Around 10 million years ago, ancient king crabs were known as Paralomis debodeorum. Scientists have recently discovered that many animals have evolved into crabs or crab-like creatures over millions of years. In other words, animals seem to evolve into crabs and did so successfully at least five times. That’s a little bit unusual. Carcinisation is when non-crab organisms evolve to a point in which they become crabs.
However, it is an example of convergent evolution in which a crustacean evolves into a crab-like. Coconut, King, and porcelain crabs look crabby, but they’re all examples of crustaceans. Many crustaceans have evolved a crab-like shape to protect their pleon. The back section of a crustacean contains an abdomen for protection from predators.
What is parallel evolution?
Parallel evolution is when certain animals or species develop similar or practically identical features without being related. A good example would be wings. After the Triassic period, birds evolved wings to fly in the air. But parallel to birds, insects have done the same their wings are slightly different. But they serve a very similar or pretty much the same purpose.
Another good example is the leaves of different plants, which over millions of years of evolution, have evolved relatively similar features and relatively similar structures entirely independently from one another. One of the more common examples is the similarities between the marsupials like kangaroos and mammals.
For example, marsupials have similar features, including hair, hands, and feet, with similar digits. Also, they do resemble mammals, but they’re not. They’re entirely different as a species.
Why do things keep evolving into crabs?
The crab body plan, characterized by a broad carapace, a pair of large pincers (chelipeds), and a sideways or “crab-like” gait, has proven successful in specific ecological niches. Here are a few reasons why this body plan has evolved multiple times:
Adaptation to Benthic Environments: Many crab-like organisms inhabit benthic environments, such as the seafloor or freshwater bottoms, where they scavenge for food and seek shelter. The crab-like body plan, with a flattened body and a set of strong pincers, allows for efficient movement across the substrate and effective predation or scavenging.
Enhanced Defense and Predation: The pincers of crab-like organisms provide an advantage in defense and predation. They can capture prey, break open shells, and ward off potential predators. The evolution of pincers in various lineages reflects the selective advantage they offer in these ecological roles.
Ecological Opportunities: Different evolutionary lineages have exploited similar ecological opportunities that favor the crab-like body plan. These opportunities may include occupying specific habitats, utilizing available resources, or adopting certain feeding strategies. The evolution of similar body forms can be an effective way to exploit these niches.
Environmental Pressures: Environmental factors, such as predation, competition, and availability of resources, can drive the evolution of specific body plans. If a crab-like body plan provides advantages in surviving or reproducing within a particular ecological context, natural selection can favor its development.
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 various crustaceans that were not crab-like or didn’t look like crabs before became crabs over time.
The actual crabs’ species are essentially entirely different from Porcelain crabs. It also differs from the largest crabs on the planet, 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.
In other words, this process of carcinization happened at least five times, possibly even more, and it seems to have been 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 environments will have a chance to become crab-like.
The parallel evolution is usually very dependent on the environment and specifics of those environments. The other thing is that all animals that evolved into crabs started as crustaceans. So the ocean’s lobster-like creatures already possess many similar features to crabs.
However, some of them are very different from anything. So far, only the crustaceans have evolved into specific shapes with particular features. But it is a little strange because evolution led these creatures not 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 similar. For example, they had highly similar reproductive organs, different from the original species, and extremely similar neurological and muscular systems. They had also different from the original species. Even when the heart works its arteries, its veins seem similar.
They evolve the hard shells typical of crabs and evolve pincers. Also, they all became flat like a typical crab, and they even learned to walk sideways because this was more efficient. But the reasons this occurs are primarily 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. Most of these creatures lived in somewhat shallow aquatic conditions with similar temperatures and food sources, and they all evolved to be relatively similar.
- Hermit crabs are not true; they’re a slightly different species. They have a soft shell to try improvising 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 from a species similar to a lobster type, different from a typical crab.
- Coconut crabs are unusual and well-known in certain regions because they live on palm trees. They’re known as coconut crabs because they resemble coconuts and stick to trees.
So, in other words, these land-living crabs are also not true. Some species evolved into something that resembles a crab but not a crab. One of the most essential parts of the crab’s body is now protected inside the shell.
At the same time, this has a very unusual effect of providing much more mobility for the creature that suddenly evolves to bend its play. It provides more defense, more chances 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 tiny crevices and hide them from predators.
Crabs keep turning into land animals
Terrestrialization is a species evolving from living in water to living on land. 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 to this.
Some Hermit crabs use the water in their shells for gas exchange. However, a more impressive adaptation is the 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 can’t stay too long or drown. Those crabs’ lungs aren’t exactly like ours.
However, they’re modified branchial chambers, 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, depending on the type of crab. In some cases, gills may get smaller and stiffer and have spacers on them, which help prevent that collapsing problem.
Many crabs can switch between gills and lungs to optimize oxygen delivery based on their activity level and environment. Another species, the gills, mostly stick 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 of whales and other vertebrates. They have hard chitinous exoskeletons instead. In the ocean, this is great for protection against predators. But it also comes in handy for walking out of the water. The crab shell is generally well-built and more than enough to deal with gravity and mechanical strain. But that’s not to say there’s no challenge.
The biggest problem is 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 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. Crabs usually support themselves in another way, involving pressurized internal fluids that help inflate the crab.
All water or land-bound crabs have this, but to stand up to the force of gravity on land, the fluid pressure must be much higher than inland crabs. 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, and was taken in the air, it would eventually get hungry. 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 be most accessible to them is vascular plants, fruits, that thing for land crabs. These 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 contain tough, hard-digest fibers like cellulose and even toxic compounds to crab-like tannins. They can be more nutrient-poor than seafood, especially when calories and nitrogen are consumed.
To deal with this, herbivorous land crabs have developed many adaptations. On the one hand, crabs 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 to help digest cellulose and other fibers.
The crabs themselves may make some of these enzymes, 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 and eating these adaptations help individual crabs survive on land.
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References:
“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.
