A crab is an animal, confirmed by its basic biological characteristics that align it with the Kingdom Animalia. Animals are multicellular, eukaryotic organisms whose cells lack walls, unlike plants and fungi. They are heterotrophs, obtaining nutrition by consuming other organisms or organic matter, a trait shared by the scavenging crab. The ability to be motile at some stage of life is also a common feature, which crabs exhibit with their characteristic sideways movement.
Where Crabs Fit in the Animal Kingdom
Crabs belong to the Phylum Arthropoda, the largest phylum, which includes insects, spiders, and centipedes. Arthropods are defined by their segmented bodies, jointed appendages, and a hard external skeleton.
Within the Arthropoda, crabs are placed in the Subphylum Crustacea, a group predominantly found in aquatic environments. The class that contains true crabs, as well as lobsters and shrimp, is Malacostraca. This class is characterized by having a body composed of 20 segments, typically organized into a head, thorax, and abdomen, though these sections are often fused.
Most species commonly recognized as “true crabs” belong to the infraorder Brachyura. This classification highlights their shared ancestry with other joint-legged creatures. True crabs are distinguished by a very short, tucked-under abdomen, a key physical difference from related crustaceans like the long-bodied lobster.
Defining Features of Crab Anatomy
The most recognizable feature of a crab is its rigid, external skeleton, known as the exoskeleton or carapace. This structure, primarily composed of chitin and calcium carbonate, acts as protective armor and provides structural support. Because the shell does not grow with the crab, the animal must periodically undergo molting, shedding its old shell to allow for growth before a new one hardens.
A crab’s body is typically divided into a fused head and thorax region, called the cephalothorax, and a reduced abdomen. They possess ten limbs, classifying them under the Order Decapoda, which means “ten-footed.” The first pair of these limbs is modified into chelipeds, or claws, used for defense, communication, and manipulating food.
The remaining eight appendages are primarily used for walking, which is why crabs often move sideways. Respiration is achieved through gills, located inside a branchial chamber beneath the carapace. This allows the crab to extract oxygen from the water, a necessity for its largely aquatic lifestyle, although some species can breathe air when moist.
Diverse Habitats and Lifestyles
The over 6,800 species of crabs have colonized a vast range of environments across the globe. While many species are marine, inhabiting oceans from shallow coastlines to deep-sea trenches, a significant number have adapted to live in freshwater and entirely terrestrial environments. Land crabs, for example, often remain dependent on water only for the reproductive stages of their life cycle.
Crabs are generally considered omnivorous scavengers, consuming a varied diet that includes algae, detritus, small invertebrates, and carrion. This opportunistic feeding behavior makes them important participants in their ecosystems, helping to keep aquatic and coastal environments clean. Their life cycle begins with larvae that float as plankton, a stage vastly different from the adult form.
The behaviors of crabs are as diverse as their habitats, ranging from solitary existences to large aggregations called “casts” or colonies. Many species are known for their burrowing habits, creating shelters in sand or mud for protection from predators and environmental changes. Their powerful claws are also used in complex visual communication and elaborate courtship rituals.