Brachyura

Crabs, animals of the infraorder Brachyura, are the most commonly occurring taxonomic group in the Yonderverse. 93% of planets in the Milky Way have at least one extant species of crab; 85% of planets have more than ten.

Most crabs are omnivorous, gregarious, and highly adaptable. They are most often seen in aquatic and marine environments, certain species also frequenting land, sky, and even space.

Those in this infraorder are considered True crabs.

Anatomy

Eclectus Crab by Mochi

Crabs are characterised by a thick exoskeleton made of chitin, the same material seen in fungi. They have six legs and two arms, also known as chelae.

Species in this infraorder vary greatly in size. Some of the smallest measure less than a nanometer, others reaching the size of small planets. Those on the larger scale are found in the depths of space, an example of deep space gigantism.

Caricinisation is a natural phenomenon seen on countless planets. Non-crab crustaceans often evolve a crab-like body plan, over the course of thousands, if not millions of years. The frequency of this occurrence leads some to believe it isn't natural.

Taxonomy

Y'know, Brachyura isn't even in the top ten in terms of how many species it contains. And yet not a single planet goes without crabs. It's remarkable!

— Excited taxonomer

True crabs are in the order Decapoda, under the class Malacostraca. They are closely related to Anomura, an infraorder containing hermit crabs.

Planet Earth is home to approximately seven thousand species of crabs. The vast majority of the families in Brachyura were designed by humans, then used to classify crabs on other planets.

One of the most well known families of crabs are commonly known as space wanderer crabs. The family, Volumundidae, contains nearly nine thousand species and can be found in nearly every corner of space. The type species, the Molten star crab, swarm asteroid belts in their billions, a sea of bright orange and white infiltrating the Galactic Highway during breeding season.