Why Do Crabs Walk Sideways?

Why have crabs evolved to walk sideways?

Published Dec.20, 1994 | Updated Oct.8, 2005 Most crabs usually stroll on the beach by walking sideways. But crabs can also walk forward, backward and diagonally. Because crabs have stiff, jointed legs, they move faster and easier walking sideways. Walking sideways means that one leg never moves into the path of another.

So a crab is also less likely to trip over its feet.
That’s important when you’re keeping track of four pairs of walking legs, plus a set of claws! A crab walking sideways pulls itself along with one set of legs and pushes with the other.
Pairs of legs on opposite sides work together to carry the crab along.

But that doesn’t mean it’s a slowpoke. A ghost crab can scoot sideways 5 feet in just one second. It uses only two pairs at top speed! By 3-2-1 CONTACT Magazine For AP Special Features

Do all crabs walk sideways?

Why do crabs walk sideways? Most crabs walk sideways but some still prefer to walk forwards, such as the lumbering spider crabs, which use the sharp tips on their legs to climb sheer rock faces, and the, which carry their seashell homes around on their backs.

It is thought that crabs evolved from an ancient crustacean body plan that, like those of shrimps and lobsters, included a large tail. Though this appendage enables a lobster to swim backwards quickly when faced with a predator, it is heavy and cumbersome, especially out of water. It also means they can only walk forwards, which is not ideal – not only is it slower, but since a lobster’s long legs are arranged in two rows either side of its body, they tend to get in the way of each other.

Through, crab tails have reduced dramatically. Today, they comprise just a tiny flap held against the underside of the body. This allows the animals to walk sideways, which is considerably quicker (the fastest-moving are the ghost crabs, which live on tropical beaches).

Can a crab walk in a straight line?

Crabs walk funny – But whether crabs are actually crabby is beside the point. Aristophanes was not referring to the crab’s disposition—but to his MOVEMENT. Crabs don’t walk straight. They walk sideways. This only adds to their bad reputation. They seem sly and deceitful to us.

If you saw a PERSON walking sideways, you’d assume they had some evil plan in mind. So why do crabs walk sideways? Are they just being ornery? Are they just a bunch of nonconformist crustaceans? Are they merely determined to do things their own way? To chart their own course? Flaunt their own style? Do they simply march to the beat of a different drummer? Are they trying to set a new walking trend? NO.

Crabs walk sideways because they HAVE TO. It’s because of their anatomy. Crab legs (yum) are attached to the SIDE of a crab’s body. Their knees bend in a straight line, just like ours do. But given the location of their legs, the straight line movement is side-to-side rather than front-to-back.

Can crabs feel pain?

Decapods respond to painkillers – Studying an animal’s response to painkillers can provide useful insight into whether that animal is capable of feeling pain. If the animal responds differently to a painful situation when given painkillers, this can be used as evidence that the animal can feel pain.

The example studies below compared the response of decapods to painful situations both with and without painkillers and found that their behaviour did change, supporting the idea that decapods are capable of experiencing pain. In two scientific studies, researchers caused injury to the antennae of prawns.

The behaviour of different groups was assessed based on whether they did or did not receive painkillers. In both studies, the group that received painkillers for the procedure showed less pain-associated behaviours like tail-flicking and rubbing or guarding the injured spot (6, 9).

Are things evolving into crabs?

Sign up for Scientific American ’s free newsletters. ” data-newsletterpromo_article-image=”https://static.scientificamerican.com/sciam/cache/file/4641809D-B8F1-41A3-9E5A87C21ADB2FD8_source.png” data-newsletterpromo_article-button-text=”Sign Up” data-newsletterpromo_article-button-link=”https://www.scientificamerican.com/page/newsletter-sign-up/?origincode=2018_sciam_ArticlePromo_NewsletterSignUp” name=”articleBody” itemprop=”articleBody”> A flat, rounded shell. A tail that’s folded under the body. This is what a crab looks like, and apparently what peak performance might look like — at least according to evolution, A crab-like body plan has evolved at least five separate times among decapod crustaceans, a group that includes crabs, lobsters and shrimp. In fact, it’s happened so often that there’s a name for it: carcinization. So why do animals keep evolving into crab-like forms? Scientists don’t know for sure, but they have lots of ideas. Carcinization is an example of a phenomenon called convergent evolution, which is when different groups independently evolve the same traits. It’s the same reason both bats and birds have wings, But intriguingly, the crab-like body plan has emerged many times among very closely related animals. The fact that it’s happening at such a fine scale “means that evolution is flexible and dynamic,” Javier Luque, a senior research associate in the Department of Zoology at the University of Cambridge, told Live Science. Crustaceans have repeatedly gone from having a cylindrical body plan with a big tail — characteristic of a shrimp or a lobster — to a flatter, rounder, crabbier look, with a much less prominent tail. The result is that many crustaceans that resemble crabs, like the tasty king crab that’s coveted as a seafood delicacy, aren’t even technically “true crabs.” They’ve adopted a crab-like body plan, but actually belong to a closely related group of crustaceans called “false crabs.” When a trait appears in an animal and sticks around through generations, it’s a sign that the trait is advantageous for the species — that’s the basic principle of natural selection. Animals with crabby forms come in many sizes and thrive in a wide array of habitats, from mountains to the deep sea. Their diversity makes it tricky to pin down a single common benefit for their body plan, said Joanna Wolfe, a research associate in organismic and evolutionary biology at Harvard University. Wolfe and colleagues laid out a few possibilities in a 2021 paper in the journal BioEssays, For example, crabs’ tucked-in tail, versus the lobster’s much more prominent one, could reduce the amount of vulnerable flesh that’s accessible to predators. And the flat, rounded shell could help a crab scuttle sideways more effectively than a cylindrical lobster body would allow. But more research is needed to test those hypotheses, Wolfe said. She is also trying to use genetic data to better understand the relationships among different decapod crustaceans, to more accurately pinpoint when various “crabby” lineages evolved, and pick apart the factors driving carcinization. There’s another possible explanation: “It’s possible that having a crab body isn’t necessarily advantageous, and maybe it’s a consequence of something else in the organism,” Wolfe said. For example, the crab body plan might be so successful not because of the shell or tail shape itself, but because of the possibilities that this shape opens up for other parts of the body, said Luque, who is a co-author of the 2021 paper with Wolfe. For example, a lobster’s giant tail can propel the animal through the water and help it crush prey. But it can also get in the way and constrain other features, Luque said. The crab body shape might leave more flexibility for animals to evolve specialized roles for their legs beyond walking, allowing crabs to easily adapt to new habitats. Some crabs have adapted their legs for digging under sediment or paddling through water. “We think that the crab body plan has evolved so many times independently because of the versatility that the animals have,” Luque said. “That allows them to go places that no other crustaceans have been able to go.” The crab-like body plan also has been lost multiple times over evolutionary time — a process known as decarcinization. “Crabs are flexible and versatile,” Luque explained. “They can do a lot of things back and forth.” Wolfe thinks of crabs and other crustaceans like Lego creations: They have many different components that can be swapped out without dramatically changing other features. So it’s relatively straightforward for a cylindrical body to flatten out, or vice versa. But for better or worse, humans won’t be turning into crabs anytime soon. “Our body isn’t modular like that,” Wolfe said. ” already have the right building blocks.” Copyright 2023 LiveScience, a Future company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Why do crabs pinch you?

The powerful pinch of a blue crab Tuana Phillips, a staffer with the Chesapeake Research Consortium, holds an adult blue crab during a trip to Smith Island, Md. (Photo by Will Parson/Chesapeake Bay Program) With its bright blue claws, the is one of the most recognizable species in the Chesapeake Bay.

This colorful crustacean’s strong claws allow it to crack open or pry apart the shells of clams, snails, mussels and more in its search for a meal. But blue crabs don’t just use their claws to find food: they can also use the powerful pincers to defend themselves. Their sharp and strong grip can be quite painful, as anyone who has ever been pinched by one can confirm.

And if threatened, a crab may break off a claw or leg to try to escape predators; the limb will later regrow through a process called regeneration. Crab claws have made headlines in the past with viral images and videos showing the crustaceans wielding everything from cigarettes to knives.

And though these posts may seem silly, as Jack Cover of Baltimore’s National Aquarium told, the crabs in these images are “absolutely distressed”: either unable to let go of what they’re holding or instinctively clamping their claws in self-defense. If you see a blue crab, it’s best to avoid putting anything—especially your fingers—between its claws.

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If you see a blue crab, it’s best to avoid putting anything—especially your fingers—between its claws. If you want to pick up a blue crab, to safely handle it, pick it up from behind where its rear swimming legs connect to the shell. You can also gently step on the crab to make sure it doesn’t move as you try to pick it up.

To be extra safe, wearing crabbing gloves will protect your skin from getting cut. If, despite your best efforts, a crab has pinched you, the best method to get it off is to calmly put your hand back in the water and the crab will release its grip and swim away. Learn more about, the blue crab. Stephanie is the Web Content Manager at the Chesapeake Bay Program.

A native of the Midwest, she received her Bachelor’s in Professional Writing from Purdue University and Master of Science degree from the University of Michigan. Stephanie’s lifelong love of nature motivates her to explore solutions to environmental problems and teach others what they can do to help.

Do crabs think fish are flying?

Do the crabs think fishes could fly? They have a relatively simple nervous system that wouldn’t comprehend a concept such as flight. They would simply sense a fish swimming over them, and respond with a flight or hide reaction.

Can crabs survive upside down?

If any are upside down, they will die. Cover the crabs with a damp towel.

Why can’t crabs walk forward?

So why do crabs walk sideways when most other creatures walk forwards? It is easy for us humans to move in pretty much any direction, this is because of how our legs are attached to our bodies. Our knees facing towards the front makes walking forward way easier than sideways.

But what about crabs, do they all walk sideways? – A crab’s legs are attached to the side of its body. Its joints unlike our knees bend towards the sides which leaves the crab able to only walk one direction. Why do Crabs walk Sideways? | Mocomi Kids – YouTube MocomiKids 642K subscribers Why do Crabs walk Sideways? | Mocomi Kids MocomiKids Search Watch later Share Copy link Info Shopping Tap to unmute If playback doesn’t begin shortly, try restarting your device.

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Why do crabs make sand balls?

Give a sand bubbler crab some sand and you won’t believe what it’ll rustle up for you! After a three-hour minibus trip down the coast, to a town called Hua Hin in the northern part of the bay of Thailand, I finally found myself on a beach. It was 4am (still dark) and also overcast, which didn’t bode well for the sunrise photos I had planned.

There were a few other people around, but no other photographers – maybe they knew something I didn’t. I had a feeling I was in for a wasted trip when something caught my eye. Low tide had now reached its peak and the beach looked odd, like someone had raked it. Closer inspection revealed the tiny culprits.

The beach was covered by millions of tiny spheres of sand, all created by the sand bubbler crabs (or Scopimera ). Why? Because they’re hungry. Basically, the tiny balls are a byproduct of the crabs’ snacking. They don’t eat the sand, but they do feed it through the bottom of an adapted mouth of sorts, filtering out all of the micronutrients that the high tide has brought in and dumped on the beach since their last feeding session.

The crabs retreat into small burrows in the sand during high tide, and emerge every low tide to feed. You can actually see small trails that lead back to their burrows, with the little balls stacked up on either side. Getting close enough to the critters was pretty difficult, and generally I had to pick a spot and wait patiently for them to come out.

Being so small and numerous, they face a long list of dangers, including the local birds, larger crabs, the occasional careless beachgoer – and even their own kind ( I watched one crab drag another into its burrow and then seal it up, presumably to do something sinister without needing to worry about its neighbours’ prying eyes). The tiny crabs sieve nutrients from the sand, regurgitating the rest in the form of these small pellets, which they deposit on the beach. Image: Brett Blignaut, BRB Photo Given enough hungry crabs, the beach soon becomes littered with their sandy deposits. Image: Brett Blignaut, BRB Photo Come high tide, the crabs retreat into their burrows, where they wait for their next feeding session at low tide. Image: Brett Blignaut, BRB Photo And this is what all that furious feeding action creates countless tiny pellets surrounding the small openings of the burrows, with a telltale trail left by the crab. Image: Brett Blignaut, BRB Photo Sadly, not all of the crabs make it back to the safety of their burrows. This unlucky crustacean was crushed by an unobservant beachgoer – and it’s now become a tasty snack for others. Image: Brett Blignaut, BRB Photo For a great video of sand bubbler crabs in action in Malaysia, click here. Earth Touch News

Why are crab walks so hard?

Improves Your Focus and Coordination – Doing a move like the crab walk requires focus and coordination because you’re transferring your weight from your feet to your hands and back again, using different sides of your body, and trying to do this all quickly. You’ll improve your agility and coordination as well as your mental focus with this move.

Can crabs walk or swim?

Blue Crab FAQ | Some crabs swim. Most crabs, like stone crabs and spider crabs, walk or run across the bottom. However, crabs in the family Portunidae have specially modified back legs called swimmerettes. These paddle-shaped legs rotate at 20 to 40 revolutions per minute, allowing the crab to quickly swim through the water.

One of the most well known crabs in the Portunidae family is the blue crab.
In fact, the scientific name of the blue crab is Callinectes sapidus, which translates to “beautiful, savory swimmer.” Just like fish, blue crabs breathe using gills.
However, unlike fish, blue crabs can survive out of water for long periods of time-even over 24 hours-as long as their gills are kept moist.

When out of water, crabs will seek out dark, cool, moist places to help prevent their gills from drying out and to hide from predators. Crabs also have special articulating plates around their gills. They use these articulating plates to seal off their gills and help keep them moist.

That depends on how well they avoid predators.
Typically, the life span for a female blue crab is 1-2 years and a male is 1-3 years; however, in some tagging studies, crabs aged 5 to 8 years old were caught.
Soft-shelled crabs and hard-shelled crabs are of the same species.
Blue crabs have a hard shell or exoskeleton.

In order for the crab to grow, it must periodically shed its shell in a process called molting. Typically, a crab will seek shelter during this process because it is highly vulnerable to predators. The crab absorbs water, which causes the tissues to swell and split the shell in the back between the lateral spines.

The crab then backs out of its old shell and discards it. It continues to pump water into its tissues causing a new shell that grows approximately 33% larger than its original size. The shell hardens again within 2 to 4 days. However, the shell will only harden in water; if the crab is removed from the water, the process is halted.

To sell soft-shelled blue crabs, harvesters catch blue crabs just before the crabs are going to molt. The harvester can tell if a crab is in that “molt stage” by the color of the crab’s exoskeleton in specific areas. Harvesters place the crabs in large shallow pans, watch them carefully, and take them out of the water soon after they molt.

Can crabs breathe underwater?

Breathing Underwater – Crabs breathe underwater by drawing water (which contains oxygen) over their gills using an appendage called a scaphognathite, which is located on the crab’s underside, near the base of its claws. The water passes over the gills, which extract the oxygen. Blood passes over the gills as well and transports carbon dioxide into the water, which releases near the crab’s mouth.

Do crabs suffer when boiled?

Crabs and lobsters can feel pain when you boil them, research shows Wednesday, September 13, 2023 | Lobsters seized by the Hong Kong Customs during an anti-smuggling operation are displayed at a news conference in Hong Kong on Tuesday, Nov.16, 2021. Lobsters, octopuses and crabs will soon be classified as sentient beings, bringing on possible change for how these animals are treated in the United Kingdom.

Lobsters, octopuses and crabs will soon be classified as sentient beings, bringing on possible change for how these animals are treated in the United Kingdom., commissioned by the United Kingdom government, evaluated evidence from 300 studies to conclude that cephalopods — such as octopuses, squid and cuttlefish — and decapods — crabs, lobsters and crayfish — are capable of experiencing pain and, therefore, shouldn’t be boiled alive. “The UK has always led the way on animal welfare and our Action Plan for Animal Welfare goes even further by setting out our plans to bring in some of the strongest protections in the world for pets, livestock and wild animals,”, the Animal Welfare Minister.

The country’s Animal Welfare bill, which isn’t a law yet, already considers all animals with a backbone as sentient beings, according to, This bill will establish an Animal Sentience Committee, which will ensure that the welfare of sentient animals is considered in government decisions. For now, industry practices for shellfish catching or restaurant kitchens won’t be affected, according to the,

The studies examined things such as pain receptors, ability to learn and response to pain-relieving drugs. Pain and suffering are highly relevant because they can shape animal welfare laws, per, Copyright © 2023 Deseret News Publishing Company. All Rights Reserved : Crabs and lobsters can feel pain when you boil them, research shows

How intelligent are crabs?

Crabs can learn and remember their way through a complex maze Crabs are smarter than you might think Harald Schmidt/Shutterstock A species of crab can learn to navigate a maze and still remember it up to two weeks later. The discovery demonstrates that crustaceans, which include crabs, lobsters and shrimp, have the cognitive capacity for complex learning, even though they have much smaller brains than many other animals.

Crustaceans have a brain roughly 10 times less than the size of a bee’s in terms of neuronal count,” says Edward Pope at Swansea University in the UK.
Pope and his colleagues trained 12 ( Carcinus maenas ) to complete an underwater maze in an aquarium.
The maze had a single correct path to the end, which required five changes of direction and included three dead ends.

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The researchers placed a single crushed mussel at the end of the maze as a tasty reward. Read more: The crabs attempted the maze once a week for four weeks. They didn’t manage to complete the maze without making a mistake until their third week of training, although they improved after each training session.

“It’s interesting that the crabs can learn the maze,” says Neil Burgess at University College London, though they seem to learn more slowly than rodents or other mammals, he says.Pope says his team next wants to investigate how changing ocean conditions, such as ocean acidification and rising temperatures, might impact the crabs’ ability to learn.

Biology Letters Topics: : Crabs can learn and remember their way through a complex maze

Is it cruel to boil crabs?

Why Crabs and Lobsters Shouldn’t Be Boiled Alive Crabs and lobsters have a tough time at the hands of humans. In most countries, they are excluded from the scope of animal welfare legislation, so nothing you do to them is illegal. The result is that they are treated in ways that would clearly be cruel if inflicted on a vertebrate.

This might in part be because they are so alien to us.
It is hard to begin to imagine the inner life of a 10-legged, faceless creature with a nervous system distributed throughout its body.
Worse still, crustaceans lack the headline-grabbing intelligence of the octopus.
With only about 100,000 neurons in their nervous system compared with the octopus’s 500 million, crabs and lobsters are unlikely to set the ocean alight with their cognitive prowess.

They are easy to overlook and difficult to empathise with. Nevertheless, if you care about animal welfare, you should care what happens to crabs and lobsters. Consider live boiling. The animal often takes minutes to die, during which it writhes around and sheds its limbs.

Crustaceans can be killed in seconds with knives, but most non-specialists don’t know the right technique. Electrocution using a ‘Crustastun’ takes about 10 seconds, and is probably as as it gets, but the expense of this device means it is hardly standard kitchen equipment. Some processing plants use them (and some UK supermarkets require their suppliers to do so), but many do not, and there is no legal requirement to stun.

Crabs are often still, as one recent put it, ‘processed in a live state’. ‘Processed’ here is a euphemism for ‘carved alive’. Does any of this matter ethically? For many, the key question here is whether these animals are capable of feeling anything – whether they are sentient,

If they feel nothing as they are boiled or carved alive, then ethical qualms about these practices seem as misplaced as they would be for vegetables.
But if they do feel – if they are sentient – then they are cruel and inhumane.
So what is the reality? Are crabs and lobsters sentient or not? Can science settle the issue? Before we can address this question, it helps to be clearer about what we’re looking for.

I will focus here specifically on the phenomenon of pain. There is much more to an animal’s subjective experience of the world and of its own body than pain alone – but pain is the aspect of sentience with the most obvious ethical consequences. Animal welfare scientists define pain as ‘an aversive sensation and feeling associated with actual or potential tissue damage’.

When they talk about pain, they mean pain in its most elemental, evolutionarily ancient sense – a feeling that might have some but not all of the aspects of pain in humans.
In particular, to feel pain in this basic sense, it is not necessary to be self -conscious – to be aware of oneself as being in pain.

Do crustaceans feel pain in this basic sense? Over the past few years, a series of by the biologist Robert Elwood and colleagues at Queen’s University Belfast has demonstrated impressively sophisticated behaviour in crabs. Here is one example. Hermit crabs live in shells vacated by other animals.

They prefer some types of shell to others, and will often switch from a less-preferred to a more-preferred shell in the wild.
Elwood drilled holes in the hermit crabs’ shells and poked electrodes through the holes to see how they would react to small electric shocks – not a pleasant procedure, but a necessary one to gain insight into their responses.

Unsurprisingly, crabs would sometimes vacate a shell, even a good one, if the shock became too severe. More surprisingly, the crabs traded off the quality of the shell against the intensity of the shock received within it. For a given intensity of shock, they’d be more reluctant to give up a high-quality shell than a low-quality one.

This is known as a motivational trade-off,
The crabs were balancing their need to avoid shocks against their other needs.
In another experiment, Elwood and colleagues found that shore crabs rapidly learn to avoid locations they associate with harmful experiences.
The crabs were offered a choice of two dark shelters: in one, they received shocks; in the other, they did not.

In general, crabs prefer to return to shelters that they have previously occupied. But after repeatedly receiving a shock in one of the shelters, the crabs were much less likely to return to it – a phenomenon known as conditioned place avoidance, Motivational trade-offs and conditioned place avoidance are what I call credible indicators of pain – credible because they cannot be explained away as mere reflexes, and because they tie in with a reasonable theory about the function of pain for animals that feel it.

The idea in the background here is that pain is a guide to decision-making, To make flexible decisions, animals need to be able to weigh the seriousness of an injury against other things they need. Sometimes fleeing is the right thing to do; sometimes carrying on as normal is the right thing to do; sometimes tending the injury is the right thing to do – it depends on the situation.

Pain is the currency in which the need to stop, or the need to flee, is measured. When we find an animal making flexible decisions by integrating information about past or present injury with information about its other needs, that is a credible indicator of pain.

Is it conceivable that motivational trade-offs and conditioned place avoidance occur without any pain? Of course, but no one is suggesting that pain is conclusively established by these experiments. We are talking about credible indicators, not conclusive proof. If we demand conclusive proof, this will never be attained – not for any animal, not even for other human beings.

W hat should we do, then, in this state of uncertainty? I suggest a common-sense approach: apply a version of the precautionary principle, The precautionary principle was originally devised for environmental policy. It says, in effect: when you’re uncertain about the link between human actions and a seriously bad outcome, don’t let your uncertainty prevent you from taking effective precautions.

The principle has been applied to environmental threats as diverse as climate change and neonicotinoids (or neonics), the pesticides linked to colony collapse in bees. It should also be applied to the question of animal sentience. I have recently the following ‘animal sentience precautionary principle’: Where there are threats of serious, negative animal welfare outcomes, lack of full scientific certainty as to the sentience of the animals in question shall not be used as a reason for postponing cost-effective measures to prevent those outcomes.

In short: when the evidence is suggestive but not conclusive, give the animal the benefit of the doubt. The phrase ‘lack of full scientific certainty’, taken from the 1992 Rio Declaration on Environment and Development, is, admittedly, far too vague.

It does not specify the evidential standard that has to be met.
This is why I have also proposed a specific, pragmatic standard: we should act to mitigate risks to animal welfare when there is at least one credible indicator of sentience in at least one species of the order of animals in question.
The decapod crustaceans meet this standard.

Arguably an even stronger case can be made for and cuttlefish, which already receive some protection in the European Union. The phrase ‘cost-effective measures’ is also vague. So here is my specific, pragmatic proposal: when the evidential standard is met, we should bring the order of animals within the scope of animal welfare legislation in a way appropriate to their particular welfare needs.

In the case of crustaceans, that means banning processing methods with a substantial risk of inflicting pain, such as live carving and live boiling.
To be clear, the precautionary principle is a guide to policy, not to individual action.
In light of evidence of pain in crustaceans, you might feel it appropriate to stop eating them.

That would be a reasonable reaction, but it isn’t entailed by my proposals, which are concerned with the law rather than with individual behaviour. What my proposals do entail is that decapod crustaceans deserve a basic level of legal protection. We could wait for evidence to pile up, demanding more credible indicators in more species, while decapods continue to be ‘processed’ alive around the world.

But there’s a good chance we’d regret our inaction, just as we might well come to regret our inaction over climate change and neonics. Alternatively, we could take precautions now. On climate change and neonics, it is common sense to act now to mitigate the risk of environmental disaster. Likewise, we should to protect decapods, to mitigate the risk of a continuing animal welfare disaster.

Jonathan Birch is an assistant professor in philosophy at the London School of Economics and Political Science. He is the author of (2017). This article was originally published at and has been republished under Creative Commons. : Why Crabs and Lobsters Shouldn’t Be Boiled Alive

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Will humans evolve again?

READER QUESTION: If humans don’t die out in a climate apocalypse or asteroid impact in the next 10,000 years, are we likely to evolve further into a more advanced species than what we are at the moment? Harry Bonas, 57, Nigeria Humanity is the unlikely result of 4 billion years of evolution.

From self-replicating molecules in Archean seas, to eyeless fish in the Cambrian deep, to mammals scurrying from dinosaurs in the dark, and then, finally, improbably, ourselves – evolution shaped us.
Organisms reproduced imperfectly.
Mistakes made when copying genes sometimes made them better fit to their environments, so those genes tended to get passed on.

More reproduction followed, and more mistakes, the process repeating over billions of generations. Finally, Homo sapiens appeared. But we aren’t the end of that story. Evolution won’t stop with us, and we might even be evolving faster than ever, It’s hard to predict the future.

The world will probably change in ways we can’t imagine. But we can make educated guesses. Paradoxically, the best way to predict the future is probably looking back at the past, and assuming past trends will continue going forward. This suggests some surprising things about our future. We will likely live longer and become taller, as well as more lightly built.

We’ll probably be less aggressive and more agreeable, but have smaller brains. A bit like a golden retriever, we’ll be friendly and jolly, but maybe not that interesting. At least, that’s one possible future. But to understand why I think that’s likely, we need to look at biology.

Are humans still evolving?

Are Humans Still Evolving? – Perhaps we haven’t stopped after all. Broadly speaking, evolution simply means the gradual change in the genetics of a population over time. From that standpoint, human beings are constantly evolving and will continue to do so long as we continue to successfully reproduce. What has changed, however, are the conditions through which that change occurs.

What is a false crab?

‘False crabs’ is a name we have given to a group of species known as anomarans by scientists. It is the sister group to true crabs (brachyurans) and includes familiar species such as hermit crabs, porcelain crabs and squat lobsters.

Why can’t crabs walk forward?

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Why do crabs walk sideways compared to crayfish?

Crabs have a unique sideways walking pattern due to the shape of their body and the location of their legs. Sideways walking helps crabs navigate rocky and unstable surfaces and climb trees.

Why do you think horseshoe crabs have not evolved?

Background – Synopsis: Most living things adapt over time, but horseshoe crabs found their perfect form more than 400 million years ago and haven’t needed to evolve much ever since. They are long-lived marine scavengers with a highly effective, yet primitive, immune system that has protected them through the eons. Comparison of modern Limulus Polyphemus (left) and the oldest known member of the genus, Limulus darwini (right), which is 148 million years old (Late Jurassic Upper Tithonian). The Jurassic specimen came from a quarry in central Poland. Credit: Adrian Kin, Błażej Błażejowski ( CC BY )

Life continually evolves, adapting to the changing circumstances of environments over time, but there are a few exceptions that seem frozen in time—like organisms from the order Xiphosura’s only remaining family, Limulidae, the family of the horseshoe crab.

These organisms are referred to colloquially as “living fossils,” but scientists call them stabilomorphs because they retain a stable form over long geological time frames. Their soft tissues may have evolved over time, but the parts that are preserved as fossils have kept the same characteristics. Horseshoe crabs are creatures of the Paleozoic Era that survived all five major extinction events including the Ordovician (444 Ma), Devonian (375 Ma), Permian (251 Ma), Triassic (200 Ma), and Cretaceous (66 Ma). We covered these extinctions in a previous EarthDate, The oldest fossils of horseshoe crabs, found in 445-million-year-old Ordovician rocks in Manitoba, Canada, look similar to the creatures we find living on beaches today. Younger 148-million-year-old Jurassic fossils from a Polish quarry have shapes and sizes virtually identical to today’s horseshoe crabs. When these fossil animals lived alongside the dinosaurs, their ancestors had already lived on Earth for about 300 million years. They have scrambled over Earth’s surface more than 1000 times as long as Homo sapiens have, and their niche for all this time has been nearshore brackish to marine shallow subtidal environments. They have evolved to be extremely well suited to these environments without the need for further adaptation, even as global temperatures, atmospheric CO 2 levels, and ocean salinites have varied widely. Scientists are currently studying the horseshoe crab’s genome to try to understand how it has survived for so long, even through some of the direst times for life on Earth.

Horseshoe crabs are not actually crabs—their closest relatives are arachnids like spiders, ticks, and scorpions.

Horseshoe crabs and crabs are both arthropods: invertebrates with exoskeletons, segmented bodies, and jointed appendages. Insects, arachnids (spiders), centipedes, and crustaceans are all arthropods. Horseshoe crabs are most closely related to modern arachnids called hooded tickspiders and to the extinct giant arachnids known as sea scorpions. Today, there are four living species of horseshoe crabs in the family Limulidae : one native to the North American Atlantic and Gulf of Mexico coasts, and three that are indigenous to Indian and Pacific Ocean coastlines of Asia.

A boat captain displays the underside of a horseshoe crab from the northern coast of Mexico’s Yucatan Peninsula. The front of the animal is pointed toward the camera. The mouth is surrounded by six pairs of legs at the front, and the linear book gills of the respiratory system are at the rear of the animal. Credit: Angel Schatz ( CC BY )

As animals that completed most of their evolution during the Paleozoic Era, modern horseshoe crabs are very unusual creatures compared to most modern marine life.

Their whole body is covered by a hard, shell-like carapace that is shed each year during molting. They respire through book gills located behind their legs but can live for up to 4 days out of the water as long as the gills are kept wet. Their mouths are surrounded by their twelve legs, of which the smallest pair is dedicated to sweeping food into the mouth. They eat algae, clams, crustaceans, worms, and other animals by crushing hard food between their legs before passing it to their mouths and gizzards for further grinding. The five larger pairs of legs are used for walking and swimming. Horseshoe crabs swim upside down at up to 0.3 miles (0.5 kilometers) per hour with their body at an angle of about 30 degrees, carapace headed forward to plow through the water. The tail, known as a telson, is long and straight, serving as a rudder during swimming and as a lever to help right the animal if it flips upside-down on land. Horseshoe crabs have two compound eyes with around 1000 photoreceptor units each and another pair of eyes that can see both the visible spectrum and ultraviolet light. They have several other rudimentary eyes that are simple photoreceptors, including some on their tail. They are extremely light sensitive. Their rods and cones are 100 times the size of those in humans, making their eyes a million times more sensitive to light at night than during the day. In the wild, horseshoe crabs live for 20 to 40 years. Adults of the North American species can weigh about 10 lb (4.5 kg) with bodies that are 1 ft (30 cm) in diameter with a foot-long telson.

The circulatory system of the horseshoe crab. Notice the structures of the system do not form a closed loop like the blood vessels of our closed circulatory systems do. Instead, horseshoe crabs have sinuses that receive the hemolymph and bathe tissues and organs directly. Credit: Jordan Krisfalusi-Gannon

In the closed circulatory system of humans, iron-based hemoglobin carries oxygen to tissues and organs through closed blood vessels. But horseshoe crabs have primitive open circulatory systems, which distribute a blood-like substance called hemolymph.

A protein called hemocyanin is suspended in the hemolymph and carries oxygen to the horseshoe crab’s tissues. The hemocyanin is copper-based, which gives oxygenated hemolymph a blue color. This is because when copper atoms are exposed to oxygen, they turn blue. Our blood is red for a similar reason—iron becomes bright red when exposed to oxygen. Instead of closed blood vessels that supply blood to their tissues, horseshoe crabs have circulatory sinuses that bathe their tissues directly in blood. It would be fatal to the horseshoe crab if the pathogens so abundant in coastal waters were to breach these sinuses. So Limulidae developed an immune response to certain types of both live bacteria and the endotoxins released when these bacteria die and disintegrate. When these materials are detected in the horseshoe crab’s system, nearby amoebocytes immediately eject granules that cause the blue blood to coagulate instantly, surrounding the foreign cells and sealing any leaks in the circulatory system with a thick gel. The gel serves as a scab-like barrier that holds even in the event a limb is severed. This has important implications for human health, as we will describe in another EarthDate episode,

Why do horseshoe crabs flip over?

9. A horseshoe crab’s pointed tail is not for self-defense. – Horseshoe crabs are easily jostled by ocean currents and waves — and each other. When a crab gets stuck upside-down, it uses its tail, called a telson, to flip over! Horseshoe crabs can also use their telson as a rudder to help steer as they swim upside down.