Brachyura, commonly known as true crabs, are decapod crustaceans characterized by a short projecting tail or reduced abdomen, typically hidden under the thorax. They inhabit various environments, from marine and freshwater to terrestrial, and are known for their sideway walking. Crabs play crucial roles in the ecosystem as both predators and prey.

Crabs of the infraorder Brachyura are found in a wide range of geographical locations around the world. They are primarily distributed along the coastlines of all continents except Antarctica. Specific regions include the Atlantic and Pacific Oceans, the Indian Ocean, as well as the Mediterranean and Caribbean Seas. Brachyura species inhabit tropical, temperate, and polar regions, with some species adapted to freshwater and terrestrial environments.

Brachyuran crabs thrive in diverse environmental conditions. Marine species typically inhabit intertidal zones, estuaries, coral reefs, and mangroves, where they experience fluctuating tides and salinity levels. Freshwater crabs are found in rivers, lakes, and streams, while terrestrial species dwell in humid forests and grasslands. Some species are adapted to extreme conditions such as hydrothermal vents and cold polar waters.

Brachyuran crabs play crucial roles in their ecosystems as both predators and scavengers. They contribute to nutrient cycling by breaking down organic matter and serve as prey for a variety of animals, including birds, fish, and mammals. Their burrowing behavior can enhance soil aeration and influence sediment structure in aquatic habitats. Some species are also important in controlling populations of other invertebrates and algae.