Crabs inhabit diverse aquatic environments across the globe, from the deepest oceans to freshwater rivers and even terrestrial landscapes. These animals display a wide array of adaptations, allowing them to thrive in various niches. Their unique forms and behaviors prompt a closer look into what defines these organisms.
Crabs’ Place in the Animal Kingdom
Crabs are classified within the phylum Arthropoda, a group characterized by their segmented bodies, jointed appendages, and external skeletons. Arthropods include insects, spiders, and crustaceans. Within Arthropoda, crabs belong to the subphylum Crustacea, which encompasses primarily aquatic organisms such as lobsters, shrimp, and barnacles. Crustaceans possess two pairs of antennae and breathe through gills.
Most animals commonly recognized as crabs fall under the order Decapoda, meaning “ten-footed.” This order includes a variety of crustaceans, all sharing five pairs of legs. True crabs are specifically members of the infraorder Brachyura, a scientific term that translates to “short-tailed,” referring to their characteristic tucked-under abdomen.
Distinguishing Features of Crabs
Crabs possess distinctive physical characteristics. Their hard exoskeleton, composed primarily of chitin and calcium carbonate, provides defense and structural support. Because it does not grow, crabs must periodically shed it in a process called molting to increase in size.
Their body structure features a broad, flattened carapace covering a fused head and thorax, known as the cephalothorax. Unlike other decapods such as lobsters or shrimp, a crab’s abdomen is very short and tucked tightly underneath the cephalothorax. Crabs have five pairs of legs; the frontmost pair is modified into chelipeds, or pincers, used for defense, catching food, and communication. The remaining four pairs of legs are used for walking, often resulting in their characteristic sideways movement, though some species can move forward or backward. Crabs breathe using gills, which are housed within their shells and require moisture to function.
Beyond the Basics: Diversity and Related Forms
While many animals are colloquially called “crabs,” scientific classification distinguishes between “true crabs” and other crustaceans that share a similar appearance through a process called carcinization. True crabs belong to the infraorder Brachyura and exhibit the fully tucked abdomen and broad, flattened body. There are approximately 7,000 known species of true crabs, displaying a wide range of sizes and habitats.
Other crustaceans, despite their “crab-like” common names, are not true crabs. For instance, hermit crabs, king crabs, and porcelain crabs are also decapods but fall into different infraorders or superfamilies, primarily within the suborder Anomura. Hermit crabs, for example, have soft, asymmetrical abdomens that they protect by inhabiting discarded shells, and their abdomens are not fully tucked beneath their bodies like true crabs. King crabs, while resembling true crabs, also have a different abdominal structure and possess only three pairs of walking legs in addition to their chelipeds. These distinctions highlight the diversity within the broader group of decapods.