Why are all animals becoming crabs? Well, all animals are not becoming crabs, but crabs have evolved separately five times, so there must be something about the crab form that is very advantageous.
We don’t see that many of them, but there are more than 7,000 species of crab. They vary hugely in habitat and in size. Some crabs like warm water, such as stone crabs, and some like cold water, such as red king crabs. Some crabs are tiny, such as the miniature false spider crab, which is 1.4 mm across, and some crabs are enormous, such as the giant Japanese spider crab, which is 40 cm across and weigh 19 kg. Some crabs like fresh water, such as the red claw crab, and some like salt water, such as hermit crabs. Some crabs don’t even live in water, such as the giant coconut crabs that climb trees. However, despite their differences, they almost all have a thick exoskeleton, five pairs of legs, and pincer claws.
All animals are becoming crabs is a little bit of a stretch. It would be more accurate to say that all crustaceans are slowly becoming crabs. This has happened so often that there is even a word for the process of evolving into a crab. The word is carcinization. There is no way that we would evolve into crabs because none of the attributes a crab has would benefit us in any way. We are also too far removed as a species. We are not jointed enough and we have vertebrae. Crabs are very jointed and have exoskeletons. Something that is close to a crab can evolve into a crab, but there is no way we would ever evolve to be crabs. So, why have some creatures evolved into crabs?
The idea that all crustaceans will eventually become crabs comes from the fact that five different species of crustaceans have evolved into crabs independently. For two species to do it, it could be a coincidence, but for it to happen five times, there must be something that gives a crab form an advantage. What are the five times? They are king crabs, coconut crabs, hairy stone crabs, patagurus rex, and porcelain crabs. All of these crabs started out as a different shape, and they have evolved to become crab-like.
Some of these crabs started out as hermit crabs, which are crab that don’t have a very tough exoskeleton. They are at risk of being eaten, so they crawl into abandoned shells and carry them around as a house. They are not symmetrical so they can fit into the shells. They also have longer and thinner bodies. These crabs have evolved to have flatter bodies and to have a much harder exoskeleton. This gives them protection and the flatter body gives them a lower enter of gravity, which means they can move more quickly.
The porcelain crab began as a relative of the squat lobster, which is a basic lobster shape. They evolved to lose the long body and developed a short, round body, which gives them the speed advantage and the ability to move sideways. They also lost their long tail. Lobsters use their tail to swim away from danger, but it is also an extra large appendage that predators can grab onto. By evolving into more of a crab shape, the lobster reduced its profile for any predators.
All of these crabs are called fake crabs because they look like crabs but aren’t actually crabs. This shape has evolved numerous time throughout the ages, but it is a case of creatures that are partly crab, or very similar to crabs, evolved to become crabs. It is not too big of a leap. And crab bodies do give a lot of advantages. They can move sideways very quickly, which is a good escape mechanism when most predators attack straight on. They can use their legs for a range of different purposes and they don’t have tails that predators can grab onto. However, it is not too dissimilar to a lot of other animals. Bats, insects, and birds all evolved flight independently. Whales and bast have both evolved echolocation independently. Sharks and dolphins both evolved their streamlined shape independently. Hedgehogs, porcupines, and echidnas all evolved the ability to grow spines independently. This is called convergent evolution and there are many other examples as well. If a mutation works very well in a specific environment and there are a lot of different animals in that environment, the law of large numbers says that the mutation will probably happen to more than one experience. And if that mutation is a real advantage, it will probably be passed on. So, carcinization is not necessarily that rare. And this is what I learned today.
Sources
https://www.floridamuseum.ufl.edu/science/remarkable-new-true-crab-like-hermit-discovered
https://www.popularmechanics.com/science/animals/a62696580/could-humans-turn-into-crabs
https://en.wikipedia.org/wiki/Porcelain_crab
https://en.wikipedia.org/wiki/Hermit_crab
https://www.scientificamerican.com/article/why-do-animals-keep-evolving-into-crabs
https://www.npr.org/2024/09/18/1200121022/crab-evolution-science-history
https://research.nhm.org/pdfs/31940/31940.pdf
https://en.wikipedia.org/wiki/Crab
https://www.mcsuk.org/news/crabs-the-crustacean-that-keeps-on-giving