When it comes to crabs, I believe everyone will be familiar with them
A pair of large pliers
Round, flat back armor
Plus symmetrical crab legs
This is what a crab looks like in our eyes
图一图源
Although seeing is believing, for crabs, there are actually a lot of "ghosts" among them, because there is a group of crustaceans who secretly imitate the appearance of "crabs", so as to mix into the team of "real crabs", so many of the crabs that we commonly eat or often eat in our lives are "fake crabs".
For example, the well-known king crab and coconut crab are actually pretending to be "crabs"!
Figure 2: Alaskan king crab source network
01
Real Crabs and "Fake Crabs"
Judging from the appearance, the "fake crab" can achieve such a realistic imitation, and if it has nothing to do with the "real crab", then no one will believe it.
In fact, whether it is a "fake crab" or a "real crab", they all belong to a member of the big family of decapods, the so-called "full", refers to five pairs of walking feet, let's take a look at the crab can quickly understand, a pair of crab claws plus four pairs of crab legs, exactly five pairs.
图三数数蟹腿 图源BHNY
Seeing this, those who are attentive may have realized that there is another class of animals that also has five pairs of legs. That's right, it's shrimp, and the common shrimp, crayfish, lobster, etc. also belong to the decapods.
图四图源BartBraun
As for the relationship between shrimp and crabs, it is not as philosophical as that of eggs and chickens, because according to current fossil research, the earliest known decapods are lobster-like decapods, which lived in the Devonian period 360 million years ago.
After about 100 million years, a branch of the decapod diverged into two groups, including Brachyura and Anomura.
In the years that followed, the two groups of lobsters went farther and farther on the road of crab making, gradually shortening the long tail little by little, hiding it under the abdomen, just like humans gradually discarding their tails.
Figure 5: Image source: joaana
At the same time, their body shape began to develop horizontally, slowly becoming round and flattened from a round body shape, and finally became the crab we see today, and this process is called "crabization" by biologists, also known as short-tailedness.
Among them, the short-tailed lower order is very specific to crabs, they completely abandon the shrimp tail, and flatten the tail under the abdomen, until the whole body is covered by the carapace, which is biologically a standard crab appearance, also known as a true crab.
Figure 6 Source: Fiddlercrab
Some crabs of the Heterodactyl order can actually see the presence of shrimp tails, such as hermit crabs, if you get it out of the "house", you can clearly see a section of the tail, so although it is also called a crab, and even looks like a crab, their spiraling abdomen and fragile exoskeleton reveal that it is not a "real crab".
Figure 7: If you look closely, when the hermit crab changes homes, there is a long screenshot of the watermark on the tail
This is also an important feature that distinguishes "real crabs" from "fake crabs".
In addition to this, we can quickly distinguish them by the number of legs. In simple terms, a real crab can clearly see four pairs of crab legs, while a fake crab can only see three pairs of legs.
Someone must be asking, are the fake crabs missing a pair of legs, and can they still stay in the decapod family?
Actually, although it is not visible on the surface, the fourth pair of legs of the fake crab still exists, but it is very short and cannot help walking, so it is hidden.
Figure 8 porcelain crab, from the appearance of only three pairs of legs Image source yammo
02
Why does everyone want to be a crab?
Although real crabs and "fake crabs" have taken different evolutionary paths many years ago, in the end, their forms have come to the same end, and they have chosen to explore the world as crabs, which is a bit like all roads leading to Rome.
Biologically, this phenomenon belongs to convergent evolution, which refers to the independent evolution of similar characteristics by different groups, which is common in the evolution of many animals, for example, we can find traces of birds in bats, and we can also see a trace of "dog" (bushi) in your good friends.
Figure 9: Source network
But interestingly, the body structure of the crab-like was "broken" in this group of crustaceans.
According to research statistics, in the long years, "crabization" is not done in one go, but at least five evolutions, and even in this process, there is also a "de-crab", that is to say, in a certain period of time, some groups have given up becoming crabs, but then returned to the road of making crabs, which is very wonderful.
So then again, why does everyone want to be a crab? Is it because lobsters are not tasty?
On the contrary, it is precisely because the lobster is so delicious, especially the lobster tail, this part is the key to the lobster's movement in the water, so it exercises many muscles, and the taste is naturally not bad.
Figure 10 Okay, let's not talk about it, I'm starting to drool Image source network
But the terrible thing is that the lobster tail, which looks delicious, is only protected by a fragile exoskeleton, because for predators, the writhing tail of the lobster is tantamount to the temptation to walk.
In order to make themselves look unpalatable, some lobsters began to shorten their tails or even hide them in their abdomen, and their cylindrical bodies also developed towards flattening, and finally turned into crabs covered with carapace and little meat, in exchange for more chances of survival.
In addition to making themselves look unpalatable, the crab-like body also brings more living space, and compared to the cylindrical body of lobsters, the flat shape of crabs can better help them move sideways, and their flexibility is greatly improved.
Figure 11 slipped away Source: quora
Moreover, in addition to the ability to walk, crab legs have evolved the ability to dig under sediment or paddle through water, allowing crabs to easily adapt to new habitats.
Figure 12: Source tiktok
At present, there are more than 7,000 species of short-tailed animals in the world, and there are only a handful of lobster species, so it is not difficult to see that the body structure similar to crabs is indeed outstanding.
Until now, the "crabization" has not stopped
There may be more crustaceans in the future
Select Crabization
In the process, their mental journey
Probably
Question the crab, understand the crab, become the crab!
Author: Yu Yu
Winner of the Silver Award for Outstanding Popular Science Works of the China Science Writers Association
Lovers of steamed fish heads, spicy fish heads, and other fish heads
Editor-in-charge: Dong Xiaoxian