Crabs are a diverse group of crustaceans, inhabiting a remarkable range of environments across the globe. From the deepest ocean trenches to tropical rainforests and sandy beaches, these adaptable creatures play significant roles in their ecosystems, contributing to nutrient cycling and serving as a food source for many animals.
The Global Count of Crab Species
The number of known true crab species (infraorder Brachyura) is estimated between 7,000 and 7,600. This figure is not fixed, as new species are regularly discovered, particularly in underexplored areas like the deep sea or remote freshwater systems.
The reclassification of existing species also influences the total count. Genetic analysis provides new insights, sometimes splitting single species into multiple distinct ones or grouping previously separate species. This ongoing taxonomic revision continually refines our understanding of crab biodiversity.
Distinguishing True Crabs from Their Relatives
True crabs (infraorder Brachyura) possess distinct morphological characteristics. They are recognized by a short abdomen tucked neatly under a broad, hard carapace. They generally have five pairs of legs: the foremost pair modified into pincer-like claws (chelae), and four pairs of walking legs. Their eyes are often mounted on stalks, allowing for a wide field of vision.
Other crustaceans, like hermit crabs, king crabs, and porcelain crabs, are colloquially called “crabs” but belong to the infraorder Anomura. Anomurans typically appear to have only three pairs of walking legs, as their fourth pair is significantly reduced or hidden. Their abdomens are also generally longer and less tucked than those of true crabs.
The Astonishing Diversity of Crabs
The vast array of true crab species showcases remarkable diversity in size, habitat, appearance, and behavior.
In terms of size, crabs range from the tiny pea crab, a few millimeters across living commensally inside bivalves, to the large Japanese spider crab. The Japanese spider crab can reach a leg span of 12 feet (3.7 meters), making it the largest known arthropod.
Crabs have adapted to diverse environments. Most species reside in marine waters, from coastal shallows to the deep sea. Over 800 species are freshwater crabs found in rivers and lakes. Some have adapted to terrestrial life, such as ghost crabs on sandy beaches or fiddler crabs in salt marshes, though many still require access to water for gill moisture or reproduction.
Appearance varies widely, with carapaces displaying different shapes, textures, and colors, often aiding in camouflage. Ghost crabs, for example, change coloring to blend with sandy surroundings, while some spider crabs decorate shells with sponges or anemones for disguise. Crab behavior is equally diverse, including the burrowing habits of ghost crabs or the distinctive waving of an oversized claw by male fiddler crabs to attract mates.