Richard Robinson

Aliens on Earth

Hundreds of millions of years ago, mantis shrimps evolved technologies we’re still trying to copy.

Written by       Photographed by Richard Robinson

It’s one of the most impressive predators in the ocean, and kills in milliseconds. It can see things you can barely conceive of. It’s the length of your index finger, which, incidentally, it will leave shredded and bloody if you even think of touching it.

The first thing you should know, though, is that it’s not even a shrimp. It’s not a mantis, either—that part of its name comes from the shape of its prominent front clubs.

Rather, it’s a stomatopod—a creature that burst from the group known as malacostracans a hundred million years before nature knew how to make dinosaurs. When you stroll along almost any New Zealand beach or estuary, the sandhoppers fleeing your footfalls are malacostracans, and they’re a product of the Cambrian explosion—nature’s psychedelic period—which occurred approximately 541 million years ago. Cambrian crustaceans were delightfully hallucinogenic to the eye, but mostly went extinct. Mantis shrimps, on the other hand, proved both trippy and tenacious—so much so that some 450 species are still with us, in every marine habitat all over the world.

All known New Zealand mantis shrimps are spearers. Heterosquilla tricarinata, pictured, lives in deep burrows in estuaries all around the country, from which it ambushes soft-bodied prey—polychaete worms, true shrimps, baby fish. The shrimps’ earthworks likely play a vital role in keeping estuarine sediments aerated.

Mantis shrimps have put those many millennia to good use, perfecting a wide range of superpowers. Their bodies are made of advanced bioceramics and biopolymers. Some of them can boil seawater. Not only do they perceive spectra we’re blind to, but they interrogate what they see using the most complex sensors in the known bestiary. They can even detect cancerous tumours.

And the mantis shrimp is as much a delight to the eye as it is to the mind. If we do eventually discover alien life forms, they will need to be pretty spectacular to top this creature: its iridescent colours, its hexnocular vision, its lethal weapons.

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Mantis shrimps are either “smashers” or “spearers”. Spearers wield spiny appendages tipped with barbs. Smashers have dactyl clubs, a structure that looks like a fist.

Most mantis shrimps worldwide are spearers, and they are the stuff of nightmares for soft-bodied victims, such as real shrimps, small fish or worms. Their two outside arms form a pincer lined with sharp spines, and in this regard they resemble mantids. Also like the mantis, they are ambush hunters. The strike of a spearer happens at 10 metres per second, and it’s over within eight milliseconds.

Those spines inside the pincers are lined with arrays of barbs, which hold the prey secure while the mantis shrimp picks at it like you might nibble meat off a skewer.

And, just for extra awesomeness, some spearers can multitask. “I was working in the lab one day,” recalls Hayley Nessia, a PhD researcher at the University of Auckland, “and I heard a loud crack.” She turned to look at her captive group of Japanese mantis shrimps, nominally spearers, and saw that “a female had grabbed a wedge shell, and she was rearing up and striking it with her elbows. It was really loud.”

By comparison, smashers hunt hard-bodied prey, such as other crustaceans or bivalves. The hammers of a smasher are always cocked, ready to release at the muzzle velocity of a .22 bullet. It takes nanoseconds for the hammers to reach 80 kilometres per hour from a standing start, working against the resistance of the water, and they land with a blow of 1500 Newtons. How much force is that? Hold your hand out flat, palm up. Now, get a friend to drop 3000 Snickers bars on it. You just got smashed.

But that’s only half the hell a smasher can unleash. The mantis shrimp’s strike is one of the fastest known movements made by an animal—so fast, in fact, that it robs the ambient pressure from the surrounding seawater. This leaves a vacuum in which cavitation bubbles instantaneously form, then collapse, briefly heating the water to as much as 4700 degrees Celsius and triggering a shockwave. In California, a mantis shrimp used its one-two punch to shatter the double-layered safety glass of the aquarium it was in.

Mantis shrimps can rotate their eyes independently to precisely align them with the angle of polarised light. The upturned tail of a Japanese mantis shrimp reveals a beauty found in many of its kind. While mantis shrimps share a body plan with crabs, crayfish, real shrimps, and the slaters under your wood pile, they have taken it to a whole other level of sophistication.

Engineers are keen to know just how a mantis shrimp can do that without destroying its own clubs—and keep doing it, too. (A particularly determined mantis shrimp in one study struck a mollusc shell more than 460 times.) The answer is that the mantis shrimp uses a composite of advanced materials. Its dactyl clubs are made of mineralised chitin, which is much the same stuff you use to grow hair and fingernails—except the mantis shrimp’s bands of chitin are arranged differently according to structure. A hard, shatterproof coating forms the outside of the clubs; within are bundles of elastic, spiral-wound polysaccharide fibres that absorb the shock forces the clubs sustain.

The secret of the mantis shrimp’s lethal velocity lies in a saddle-shaped body part that behaves like a spring—effectively a trigger mechanism. The saddle must simultaneously compress on its top surface and expand along its bottom. The top of the saddle is bioceramic—brittle in tension, but resilient to the massive compression forces of a strike. And at the bottom, a stretchy organic biopolymer copes beautifully with the equal and opposite expansion happening below.

In short, if you’re a mantis shrimp’s prey, it matters not whether it’s a smasher or a spearer bearing down on you. Either way, the game is up.

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Japanese mantis shrimps—the kind Nessia saw improvising with a wedge shell—shouldn’t actually be here. (The clue is in the name.) More rightfully, they are found off the coasts of China and Japan, where they’re caught and marketed as shako.

Nessia thinks Japanese mantis shrimps probably turned up in our waters the same way most invasive marine organisms get here: as larvae in the ballast water of ships. They were first discovered in late 2009, when some got snagged in the nets of Kaipara flounder fishers Peter and Christine Yardley. A handful have also turned up in the Hokianga, and in the shallows of the Kaipara Harbour, where they burrow into wet sand and lie in wait for polychaete worms or snapping shrimps.

As Nessia’s captive female demonstrated, Japanese mantis shrimps are opportunistic hunters, preying on a wide variety of species.

That could be a problem in New Zealand. Wedge shells, Nessia points out, are important “bioturbators”—cyclers of marine sediments and nutrients. If mantis shrimps wipe out wedge shells, this could have wide-ranging effects on the ecosystem.

As part of her thesis, Nessia looked at the guts of snapper and rig from the Kaipara, and found them full of mantis shrimps, suggesting the fish are keeping the mantis shrimp invaders in check. Then, in 2018, she says the fish were eating Asian paddle crabs instead, “which might indicate the shrimp population crashed—but there are a lot of unknowns”.

Another unknown is whether the Japanese mantis shrimps might displace some of our own. New Zealand has 19 known native species of mantis shrimp, of which eight are endemic. There are probably many, many more out there—needles that haven’t yet been found in the haystack of our marine realm. Studies just two years apart more than doubled our known mantis shrimp biota.

Peter Yardley has fished the Kaipara Harbour with his wife Christine since 1973. In late 2009, they noticed a strange new creature in their flounder nets.

Nearly half are subtropical deepwater species, found along the Norfolk and Kermadec ridges down to Northland waters, which mark the southern limit of their preference. But the other half seem just fine with colder waters—three species have been found off the subantarctic Auckland Islands. You could come across one anywhere in the ocean—from the shallows down to 1500 metres—or even out of it, says Nessia. “I’ve found a native one digging just above the low-tide mark at Omaha Beach, north of Auckland. Look for the entrance to their burrow—it’s very small, but if you dig carefully you can get them out. It’s always worth looking in the gut of any snapper you catch, too.”

Nessia’s colleague Richard Taylor met his first live mantis shrimp in the mid-90s, while doing a PhD study on animals that live in the sand at Omaha. “I always thought they were really neat animals, and I’ve kept a few in tanks.” Now a senior lecturer at the University of Auckland, Taylor says our northern estuaries are pockmarked with the industry of one local species, Heterosquilla tricarinata. “They make a distinctive burrow. The entrance is flush with the surface, so there’s no little volcano of mud next to it. You get them in really high densities in places like the Whangateau estuary—there must be millions of them there. In the daytime, you’ll only generally see their head poking out of a burrow—they’ll retract as soon as you go near them.”

For a creature barely seven centimetres long, H. tricarinata is a formidable digger. “Some of those burrows go down half a metre,” says Taylor. Their earthworks, he explains, hint at their playing a pivotal role in soft-sediment ecosystems. “By digging down into what would otherwise be anaerobic sediment, they’ll be irrigating it with oxygen-rich water.”

In high numbers, Taylor says, H. tricarinata can be a commanding predator. “They must have a big impact on those prey populations, and be responsible for a lot of the energy flowing through those systems. There’s a good chance any soft-bodied invertebrates or small fish—baby flatfish or gobies—settling on the bottom will get nailed by mantis shrimps.”

Oratosquilla oratoria, the Japanese mantis shrimp, is a native of the South China Sea. The lethal tines of its claw ensure nothing gets a second chance; Australian fishers call them “thumb-splitters”.

All 19 species of native mantis shrimps are spearers, and they seem to get along with each other better than smashers do. That may be because spearers live in DIY burrows, so they don’t need to compete for space. “I kept ten spearers and gave them little PVC pipes to live in, and they rarely fought,” says Nessia.

Meanwhile, a smasher’s default setting would be best described as “ornery”. Smashers prefer ready-made crevices in rocks and coral, but that market is tight, so they must fight for a home. However, a brawl between two smashers could mean mutually assured destruction, so any territorial skirmishes have strict rules. First, there’s a lot of ritual signalling. If a squatter shows up, a smasher will wave its clubs about, hoping the size of its guns will make the interloper think again. If that fails, it’s all on—sort of. The smashers don’t actually go head to head, as a Duke University study found. The smashers under observation only struck at one another’s tails—tough structures that can dissipate almost 70 per cent of a blow’s energy.

That same study also found that the winner wasn’t the mantis shrimp that rained the heaviest blows, but the one that was the most persistent hitter. In smasher culture, your work rate seems to matter more than your brute strength.

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Sooner or later, every mantis shrimp has to leave home in order to get something to eat. Smashers range far and wide, zig-zagging across the reef in search of prey, but they always come back to their cranny—and their trip home consistently follows a straight line. Somehow, they always know exactly where their burrows are, no matter how many turns they’ve taken while out and about.

Studies have shown that, under clear skies, mantis shrimps keep tabs on the position of the sun. On cloudy days, however, they appear to follow the orientation of polarised light—beams that vibrate in just one plane, rather than scattering like sunlight. The light receptors in a mantis shrimp’s compound eyes are stacked at alternate right angles, and the cells in each hemisphere are oriented at 45 degrees to one another—meaning it can perceive polarised light in four directions at once.

Like all crustaceans, mantis shrimps are a pincushion of appendages, each with its own function. The back end of Heterosquilla tricarinata (the tail, the frilled ranks of swimmerets, the six pereiopods, or walking limbs) is mostly devoted to enabling the front end (the six grasping maxillipeds, a lethal pair of spears, and those unrivalled eyes).

You have three kinds of photoreceptors in your eyes. They allow you to see blue, green and red wavelengths. The belt around a mantis shrimp’s eye, by comparison, contains between 12 and 16 photoreceptors—maybe all necessary so that one peacock mantis shrimp (Odontodactylus scyllarus) can fully appreciate the diaphanous splendour of another. In one instant, phosphorescent pulses of white flash across a palette of cobalt, vermilion and lime, while beneath an emerald carapace, rows of swimmerets take purple to the edge of saturation. Each shift in the light ignites a different firework.

The mantis shrimp is the only creature known to be able to see a rare form of circular polarisation. In 2014, a University of Queensland study reported that mantis shrimps had been able to discriminate between cancerous cells and healthy tissue, since cancerous cells reflect polarised light differently. Since then, scientists have been working on a camera that “sees” like a mantis shrimp does, and might therefore help doctors to spot cancer cells much sooner.

Hayley Nessia, a University of Auckland postdoctoral researcher, dons gloves to handle a Japanese mantis shrimp.
Hayley Nessia, a University of Auckland postdoctoral researcher, dons gloves to handle a Japanese mantis shrimp.

Mantis shrimps can also see deep ultraviolet, and studies suggest they might use black-and-white vision in the hemispheres of their eyes to recognise objects. We can mix maybe 10 million colours in our minds; a mantis shrimp has more than four times that power in receptors arranged across the central bands of its eyes. It sees hues we cannot imagine. And, as if that wasn’t already cool enough, some mantis shrimps can tune the sensitivity of their long-wavelength colour vision to changing light. Oh, and each eye can revolve independently. And see in hexnocular vision.

It’s the most complex visual system ever known. What does a small-brained invertebrate do with so much information?

Well, we’re not sure. Researchers think that some prey animals wear cloaks of cells that reflect circular polarised light, making them invisible to practically everything in the sea—except mantis shrimps. And, in fact, mantis shrimps themselves can do the same thing—so they might be using their remarkable vision to identify potential mates and rivals.

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Every few months, we seem to discover something new about mantis shrimps, and they blow our minds all over again. But the truth is that we actually know much more about the mantis shrimp’s superpowers than we do about the creature itself. We have no idea how many other species there might be in the seas of Aotearoa. We don’t know what roles they play in their ecosystems, what threats they pose, or what dangers they face.

“Native mantis shrimps are very data-poor,” says Nessia. “It’d be great if more people knew and cared about them, because they have an important ecological role, and the more we know about them, the more we can help conserve them.

“They’re very charismatic. Mine seemed almost to have different personalities. Some were quite docile—they’d come up and take food from my hand. What were they looking at that we can’t even see? We don’t even know why their vision is so amazing. For something to have evolved to that degree—to be so sophisticated, so early on in evolution—then to remain unchanged for so long is quite something. They’re incredibly cool.”