Limpets- Hardened by nature

by Matthew Norton

Who doesn’t enjoy a sunny day by the seaside? The fresh sea breeze, the crisp water washing in from the ocean’s edge, the moist sand caressing the spaces in between your toes. True, the sand can be an absolute nightmare to get rid of afterwards, but in that moment there is a pure, unspoiled bliss about it. Depending where you are, the coastline can take many other forms, from pebble and shingle beaches to jagged rocky shores that hold a splattering of saltwater pools at low tide. Even in predominantly sandy shores, there are sometimes isolated chunks of rock. And where there is rock, there is a good chance that you will find one the best known animals of the intertidal world. Limpets.

There are many species of limpets, from the seaweed munching blue-rayed limpet to the air breathing pulmonate limpets. But around the United Kingdom, and much of northwest Europe, the most common species is the appropriately named ‘Common limpet’. It might seem pointless for me to state this fact, but there are a lot of species, from a lot of animal groups, who are supposedly common. Such as the ‘Common seal’, the ‘Common starfish’ and the ‘Common sea urchin’. But many of these names were assigned to their species decades, if not centuries ago. And as we know, a lot can change in that time, especially when the human race is involved. 

Common or not, these limpets are tougher and more resourceful than some might expect from a little sea snail. Like any other animal on the seashore, they have to contend with the dry air and scorching sunshine every time the tide goes out. When this happens, the aim of the game is to conserve and retain as much water as possible. The limpet strategy towards this end is to clamp themselves down to the rock with their strong, muscular foot, thereby cutting themselves off from the outside world until the sea returns.

For added security, limpets will also carve a ring into the rock that is a perfect fit for their shell, rough edges and all. This ring is called their ‘home scar’ because wherever they may venture when the tide is in, they will at least try to return to the same ring before the sea moves back again. Assuming nothing catastrophic happens, they can usually retrace their steps back by using a trail of mucus slime they leave in their wake. Some studies even suggest that limpets can think strategically and modify their feeding excursions based on certain environmental factors. For example, limpets have been observed to stray further from their home scars during spring tides, when the rising tide pushes further inland compared to the more modest neap tides. But this is just one example, foraging behaviour can also be affected by a limpet’s position on the shore, the kind of habitat they find themselves in and individual preferences. 

Finding food is relatively straightforward for a limpet, for the rock on which they live is usually covered in a film of algae and other microscopic organisms on which they gorge themselves. Their weapon of choice for extracting their food is a radula, a feeding organ that most molluscs possess in one form or another, which in limpets is basically a tongue with built in scraping teeth. What is particularly remarkable about a limpet’s radula is the tensile strength of these teeth, which even outperforms the incredibly durable silk threads produced by spiders. The secret appears to be goethite, an iron oxide compound usually found in rusting metal, but which limpets have moulded into tiny nanofibers that reinforce their teeth. We humans have done something similar with glass fibres, which we have used to reinforce concrete and plastic polymer mixes and to create materials with a greater tensile strength than steel. 

In conclusion, limpets are brilliant examples of how even the smaller animals in the sea can surprise us with their ingenuity. We humans can be prone to believing that we are the smartest animals on the planet by a huge margin. We have certainly compensated for our squishy bodies and lack of sharp claws with tools and machines that suit our needs. But in some cases, our designs and inventions bear striking similarities to what the natural world had already evolved millions upon millions of years ago. Back when humanity was little more than a speck in the future of a very distant ancestor. 

From a human perspective

The ocean is, and always has been, a major influence on our existence on this planet. Even something as intrinsically man made as modern warfare can include aspects that originated in the natural world. This feels a particularly relevant topic at the moment, given the ongoing war in Ukraine. While there won’t be any further references to that particular conflict in the following paragraphs, I would like to stress that this section is not intended to celebrate, or glorify violence and death on this scale, or any scale really. Instead, this is more of an acknowledgement of how the ocean can be a source of inspiration in every human endeavour. Including those we hope never to experience first hand. 

Where do limpets fit into this picture you may ask? They are one of the most straightforward examples of ocean inspired weaponry because of the limpet mine. First deployed during the second world war, these weapons are designed to be stuck to the hull of an enemy vessel using magnets instead of powerful sucking muscles. Typically, they are applied manually by divers and fitted with a delay timer to allow them to clear out before detonation. Assuming all goes to plan, the result would be catastrophic damage to its target.

Some sources might claim that limpet mines are primarily used to disable a vessel rather than outright sinking it. But that didn’t exactly pan out when the Rainbow Warrior, Greenpeace’s flagship vessel, was sunk in 1985. It was due to sail out to the Mururoa Atoll to protest against French nuclear tests in the region, but instead it was hit by two explosions to the stern while docked in Waitematā Harbour, Auckland, New Zealand. 

Since there was only a small engine and no explosive materials on board, it wouldn’t have taken much imagination to suspect foul play. Especially when divers investigated the remains of the Rainbow Warrior and recovered the remnants of limpet mines. Two French secret agents were soon caught attempting to leave the country and they ultimately plead guilty to manslaughter for the attack and were sent back to France in 1986 to serve their lengthy prison sentences. 

The French government dismissed them as ‘rogue agents’ in an attempt to distance themselves from the attack. But further evidence has emerged in the years since, especially from documents released in 2005, that proves it was all an official operation to deter the nuclear protests by force. In the end, they made considerable compensation payments to both New Zealand and Greenpeace, the latter of whom put the money towards building a new Rainbow Warrior, which was eventually launched in 1989. Even so, the country’s damaged reputation from this incident wasn’t entirely salvaged. The two implicated agents only served two years of their respective seven and ten year sentences. A move that prompted a b******ing from the United Nations and probably caused further strain between the two countries.

Moving away from limpet mines, there are other examples of marine animals inspiring  real life weapons of war. Excoet missiles, which first entered service in 1975 and are designed to fly towards its target at very low altitude, are based on, and named after the flying fish. Going back further in time, or to certain museums, you might find sawfish snouts repurposed into swords. Or turtle shells repurposed into shields.

But it doesn’t always have to be this way. There can be peaceful applications for nature’s inventions. Jellyfish for example, their slow and steady swimming style has been adapted for underwater drones, complete with hydraulic ‘tentacles’. More stable and energy efficient than other designs, these jellyfish drones could be used for monitoring the oceans, particularly the environments surrounding vulnerable habitats like coral reefs, or exploring shipwrecks and other submerged structures. They could still be used for military surveillance, but unlike a gun, mine, or missile, they can be used for so much more. 

And in the end, there are very few ideas or inventions that are inherently good or evil. It’s how they are applied that really matters. Take Darwin’s theory of evolution by natural selection. For decades now, it has formed the basis of how we understand the natural world and has been applied to our benefit, such as tackling the causes of antibiotic resistant bacteria. But throughout the first half of the twentieth century, it was also exploited and used to justify discrimination, forced sterilisation and genocide. Whatever ideas we come up with, inspired or not by the world around us, it is our responsibility to use them for the right reasons.

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