Crabs, belonging to the infraorder Brachyura, are one of the most well-known and recognizable groups of crustaceans in the marine and coastal world. Currently, more than 7,000 species of crabs have been described, distributed across a huge variety of habitats ranging from tropical reefs to cold deep waters, and even on land in tropical areas and mangroves.
Within the infraorder Brachyura, there are multiple families that group the different types of crabs. Among the best known are the freshwater crabs (Gecarcinucidae and Potamidae), the fiddler crabs (Ocypodidae), the spider crabs (Majidae), the stone crabs (Xanthidae), and the popular blue crabs such as Callinectes sapidus. Each of these groups presents unique adaptations, but all share the basic morphology that characterizes crabs.
The main characteristic that distinguishes crabs from other crustaceans is their wide and flattened body, with a carapace that protects their internal organs. Unlike other decapods, such as lobsters, crabs have a very reduced abdomen folded under the body, giving them their distinctive compact shape. They also have five pairs of legs, with the first pair modified into claws (or chelae) used for feeding, defense, or interacting with other individuals.
The distribution of crabs is almost global. They are found in both saltwater and freshwater, and many species have developed the ability to live out of water for long periods. Some terrestrial crabs, such as those of the genus Gecarcinus, spend most of their lives on land, though they return to the sea to reproduce.
Regarding their diet, most crabs are omnivorous and opportunistic. They feed on algae, small animals, organic matter, and carrion, making them excellent recyclers in the ecosystem. Some species have more specialized diets: for example, fiddler crabs mainly feed on detritus and microorganisms filtered from the sand.
Reproduction in crabs is sexual, with clearly differentiated males and females. During mating, the male transfers sperm to the female, who then carries the fertilized eggs under her abdomen until they hatch. The larvae go through several planktonic stages before settling on the seabed and transforming into juvenile crabs.
Besides their biological diversity, crabs have great ecological and economic importance. Many species are key in marine food webs, and others, such as Portunus trituberculatus or Scylla serrata, are the target of fisheries and aquaculture in many regions of the world. Their behavior, adaptability, and variety of shapes and colors make them one of the most fascinating groups in the animal kingdom.
