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A Human Holding a Small Limpet

A Human Holding a Small Limpet

We live on the threshold of an era of stupendous nanomaterials! In the near future, molecules will be engineered to be harder than diamonds or stronger than steel…yet these miracle materials will also be workable and light.

Artist's Conception of a Space Elevator--one of the miracles which should be possible through nanotechnology

Artist’s Conception of a Space Elevator–one of the miracles which should be possible through nanotechnology

Well, at least that’s what they keep telling us. In practice our best nano-materials do not seem capable of besting nature in the truly important categories—like hardness, tensile strength, or elasticity (or, if our synthetic materials are superior, they prove difficult to build into structures which fully exploit their strengths). A case in point comes from the lowly yet resilient limpet. Limpets are marine gastropods (snails) which have shells without visible coils. Actually, the name “limpet” is an informal common name—scientists have a very different way of characterizing these mollusks.

Common limpets (Patella vulgata) adhering to tidal zone rocks

Common limpets (Patella vulgata) adhering to tidal zone rocks

Limpets cling tenaciously to rocks at the tidal line by means of a muscular foot designed to create suction. They also produce an adhesive mucus which helps the foot adhere to whatever surface the limpet wishes to cling to. They carefully scour their ocean rocks for nutritious algae with a radula—a tongue-like rasping organ covered with teeth. Limpets have been of note to humans principally as a metaphor for resilience…or as a nuisance. Yet scientists experimenting on a common limpet, Patella vulgata, found that the little snail’s teeth had greater tensile strength than spider silk. Indeed, limpet teeth are the strongest known material in the natural world and approach the tensile strength of our strongest carbon fibers. With these teeth the little snail can (and does!) chew through rocks.

A (horrifying) microscopic picture of limpet's teeth

A (horrifying) microscopic picture of limpet’s teeth

The secret to the limpet’s mighty teeth is a miracle of molecular design in its own right. The cutting portion of the teeth are composed of fibers of goethite (a sort of iron hydroxide named after the great German poet). These fibers are under 60 nanometers in diameter—a size which allows them to be tremendously strong. The teeth are technically a composite–since the tiny goethite fibers are held together by chitin, a natural polymer (which the exoskeletons of insects are made of).

This evil henchman was the best character.

This evil henchman was the best character.

Technically there are human-created carbon fibers stronger than the astonishing teeth of the limpet, but these fibers can only be utilized in certain configurations and fashions–so the limpets’ teeth are of very real practical interest to materials scientists. Engineers are already working on duplicating the little snail’s teeth for mining and cutting equipment…and for human dental uses. Perhaps we really could someday have some of the powers of Jaws, the lovable hulking henchman from seventies James Bond movies. With our synthetic chompers we could bite through rocks and steel cables. Uh, wouldn’t that be wonderful?

The Common Limpet (Patella vulgata)

Limpets are gastropods with simple cone-shaped shells.  The majority of limpets are herbivores which live by grazing up films of algae by means of radula—an organ which is a tongue covered in rows of teeth.  The most salient feature of limpets is their incredible tenacity. They cling to rocks and other haed surfaces with incredible tenacity—to such an extent that they have become synonymous with tenacity and obduracy.  Brute force will not compel a limpet to stop clamping to a favored surface and the animal will allow itself to be destroyed before giving up its grip.

There are a number of limpet-like gastropods but the true limpets are named Patellogastropoda.  The majority are tiny—less than 8 cm (3 inches) although a few larger species are known.  Some limpets have a “home scar” on their favorite rock—a niche which fits them carefully and where they are perfectly camouflaged.  Such limpets tend to be territorial and will fight for the grazing rights to algae near their scar.

Although limpets are most familiar to people at the tidal line (where rockbound limpets survive above the low tide line by adhering to rocks and tightly maintaining moisture) the creatures exist in numerous different marine ecosystems, including the depths of the sea, coral reefs, whale skeltons, seagrass forests, black smokers, and cold seeps.  The deepwater limpets are probably detritivores.  Limpets begin sexual life as males, but when they mature to a couple of years old they turn female. Young limpets undergo a brief larval stage before clamping down on a hard surface and stating to graze or forage.

A limpet shell pillbox

Marbled cone snail (Conus marmoreus) by shadowshador

In olden days, in Australia, young healthy beachgoers were sometimes found lying on the shore dead.  Their bodies gave no evidence of trauma, indeed they had not even gone into the water. Something just struck them down as they sauntered along the beach.  It was not until 1936 that the mysterious killer was finally revealed when a beachcomber picked up a colorful snail and began to scrape its shell with his knife.  The unlucky young man uttered a cry as the snail somehow pricked him.  He then fell down, went into a coma, and shortly died.  Because of witness testimony, coroners knew what to look for and they removed a tiny poisonous harpoon the size of a small hair from the victim’s hand. The culprit turned out to be a cone snail, one of a diverse group of deadly gastropod mollusks.

The Geographic Cone Snail (Conus geographus) shows its siphon and proboscis. This snail is also humorously called “the cigarette snail” since if one stings you, you allegedly have time for one cigarette before dying.

There are over 600 different species of snail within the genus Conus and they are all poisonous predatory hunters.  The smaller cone snails hunt tiny mollusks and worms but the larger snails feed on fish, which need to be quickly subdued (so that they do not injure the snail by thrashing about) and then consumed with equal dispatch so that other ocean creatures do not steal the meal.  In order to quickly dispatch their prey (and defend against larger predators), Cone snails have a sophisticated weapon–a modified radula tooth which directly injects potent venom by means of a tiny harpoon-like “dart.”  The snail finds prey by carefully testing/sniffing the water with a siphon.  It then stretches out a long flexible proboscis and fires the disposable hollow radula tooth (filled with venom) into the prey by means of a powerful muscle contraction.  Below is a shocking film which shows a cone snail killing and consuming a clown fish by such means.  It is not for the faint of heart!

Although cone snails are obviously alarming to divers and shell collectors (particularly in warm tropical reefs where the large poisonous specimens live), the potent cocktail of neurotoxins utilized by the creatures is of great interest to pharmaceutical researchers.  Since each species of cone snail has a very large number of different “conotoxins” in its poison, scientists have been struggling to catalog and understand the dangerous mixtures. These conotoxins are generally peptides which interfere with the ability of nerve cells to communicate with one another.  Not only might such chemicals provide the key to curing neurodegenerative diseases and brain cancers,  conotoxin research is now the most promising avenue towards effective medications to deal with certain sorts of chronic pain.

A lovely diagram of Conotoxin Peptides from “The Journal of Neuroscience”

Unfortunately all of this research has not provided any effective antitoxins for victims of cone snail stings.  If a person is fully darted by one of the large poisonous specimens, their best hope is to go on a ventilator until their body expunges all of the poison—an uncertain prospect at best.

A Tiny Sample of the Exquisite Variety of Cone Snail Shells (Photo by Pet/Wikimedia Commons)

Many cone snails have beautiful colorful shells marked with vivid abstract patterns.  Some of the most valuable shells ever came from cone snails–which continue to fascinate conchologists and shell collectors.  Even today divers and beach combers are sometimes overwhelmed by the beauty of cone snails and reach out to grab the lovely creatures.  Hopefully this article has convinced you that doing so is a very bad idea.

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