The strength in a limpet’s tooth

AS a boy, I often prised limpets with my penknife off the rock pools in West Cornwall, United Kingdom, to scrape out their ‘innards’ to use as fishing bait on my hooks. Over several years, I only managed to catch one sea bass.

Wherever we are on rocky seashores worldwide, limpets abound and their empty shells lie washed up on beaches or fossilised in many types of rock millions of years old.

Limpets are hardy creatures, living in the stormiest of marine environments on the inter-tidal zone of our shores. Their cone-like shell is structurally strong and resistant to predators, with their large surface area firmly connected to the rock face.

With their sucker-like disc-shaped feet, their tenacity to grip onto rocks is exceptional for they are sealed in their shells at low tide to prevent bodily moisture loss from the overhead sun.

At night limpets come alive, moving along rock surfaces whilst grazing on algae. They use their radula, extruded from their mouths, with their rasp-like teeth to scrape off the algae from the rock surfaces. With their heads protruding beyond their shells, they nibble more than a good meal.

Seeing limpets in rock pools at low tide, the seawater seems crystal clear with little evidence of algae growth other than that on a limpet’s shell. Limpets are like bulldozers in rasping algae away from rock surfaces.

When grazing and moving they emit a slimy substance, almost snail-like, marking their trails to guide them home to their original abode in order to settle on the same spot each time after they have gone in search of algae pastures. A homing instinct.

Even in resistant rocks one can observe ring-like gouges, which for the 17 years of their lifespan, limpets call home.

When next on the beach do examine limpet shells and, particularly, look at the edges of each shell. If the edges are relatively smooth then those limpet shells will tell you that the local geology is comprised of less resistant rocks.

If the shell-edges are heavily serrated then you can bet your bottom dollar that the local rocks are relatively hard. All limpets, like humans, wriggle down on the exact spot to rest peacefully and, in a limpet’s case, doing so provides a perfect seal to the rock.

Limpets belong to the family of gastropods but they are pretty exceptional in that they start life as males and can change to females as they age.

When females exude their eggs, males emit their sperm and the meeting place of the fertilisation process depends upon tidal and sea currents. Lunar phases no doubt attribute to the time when this all happens.

The new larvae start to feed on plankton and microscopic algae before settling in rocky pools with their shells evolving. It is well beyond my knowledge to identify the many types of limpet beyond two common species: the Common Limpet (Patella vulgata) and the China Limpet (Patella aspera). Both of these are found in the warm waters of South West England and in the rock pools of Damai and Santubong beaches.

Interestingly, in March 1987, the world’s first oil supertanker disaster occurred when the SS Torrey Canyon broke into two pieces as it hit the notorious Seven Stones reef, just off Land’s End in West Cornwall, spilling 120,000 tonnes of crude oil into the sea. Locally, and on the coasts of Brittany, France, this was an environmental catastrophe as the oily brown sludge invaded all coastal inlets, beaches and coves, coating rock and sand surfaces. I witnessed this first-hand.

A very heavy dosage of strong detergent was used to try to break up the oil slick but this merely compounded an already serious situation. On the sea surface the crude oil began to disintegrate but sank in huge globules to the seabed. Rock pools were contaminated and the limpets died. Within a year of this disaster, the rocky shorelines were plastered in slimy green algae for there were no limpet grazers.

In a recent conversation with a cousin of mine, who was a scuba diver at the time of the Torrey Canyon disaster, voluntarily spraying detergents from his boat onto the oil slick, he astutely commented, “It was surprising how quickly such a fragile marine ecosystem re-established itself, all within 12 years of that disaster, and limpets were again evident.”

It is the limpet Patella vulgata, which has astonished scientists and structural engineers worldwide through the investigations of Professor Asa Barber and two other renowned scientists in his team in the School of Engineering at Portsmouth University, UK.

Their joint paper, to be published next month in the Royal Society Journal, ‘Interface’, will reveal to the world some exceptional findings. Up until now we have regarded spider silk as having superlative strength to withstand windblasts and as the world’s strongest natural material.

Spider silk has been woven together with synthetic fibres to produce bullet-proof jackets and Kevlar helmets for military personnel. What is the limpet’s secret which has just been discovered?

A limpet’s tooth, whilst only a millimetre long, contains strands of a mineral called goethite. This iron-based mineral is found not only in igneous rock formations worldwide but also in the lateritic soils of tropical countries.

Goethite is not a new substance. It was discovered by a German mining geologist, Herr Lenz, in the Rhine-Palatinate in 1806. Lenz named it after Goethe, the German poet, author and politician.

It is a common product of the weathering of iron bearing rocks and frequently found in deposits at thermal springs, bogs, and in marine environments.

Interestingly the colour ochre is derived from this, and because of its strength, I am not at all surprised as to why so many ancient cave paintings still exist. Here, in my study, I have a chunk of tin ore extracted 50 years ago from a local tin mine in West Cornwall. It is only today that I have noticed goethite crystal embedded in its texture.

In laboratory conditions, Barber and his colleagues, using an atomic force microscope, applied a pulling force to the limpet teeth samples. The limpet teeth material averaged a strength of 5 Gigapascals, surprisingly five times greater than most spider silks could withstand, yet one Southeast Asian spider silk measured 4.5 Gigapascals.

We may well ask how this scientific knowledge can be applied to our everyday lives. The goethite fibrous strands embedded in the teeth of a limpet, subjected to such extreme forces, suggest that these fibres can be used in the production of the chasis of cars, to strengthen the hulls of boats and the fuselage of aircraft or even in dental fillings.

No matter, whatever the size or scale of structural engineering may be, we now know that a detailed study of limpet teeth has provided us with the vital knowledge that a product with goethite fibres encased can withstand very high forces and with very few flaws in it.

Barber has most pertinently stated, “Nature is an ideal source of inspiration for structures that have good mechanical properties and engineers are always on the lookout for ‘bioinspiration’ from nature to produce useful material.”

A mineral that was first discovered in 1806 has now excited engineers worldwide, thanks to recent experiments on limpets’ teeth.