Crabs are captivating crustaceans found across diverse environments, from the deepest oceans to freshwater rivers and even land. Understanding crab types involves exploring their biological characteristics and distinguishing them from other crab-like creatures.
What Makes a Crab a Crab?
True crabs belong to the scientific infraorder Brachyura, a name meaning “short-tailed” in Greek. These animals are characterized by a flattened body and a short, projecting abdomen that is typically tucked entirely underneath the thorax.
True crabs possess a hard, protective exoskeleton. They generally have five pairs of legs. The first pair is modified into powerful pincers, known as chelipeds, used for defense, feeding, and social interactions. The remaining four pairs of legs are typically used for walking, allowing for their characteristic sideways scuttling motion.
The True Crab Lineage
The infraorder Brachyura encompasses an immense diversity, with approximately 7,000 to 7,600 known species across nearly 100 families. These true crabs exhibit a wide range of sizes, from tiny pea crabs just a few millimeters wide to the colossal Japanese spider crab with a leg span exceeding four meters.
Many species are marine, but a significant number, around 850 species, are found in freshwater. Some groups have even adapted to life on land, such as fiddler crabs, which are semi-terrestrial, or land crabs like the coconut crab. Well-known examples illustrating this diversity include the blue crab, often found in estuaries, and various spider crabs, which typically reside in deeper waters.
Crab-Like Relatives and Their Distinctions
Many creatures are commonly called “crabs” but are not true crabs. These include familiar animals like hermit crabs, king crabs, and porcelain crabs. These crab-like relatives belong to a different infraorder called Anomura, which means “differently-tailed” due to their varied abdominal forms.
A key difference between true crabs (Brachyura) and anomurans is the number of visible walking legs. While true crabs typically have four pairs of walking legs in addition to their claws, many anomurans only appear to have three pairs of walking legs. Their fourth pair of walking legs is often significantly reduced in size and may be tucked away or used for cleaning gills. Additionally, the abdomen of anomurans is often more prominent or elongated, unlike the small, tightly tucked abdomen of true crabs.