Science Facts

Why Do Things Keep Evolving Into Crabs? – Carcinisation


Around 10 million years ago, ancient king crabs were known as Paralomis debodeorum. Scientists have recently discovered many different animals have evolved into crabs or crab-like creatures over millions and millions of years of evolution. In other words, it seems that animals have a tendency to evolve into crabs and did so successfully at least five times. That’s a little bit unusual. Carcinisation is the process of non-crab organisms evolved to a point to which they become crabs.

However, it is an example of convergent evolution in which a crustacean evolves into a crab-like. Coconut crabs, King crabs, and porcelain crabs all look crabby, but they’re all examples of crustaceans. The reason many crustaceans have evolved a crab-like shape was to protect their pleon. It is the back section of a crustacean that contains an abdomen that for safety purposes from predators.

What is parallel evolution?

Parallel evolution is when certain animals or certain species develop very similar or even practically the same features without being related to one another. A good example would be wings. Sometime after the Triassic period, birds have evolved wings and evolve to fly in the air. But parallel to birds, insects have done the same their wings are a little bit different. But they serve a very similar or pretty much the same purpose.

Another good example is the leaves of different plants, which over the millions of years of evolution have evolved relatively similar features and relatively similar structures completely independently from one another. One of the more common examples is the similarities between the marsupials like kangaroo and mammals.

For example, marsupials have relatively similar features, including hair, hands, and feet with relatively similar digits. And they do resemble mammals, but they’re not. They’re entirely different as a species.

Why do things keep evolving into crabs?

The parallel evolution is the one known as carcinization. At least five and possibly even more different species of animals have evolved to look and behave similarly to crabs. In other words, many different crustaceans that were not crab-like or didn’t look like crabs before became crabs over time.

The actual species of crabs are essentially an entirely different species different from Porcelain crabs. It is also different from the largest crabs on the planet, known as King crabs. They are not true crabs, but they’re a derivative of a type of crustacean similar to a Hermit crab. They also seem to possess very similar internal structures and internal organs as well.

In other words, this process of carcinization happened at least five times, possibly even more, and it seems to have been very consistent in what is created at the end. These animals seem to resemble each other possess very similar internal and external features and even have very similar behavior. It is, of course, why it got so popular on the internet.

  • Only certain animals in certain conditions and certain environments will have a chance to become crab-like.

The parallel evolution is usually very dependent on the environment and specifics of those environments. And the other thing is that all of the animals that evolved into crabs started as crustaceans. So the lobster-like creatures in the oceans already possess many very similar features to crabs anyway.

However, some of them are very different from anything. So only the crustaceans so far have evolved into these very specific shapes with particular features. But it is nevertheless a little bit strange because the evolution led these creatures not just to share similar features from the outside. It also reshaped their internal organs.

Suppose you were to put five different species into the same environment. In that case, they have an extremely high chance to evolve into something that looks, acts, and even is extremely similar to one another. For example, they had highly similar reproductive organs, different from the original species, and extremely similar neurological and muscular systems. They had also extremely different from the original species. Even when the heart works its arteries, their veins seem to be extremely similar to one another.

They evolve the hard shells that are typical of crabs and also evolve the pincers. Also, they all became flat like a typical crab, and they even all learned to walk sideways because apparently, this was more efficient. But the reasons why this occurs are mostly environmental and pretty much entirely physical.

Essentially all these creatures were going through the same external stresses, and there seemed to be only one way to evolve to be more efficient. For the most part, all of these creatures lived in somewhat shallow aquatic conditions with similar temperatures, similar food sources, and they all evolved to be relatively similar.

  • Hermit crabs are not true crabs, and they’re a slightly different species. They do have a soft shell to try to improvise and steal a shell from somewhere else. They don’t move sideways and have many other features that a typical crab does not have.
  • Porcelain crabs are originally from a species similar to a lobster type, different from a typical crab.
  • Coconut crabs are unusual crabs well known in certain regions because they live on palm trees. They’re known as coconut crabs because they resemble coconuts and they kind of stick to the trees.

So, in other words, these land-living crabs are also not true crabs. Some species evolved into something that seems to resemble a crab but not a crab. One of the most important parts of the crab’s body is now protected inside the shell. At the same time, this has a very unusual effect of providing a lot more mobility for the creature that suddenly evolves to bend its play. It provides more defense, more chance to escape any prey that tries to catch the crab, and other changes such as the flattening of the body. It allows various crabs to fit into very tiny crevices and hide them from predators.

Crabs keep turning into land animals

The process of a species evolving from living in water to living on land is called terrestrialization. It could involve anything from food to predators to oxygen availability. But one way or another, it wasn’t an easy switch fresh realization requires nearly everything to change from how an animal eats to how it has babies.

Aquatic crabs get oxygen. They need a series of gills feathery structures that are chock full of blood vessels and extract oxygen from the water. On land, gills often don’t work so well in the air. The main problem is those feathery structures that make up the gills can collapse in on themselves, leaving the animal unable to breathe and therefore dead. But land-going crabs have found ways to adapt around this.

Some Hermit crabs use the water in their shells for gas exchange, for instance. However, a more impressive adaptation is lungs, which appear in many land-bound crabs like the robber crab, which only breathes air. They’ll enter the water to drink or release eggs, but they can’t stay too long, or they’ll drown. Those crabs’ lungs aren’t exactly like ours. However, they’re modified branchial chambers, which is also where the gills are. So these crabs breathe in through openings near the base of their walking legs. Then out through their mouths instead of the in and out mouth breathing.

In inland crabs, the tissue inside the chamber has become convoluted and spongy, increasing surface area and allowing for gas exchange. So their lungs aren’t entirely new organs. They’re more like heavy modifications to the structures that were already there. Meanwhile, the gills also tend to change slightly, although how depends on the type of crab. In some cases, gills may get smaller stiffer and have spacers on them, which help prevent that collapsing problem.

Many crabs can also switch between gills and lungs to optimize oxygen delivery based on their level of activity and environment. Another species, the gills, mostly sticks around to regulate the crab’s internal ph levels and eliminate waste products. They also do that as a secondary function in marine crabs, but it becomes their main job in these land crabs.

Crabs don’t use the same internal bony skeletons whales and other vertebrates use. They have hard chitinous exoskeletons instead. In the ocean, this is great for protection against predators. But it also really comes in handy for walking out of the water. In general, the crab shell is pretty well built and more than enough to deal with the gravity and mechanical strain. But that’s not to say there’s no challenge.

The biggest problem is what happens when land crabs outgrow their exoskeleton and molt. When this happens, the crab sheds its exoskeleton unveiling a new soft version that will harden and expand over the next few days. The problem is that they lack a strong support system holding them up during that time.

If this happens, their new shell will harden in the wrong shape impeding further molting and killing the crab. The way they’ve adapted to this is through what’s their other skeleton, the hydrostatic skeleton. It’s another way crabs usually support themselves involving pressurized internal fluids that help keep the crab inflated.

All crabs water bound or land bound have this, but to stand up to the force of gravity on land, the fluid pressure must be much higher inland crabs. For one thing, the greater internal pressure puts them at a higher risk of the newly grown exoskeleton rupturing. It also becomes more expensive energy-wise to move around compared to marine crabs. How strong this hydrostatic skeleton can get might be the major limit on the size of crabs like coconut crabs. Once an animal got onto land, walked around a bit, and was taken in the air, it would eventually get hungry. And it turns out that open-air dining is a bit different than eating underwater.

Marine crabs mostly eat algae and small animals, but the food source that seems to come most accessible to them is vascular plants, fruits, that kind of thing for land crabs. These kinds of foods are a lot tougher than what’s in the sea. Compare a nice soft clam to something crunchy and tough like celery. They can also be full of tough hard to digest fibers like cellulose and even compounds potentially toxic to the crab-like tannins. And they can be pretty nutrient-poor compared to seafood, especially when it comes to calories and nitrogen.

To deal with this, herbivorous land crabs have developed many adaptations. For one land, crabs tend to grow more slowly and live longer, which lets them deal with the low nitrogen. They’d otherwise need to grow like the coconut crab can live to be 60 years old. They also have enzymes in their gut that can help digest cellulose and other fibers.

Some of these enzymes may be made by the crabs themselves, but microbes may be one of the keys to exploiting this new food source. Microbes in their guts produce enzymes called carbohydrate-active enzymes. It helps digest and gets energy from woody plant fibers. They may also help in other ways like reducing water loss and digesting nitrogen. So breathing, walking eating these adaptations all help individual crabs survive on land.

More articles:


“Carcinization in the Anomura – fact or fiction? I. Evidence from adult morphology”. Contributions to Zoology.
Porcellanopagurus: An instance of carcinization Natural History Report.
“One hundred years of carcinization – the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea).” Biological Journal of the Linnean Society.
“Evolutionary morphology of the hemolymph vascular system in hermit and king crabs (Crustacea: Decapoda: Anomala).” Journal of Morphology.
“The anatomy of the king crab Hapalogaster mertensii Brandt.

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