Why do things keep evolving into crabs,like what’s so good about it?
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The wide, box like shape that allows them to quickly move sideways is evolutionarily advantageous. They’re armored, they can have defenses that don’t put them in danger, and most of the time they’re small enough to hide in cramped places.
It’s not so much that things in general wind up looking like crabs, instead it’s more the case that shrimp-shaped things often wind up looking like crabs. Take a shrimp, fold the tail under the body, maybe increase the pinchers a bit, and you basically get a crab. Shrimp shapes are better for swimming around, since the tail is lined with swimming paddles, but on the ground being crab shaped often has advantages because the tail isn’t getting in the way or vulnerable to attack.
Carcinization is famous, but it’s just one more example of convergent or parallel evolution. Lizards quite frequently lose their legs and wind up looking like snakes. Large swimming vertebrates tend toward the tuna/shark/dolphin/ichthyosaur torpedo shape. Plants have independently hit on “tree” shapes multiple times. So you often see animals or plants converging on a shape that fits their environment.
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