CRAB-IGAN

Tavishi I love myself a good invertebrate (will still choose pinnipeds over inverts any day, but it remains that I do like myself one of those spineless fools.) Crabs are some of the most superior invertebrates of all time. Crabs belong to the clade crustacea, a mostly decapod arthropod group. Crustacea not only includes the crabs, but many other crispy decapods like shrimp, copepods, lobsters, and isopods.

Crustacea is a clade within the larger group pancrustacea, which also includes hexapods like insects. These guys aren't technically considered crustaceans, but I'd argue that the real crustaceans are the friends we made along the way.

Within crustacea, in the class malacostraca, we find the decapods, within which we find crabs. Decapoda is an order of ten-legged crustaceans including shrimp, crabs, and lobsters. Decapoda further divides into suborders dendrobranchiata and pleocyemata. Dendrobranchiata consists of prawns, and pleocyemata consists of everything else.

A prime example of dendrobranchiata!

The suborder pleocyemata further divides into a group of walking decapods, or reptantia, and the shrimp that swim along (true shrimp and the punchy shrimp). Within pleocyemata, we find two infraorders: anomura, or false crabs, and brachyura, or true crabs. Anomurans and brachyurans are both kinds of crabs, but technically, anomurans aren't crabs? Taxonomy is strange. But the main differences between anomurans and brachyurans are that anomurans hide one pair of legs, have abdominal differences, and a segmented carapace. All crabs, whether true or false, have ten legs. In anomurans, not all the legs are visible, and rather, are tucked away into the gill chamber. In the hermit crab below, you can count only three pairs of legs, but in the ghost crab, you can count all five.

(Hermit crabs are FAKE.) Next, both fake and true crabs have a carapace, or their outer covering. In true crabs, this is usually a smooth, but false crabs generally have segmented carapaces. In the FALSE crab below (porcelain crab!), you can see a rocky carapace.

A third sign that can't always be used to identify a false crab is the presence of antennae between the eyes, which is typically present in anomurans but not in brachyurans. And finally, a fourth sign may be the visibility of the last abdominal plate sticking out of the carapace- this is the crab's "tail fan". In the king crab below, you can see the pleon sticking out.

Unfortunately, we find spies and traitors to the good crab everywhere. Among the subphylum crustacea, invertebrates long to be worthy of the respect a crab can command. And through the process of carcinization (become crab), develop more and more crab like features. Carcinization is a threat to national security which will not be forgotten- no hermit crab can soil the good name of the ghost crab.