Crabs are crustaceans known for their hard exoskeletons and ten jointed legs, with the front pair often modified into claws. They exhibit a wide array of sizes, shapes, and colors, adapting to various global environments. Crabs are found in nearly every habitat, from the deepest parts of the ocean to high mountain streams and even tropical forests.
Understanding True Crabs and Their Look-Alikes
The term “crab” refers to many decapod crustaceans, but “true crabs” belong to the infraorder Brachyura. A defining characteristic is their small abdomen, tucked neatly underneath the cephalothorax (the fused head and thorax). This compact body plan provides a streamlined shape.
In contrast, other crustaceans commonly called crabs, such as hermit crabs, king crabs, and porcelain crabs, are not true crabs; they belong to a different infraorder called Anomura. They possess an abdomen that is not fully tucked under their body and may be asymmetrical or soft, often requiring protection from a shell or external structure. While both Brachyura and Anomura are decapods, their abdominal structure serves as a primary distinguishing feature.
Crabs of Marine Environments
Most crab species inhabit marine environments. Swimming crabs, such as the blue crab (Callinectes sapidus) found along the Atlantic and Gulf Coasts, possess flattened, paddle-like hind legs. These allow them to efficiently propel themselves through water, aiding in hunting and escaping predators.
Other marine crabs are adapted for life on the seafloor or within sediments. Dungeness crabs (Metacarcinus magister), common along the Pacific coast of North America, are bottom dwellers that use robust claws to crush prey and can burrow into sandy substrates. The Japanese spider crab (Macrocheira kaempferi), residing in the deep waters off Japan, is known for having the longest leg span of any arthropod, reaching up to 12 feet. Its slender legs help it navigate the deep ocean floor, where it scavenges for food. Marine crabs also include species adapted to rocky shores, like shore crabs (Carcinus maenas), which are highly resilient to wave action and changes in salinity.
Crabs That Live in Freshwater and On Land
While most crabs are marine, some species have adapted to freshwater and terrestrial environments. Freshwater crabs are found in rivers, lakes, and streams across tropical and subtropical regions. These crabs have developed specialized gills for extracting oxygen from freshwater and can tolerate lower salinity. They often construct burrows along riverbanks or lake shores, which provide shelter and help them maintain moisture.
Terrestrial crabs, while living on land, still retain a connection to water for gill moisture or reproduction. The coconut crab (Birgus latro), the largest terrestrial arthropod, lives on islands across the Indo-Pacific. It breathes using modified gills and lungs. These crabs are known for their ability to climb trees and crack open coconuts with their powerful claws. Other land crabs migrate to the ocean to release their larvae.
Crabs with Unique Features and Behaviors
Many crab species exhibit distinctive features and behaviors. Decorator crabs showcase an unusual form of camouflage, actively attaching sponges, anemones, algae, and other marine organisms to their shells using specialized hooked bristles. This helps them blend with surroundings, protecting them from predators and aiding in ambushing prey.
Fiddler crabs (Uca species) are recognized by the males’ oversized claw. This claw is not primarily for feeding but serves as a visual signal during courtship rituals, attracting females and deterring rival males. Ghost crabs (Ocypode species), found on sandy beaches worldwide, are named for their pale coloration and their ability to disappear quickly. They are nocturnal hunters, known for their exceptional speed and agility in capturing small prey.