Why do animals keep evolving into crabs?

An interesting and recurring trend observed in evolutionary biology is the seemingly inevitable occurrence of a crab-like anatomy.

This is actually an understood and fairly simple to explain phenomenon, resulting from the convenience of their anatomy, which is excellent both offensively and defensively.

A crab is a shrimp-like form in which the abdomen is tucked under the body, and in which the head and thorax are fused into a thick, heavily armored and calcified plate.

All of this makes the crab a difficult nut to crack for its predators.
Placement of the abdomen under this armor plating is design genius by evolution, because the abdomen is the most vulnerable region in case of a predator attack. By eliminating this weakness, the crabs rapidly rose to marine prominence.

The large claws allow them to crack open even mollusk shells. Prior to this, few predators were able to break into shelled organisms. Crabs evolved the morphological means to make many previously impregnable skeletons vulnerable. This gave them a significant weapon.

A final advantage of crab’s body plan is the ability to increase respiratory efficiency.

The crab’s body plan increased respiratory efficiency by placing the gills in an enclosed space below the cephalothorax (the head-thorax) and then evolving a pump to move water over the now-enclosed gills.

Crabs evolved from shrimp-like organisms, and in these ancestors we can see a progression toward the crab gill system, a marvelous way to increase the passage of water over the gills and maximize the oxygenation of the body.

All of this makes the body plan of these creatures an evolutionary achievement toward which many organisms have converged, and will likely continue to converge, throughout the history of life on earth and maybe even on other planets.