fishes, which are more active than mollusks and worms, obviously pose a special problem for the cone snails that prey on them. This has been solved by modifying the siphon into a lure. When the a fish approaches this lure the snail shoots a harpoon directly into its face thus paralyzing it in a position such that it can be easily swallowed.
Humans obviously do not constitute a food item for these snails but if one is roughly handled it may shoot a harpoon into its tormentor causing a variety of symptoms including local numbness, pain, paralysis, breathing problems, and death. Indeed, it is said that in the past, maidens on certain South Sea Islands who were disappointed in love, used to end it all by tucking a particularly lethal species of cone snail next to their skin under their lava lavas before retiring for the night. But I can't vouch for the truth of this romantic tale.
Investigators have shown that these toxins produce paralysis by blocking sodium and potassium ion channels in the membranes of nerve and muscle cells thus inactivating them. In addition, they block calcium ion channels thus preventing impulses from crossing synapses between nerve cells or between nerve and muscle fibers. Without getting technical about this, each toxin is a peptide molecule composed of between 12 and 30 amino acids and each one has a specific mode and site of action. The chemical simplicity of these toxins and the specificity of their action makes them attractive to medical researchers as candidates for helpful drugs with minimal side effects; painkillers for example. And since each species of snail has between 50 and 100 toxins in its noxious stew, and since there are about 500 known species of snail there are lots of candidates to investigate.
But this kind of research requires a great many snails, 10,000 in one study. So now cone snails are being collected in large numbers, not only for their ornamental but also for their medical value, and some people fear that this added pressure may constitute a danger for many species, particularly those that have very specific habitat requirements and limited distributions.
So cone snails have joined that large company of creatures whose adaptations for survival have been turned against them by the avarice of humans.
References:
Henry Fountain. Snails, Desired for Beauty and Venom, May be Threatened. New York Times Science Section. Oct. 28, 2003.
The Cone Shells (Conidae) Venom Apparatus. http://members.lycos.co.uk/Mollusks/Schnecken/meer/conotoxin.html.
Photos: Collection Giancarlo Paganelli. www.coneshell.net
Chivian, Roberts, & Bernstein. The Threat to Cone Snails. Science Vol. 302 17, Oct. 2003.