True crabs hid their tails and conquered the seafloor
Brachyura—true crabs—tuck a short abdomen under a hard thorax and brandish clawed first legs. Fossils appear by the Jurassic roughly two hundred million years ago; today some seven thousand species in ninety-six families range from pea crabs to four-meter spider crabs.
True crabs are decapod crustaceans with thickened exoskeletons and five leg pairs, the foremost ending in claws. Behind the chelae lie walking legs and, in many swimmers, paddle-like rear legs. Gills under the body must stay moist to work. They form the largest group of "crab-shaped" decapods; king crabs and porcelain crabs look similar through convergent evolution, not shared brachyuran ancestry. Most walk sideways because the way their legs are jointed makes a sidelong gait more efficient, though a few, such as raninids, move forward or backward.
Sexual dimorphism is common: males often carry bigger claws, extreme in fiddler crabs (Uca), where one oversized claw waves to court and signal. Mating cues include pheromones in fully aquatic species and visual displays ashore. Females release larvae into water timed to tides and day-night cycles. Their claws are formidable: no animal on record beats the Florida stone crab for muscle force relative to cross-section, roughly 2,000 kilonewtons per square metre, against 100 to 300 in other animals.
Zoea larvae drift in plankton through a fixed number of molts, then become megalopae that still trail an abdomen before a final molt yields bottom-living juveniles. The group had already become highly diverse by the Cretaceous. Many land crabs have to trek down to the sea to release their larvae, and the young later repeat the journey in reverse. Crabs live in every ocean as well as on land and in fresh water, where about 850 species occur. Pierre André Latreille named the group in 1802, and a 2014 revision by Ling and colleagues found that many traditional superfamilies are not natural groups.
Source: Brachyura
