Crabs are a diverse group of crustaceans found across the globe, adapted to various environments. Their presence ranges from the deepest parts of the ocean to terrestrial landscapes. Understanding what defines a true crab is key to appreciating their species diversity.
What Defines a True Crab
True crabs belong to the infraorder Brachyura, within the order Decapoda. They are distinguished by specific physical attributes. Their short, flattened abdomen is typically tucked completely underneath their body. True crabs also possess ten legs, including two prominent claws, known as chelipeds, used for grasping food and defense. Their bodies are protected by a hard exoskeleton, which provides structural support and protection for their internal organs.
The Extensive Diversity of True Crabs
The infraorder Brachyura encompasses over 7,000 known species across 96 families. This vast diversity is evident in their size, spanning from the tiny pea crab, merely a few millimeters wide, to the colossal Japanese spider crab, which can have a leg span exceeding 4 meters (13 feet). True crabs exhibit a wide array of forms, including swimming crabs with paddle-shaped legs, land crabs adapted for terrestrial life, and fiddler crabs known for their oversized claws.
Crab Habitats Around the Globe
True crabs inhabit a wide range of environments, demonstrating adaptability. Many species are marine, thriving on the ocean floor, within coral reefs, or in intertidal zones. They are also found in freshwater habitats like rivers, lakes, and streams, with over 800 species adapted to these conditions.
Many true crabs have adapted to terrestrial life, residing in forests, burrowing into land, or climbing trees. While many land crabs must periodically return to water for reproduction, some species exhibit direct development, meaning their offspring hatch as tiny, fully formed crabs, eliminating the need for aquatic larval stages. This broad distribution highlights their ability to colonize and thrive in aquatic and terrestrial environments.
Animals Mistaken for True Crabs
Many creatures resembling true crabs are not classified within the Brachyura infraorder. Hermit crabs, for instance, are often mistaken for true crabs but belong to the infraorder Anomura. Unlike true crabs, hermit crabs possess a soft, asymmetrical abdomen that they protect by occupying discarded mollusk shells.
King crabs, also part of the Anomura, superficially resemble true crabs but are believed to have evolved from hermit crab ancestors. They typically have five pairs of legs, but one pair is often small and tucked away, making them appear to have fewer walking legs than true crabs.
Porcelain crabs are another group that looks similar to true crabs but are classified differently. They are distinguished by having only three visible pairs of walking legs, with the fourth pair being reduced and folded against their carapace, and typically have long antennae.
Horseshoe crabs, despite their name, are not crustaceans at all; they are chelicerates, making them more closely related to spiders and scorpions. These distinctions highlight the convergent evolution, known as carcinization, where various non-crab crustaceans have evolved crab-like body plans.