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The deadly, life-saving cone shells

Imagine being on the Great Barrier Reef. Fossicking in the shallows you find a large, beautifully coloured shell. You reach down to pick it up and are surprised to get a tiny prick on the finger. Then you die. The only upside is that you don't suffer very much.

Cone shells (genus Conus) are famous shells, for all sorts of reasons. Prized by shell-collectors for their often brilliantly and intricately coloured shells, cone shells need to be handled with extreme care as all species are toxic and some can kill a human.

There are around 600 species of cone shells, although extreme variation in shell colour and patterns has resulted in much splitting and the description of many colour variants as species. Most are tropical though a few extend to temperate coasts. Around 150 species are known from Australia. Shallow-water species are relatively well-known, but many new species are still being discovered and described from deeper waters.

Arguably the most famous cone shell, and one which occurs widely in northern Australian waters, is Conus geographus, the geographers cone. The shell has a pale violet or purple ground colour and an intricate pattern of fine brownish stripes like a crazy map. This is one of those species often mentioned as evidence of how dangerous Australia is. Stings from geographers cones have resulted in over 30 fatalities world-wide.

Conus geographus has potent, fast-acting toxins because it's a specialised predator of fish. This is tricky, because fish can swim and are fast, while cone snails cannot and are slow. The solution for Conus geographus and its relatives is a long, extensible, and fast harpoon-like dart and a suite of incredibly fast-acting toxins. A sting from Conus geographus can paralyze or kill a fish almost instantly, giving the snail plenty of time to consume it at leisure.

An encounter with a fish-eating cone shell usually ends badly for the fish. Image: U.S. National Institutes of Health

While all cone shells are toxic to some degree, only a few are fish-eaters and spectacularly toxic like C. geographus. Most feed on slower-moving prey like marine worms.

And as so often with biodiversity, toxicity brings opportunities for medicine. The saliva of Conus geographus contains literally hundreds of toxins, a rich trove for biodiscovery and the development of new pharmaceuticals. The most promising targets are small peptides, an area of biochemistry with great promise. Some of these are hundreds of times more powerful as painkillers than opioids but are not addictive. Others have potent neurological effects. One of the best-characterised, conantokin-G, shows great promise in the treatment of epilepsy and other brain-excitation disorders and conditions.

Luckily for this work, the taxonomy of Conus is fairly well-understood. The most toxic, fish-eating species appear to be closely related, comprising six species in the subgenus Gastridium. Three of these, including C. geographus, are well-known and well-studied, while three others are much more poorly known. Intriguingly, though, this paper suggests that the fish-eating habit may have evolved independently on two separate occasions in the evolution of cone shells. If this is correct, there may be a whole new treasure-trove of chemicals for us in the other species.

In the meantime, please don't pick up a cone shell. You may not live to regret it.

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