The world of crabs is diverse, encompassing creatures found across nearly every corner of the planet. From the deepest ocean trenches to tropical rainforests, these adaptable crustaceans are widespread, leading to questions about their classification and the number of types that exist.
What Makes a Crab a Crab?
To understand crab diversity, it is important to define a “true crab.” True crabs belong to the infraorder Brachyura, meaning “short-tailed” in Greek, referring to their distinguishing feature: a short, tucked-under abdomen. This contrasts with other crustaceans that might appear crab-like but possess a more prominent, extended abdomen.
True crabs are decapods, meaning they possess ten legs. The first pair is modified into pincers, also known as chelae, used for defense, feeding, and communication. Their bodies are covered by a hard, protective exoskeleton called a carapace, which provides structural support and protection.
The Sheer Number: How Many Species?
There are over 7,000 known species of true crabs within the infraorder Brachyura. This number represents recognized species. The count of known species is not static; it constantly evolves as new species are discovered and existing ones are sometimes reclassified based on ongoing scientific research. Ongoing discovery highlights the vastness of biodiversity within this group.
Beyond the Number: Understanding Crab Diversity
Understanding crab diversity extends beyond a simple count. Many animals commonly referred to as “crabs,” such as hermit crabs, king crabs, and porcelain crabs, are not true crabs; they belong to a different infraorder called Anomura.
These “false crabs” exhibit a phenomenon called carcinization, where various crustacean groups independently evolved a crab-like body form. Despite their similar appearance, anomurans often have a reduced or asymmetrical fifth pair of legs, which might be hidden or used for specialized purposes like cleaning gills or holding onto a shell, unlike the four visible pairs of walking legs in true crabs. Within Brachyura, diversity encompasses varied forms, sizes, and behaviors. Examples include the tiny pea crab, only a few millimeters wide, to the colossal Japanese spider crab, which can have a leg span of up to 13 feet.
Crabs Across the Globe: A Look at Varied Habitats
Crabs thrive in a wide array of habitats across the globe. They are found in all of the world’s oceans, from shallow intertidal zones and coral reefs to the deep sea. Marine crabs, such as the Dungeness crab or the blue crab, are adapted to saltwater environments.
Beyond marine environments, true crab species have adapted to freshwater and even terrestrial habitats. Approximately 850 species are freshwater crabs, inhabiting rivers, lakes, and estuaries. These crabs have evolved physiological adaptations, such as specialized gill structures and osmoregulation mechanisms, to cope with lower salinity environments.
Terrestrial crabs, like the coconut crab or fiddler crabs, have adapted to living primarily on land. These land-dwelling species have developed lung-like structures or modified gills to breathe air, and some can even retain water for extended periods to prevent desiccation.