Why does it grow like a crab
In the waters at the junction of the Indian Ocean and the Pacific Ocean, there is a species of spotted sea anemone crab called the red spotted new rock porcelain crab. But few people know that although porcelain crabs are called crabs, they also have the appearance of crabs, but they are actually shrimps, belonging to the superfamily of Armored Shrimp. In fact, not only erythematous new rock porcelain crabs, but also animals such as king crabs, hermit crabs and coconut crabs are not crabs, but belong to the order Heterodactylus. Only short-tailed animals are real crabs.
The above-mentioned “fake crabs” and “real crabs” all evolved from decapod animals. But for some reason, the “fake crabs” have evolved and screened over and over again, and they have all developed into a crab-like shape-the abdomen becomes shorter and the breast becomes wider. This phenomenon is called “crabization”. Crabization is a kind of convergent evolution.
The crab himself is the strongest representative of crabization
Coconut crab is not a crab
Porcelain crab is a dwarf lobster belonging to the superfamily Armored shrimp (left is the red spot new rock porcelain crab, and the right is the blue porcelain crab)
Since real crabs can grow strangely, why are creatures that look more like crabs instead of real crabs? This is because taxonomists have to consider many factors when classifying an organism. And whether there is a common ancestor is much more important than whether they are similar. Therefore, even if a certain creature looks like a crab, if it has no direct evolutionary relationship with other crabs, then it does not belong to a crab.
When did “crabization” start?
Decapod animals are named for their ten legs. Scientists speculate that the oldest decapod animal discovered so far is likely to live on the bottom of the sea like modern lobsters. About 260 million years ago, decapod animals began to gradually differentiate into Hypocura (pseudo crabs) and Hypocura (true crabs). Brachyura and Heterocura are two different evolutionary routes of decapod animals on the evolutionary path.
The degree of evolution of the clade groups of true crabs of the order Decapoda
The oldest fake crab lived about 200 million years ago, and was more like a lobster. The oldest real crabs first appeared 185 million years ago, and their bodies are slender, not like crabs. However, after a long evolutionary process, the animals of Hypomorpha and Brachyura are getting closer and closer to what we now think of as crabs. So why do these animals continue to become crabs? Is the crab’s body structure more suitable for underwater life?
Too much pressure leads to the evolution of “copy homework”
The British zoologist Boladaye first proposed the concept of “crabization” in 1916. Crabization originally refers to the phenomenon that the abdomen of the species shrinks and folds, and the carapace is stretched and flattened horizontally. In Boradaya’s words, “it seems as if nature is doing its best to create a crab”.
The tail abdomen of decapod animals is easy to be attacked, and shortening the tail abdomen makes it difficult for decapod animals to be captured by natural enemies. The abdomen is a very important body part of crustaceans. Important organs such as the intestines and part of the gonads of crustaceans are located in the abdomen. In order to prevent their abdomen from being bitten by a predator and being eaten, the decapod folds its abdomen under the breastplate to protect it from various attacks. Abdominal atrophy or folding reduces the center of gravity of decapod animals and also reduces their basal metabolism (because muscles need energy) rate.
The half-crab armour shrimp is evolving towards the “crab-shaped”
The shape of the bean crab contrasts sharply with the oyster
Asian spider crab with legs a few meters long
However, “crabization” also has disadvantages. Most crabbed decapod animals have lost the ability to swim. This is because when they fold or shrink their abdomen, they also lose the tail fan for swimming, and their bodies have become flatter. These changes do not change. Conducive to swimming. However, the body shape of crabs makes certain decapods more flexible and maneuverable on the bottom of the sea: crab-shaped bodies are more suitable for moving and digging on the bottom of the sea; crab-shaped animals can easily move sideways, which helps them expand their activities. Range and avoid predator attacks; compared to non-crab-shaped animals, crab-shaped animals have lighter abdomens and require less energy to move. Therefore, over time, the dominant form of this “crab” was selected in evolution, and many slender decapod animals evolved into the shape of a crab.
The habitats of decapod animals have been expanding, and many of them have adapted to the new environment. Nevertheless, their crab-shaped basic body structure has been maintained. It can be seen that the crab-shaped body structure is suitable for most environments.
Although there are so many “crab-like” decapods, the real crabs are actually only those short-tailed species. There are about 7,000 species of Brachyura. They are widely distributed, from the ocean to fresh water, and even land. They vary in size, including bean crabs measuring a few millimeters, and Asian spider crabs with legs up to several meters. When you see the mighty appearance of king crabs in the aquarium next time, please don’t forget that they and crabs separated as early as 200 million years ago.
The oldest heterocera
About 10 million years ago, on the coast of what is now New Zealand, a creature with claws, spines and many legs lived. This creature is the ancient ancestor of modern king crabs, and scientists call it “Imitation Rock Crab.” The shape of the rock crab is about the size of a baseball, with many spikes on its round carapace. The imitation rock crab is the ancestor of all Heteromorpha animals today.
