Crabs primarily rely on gills for respiration, similar to fish. However, their ability to survive in diverse habitats, from deep-sea trenches to high-altitude forests, has led to the evolution of specialized adaptations. In the most terrestrial species, these adaptations are functionally similar to lungs. The confusion stems from the fact that many species have transitioned to a semi-terrestrial or fully terrestrial lifestyle, necessitating a change in how their respiratory structures function.
Gills: The Primary Respiratory System
Crabs possess gills housed within the branchial chamber, located under the carapace near the walking legs. These gills are composed of numerous delicate, feathery, or lamellar structures that provide an immense surface area for gas exchange. This large, thin surface is where oxygen is extracted from the surrounding water and carbon dioxide is released from the crab’s hemolymph.
The process of aquatic respiration is managed by a paddle-like appendage called the scaphognathite, or gill bailer, located near the mouthparts. This structure beats rhythmically, creating a continuous current that draws water into the branchial chamber and pumps it over the gill filaments. The oxygen in the water then diffuses across the thin gill membranes into the hemolymph.
Breathing Air: Strategies for Semi-Aquatic Crabs
Many species, such as shore crabs inhabiting the intertidal zone, spend significant time out of water but still use their standard gill apparatus for air breathing. For these gills to remain functional in air, they must be kept moist, since gas exchange requires a wet surface. Semi-aquatic crabs retain a small amount of water within the branchial chamber when they emerge from the sea.
The retained water serves to keep the delicate gill tissue from collapsing or drying out, which would prevent oxygen uptake. These crabs then draw air into the moist chamber, and oxygen from the air dissolves into the thin water film covering the gills before diffusing into the hemolymph. The branchial chamber is sealed by articulating plates, helping to preserve the internal moisture required for this temporary aerial respiration.
True Land Crabs and Specialized Adaptations
Crabs that are almost entirely terrestrial, like the Gecarcinidae family or the coconut crab (Birgus latro), have evolved sophisticated modifications to their respiratory system. These species have developed structures known as “branchiostegal lungs,” which are functionally similar to the lungs of vertebrates. The specialization involves a significant reduction in the number and surface area of the traditional gills, which is an adaptation to reduce water loss through evaporation.
Instead of relying solely on the gills, the walls of the branchial chamber itself have become highly vascularized with a rich network of blood vessels. This thickened, folded lining of the chamber acts as a dedicated air-breathing surface, essentially converting the branchial chamber into a lung. In these specialized land crabs, the remaining gills often shift their primary role to osmoregulation, helping the crab manage salt and water balance rather than just respiration.