Most crustaceans breathe through gills that extract dissolved oxygen from water, but the group is so diverse that breathing methods range from simple diffusion through the skin to air-breathing lung-like structures. How a crustacean gets its oxygen depends largely on its size, where it lives, and how far its lineage has ventured onto land.
How Gills Work in Crabs, Lobsters, and Shrimp
The gills of large crustaceans like crabs and lobsters sit inside a protected space called the branchial chamber, tucked beneath the shell on either side of the body. Each gill has a central shaft with a double row of thin, flat plates (called lamellae) extending outward, creating a huge amount of surface area packed into a small space. Inside each lamella, a thin wall separates the blood flow into two compartments, keeping incoming and outgoing blood organized so oxygen transfer stays efficient.
Water doesn’t just passively wash over these gills. Crustaceans actively pump it. A paddle-shaped structure near the mouthparts, called the scaphognathite or “gill bailer,” beats rhythmically to draw water into the branchial chamber and push it back out. In shore crabs, this pumping system is surprisingly costly: it accounts for roughly 30% of a resting crab’s total oxygen consumption. That’s a significant energy investment just to breathe, which helps explain why crustaceans are sensitive to anything that makes gas exchange harder, like warming water or low oxygen levels.
The gills use a countercurrent exchange system, where water flows across the gill surface in the opposite direction to the blood flowing inside it. This arrangement is remarkably effective. Because there’s always a difference in oxygen concentration between the water and the blood at every point along the gill, the system can absorb up to 90% of the dissolved oxygen passing through.
Copper Blood Instead of Iron
Crustacean blood looks nothing like yours. Instead of the iron-based hemoglobin that makes mammalian blood red, crustaceans carry oxygen using hemocyanin, a large copper-based protein. When hemocyanin binds oxygen, it turns the blood pale blue. When it releases oxygen to the tissues, the blood becomes colorless or slightly gray.
Hemocyanin molecules are enormous compared to hemoglobin. A single hemocyanin complex in a horseshoe crab (a close relative of crustaceans) contains 96 pairs of copper atoms, each pair binding one oxygen molecule. That means one hemocyanin complex carries 96 oxygen molecules, compared to the four that a single hemoglobin molecule handles. This large carrying capacity per molecule helps compensate for the fact that hemocyanin floats freely in the blood rather than being packed inside red blood cells the way hemoglobin is. The overall oxygen-carrying capacity of crustacean blood is lower than mammalian blood, but it works well for the metabolic demands of cold-blooded animals living in water.
Tiny Crustaceans Skip the Gills Entirely
Not every crustacean needs dedicated breathing organs. Zooplankton like copepods and the larvae of larger species are small enough that oxygen simply diffuses through their body surface. No gills, no pumping structures, no branchial chambers. At their scale, the ratio of skin surface to body volume is high enough that passive diffusion supplies all the oxygen they need.
This has interesting consequences in low-oxygen environments. Smaller copepods have an advantage over larger ones because their higher surface-area-to-volume ratio means more efficient oxygen uptake per unit of body mass. In waters where oxygen levels drop, communities tend to shift toward smaller species for exactly this reason.
How Land Crustaceans Breathe Air
Several crustacean lineages have moved onto land, and each has solved the breathing problem differently. The most familiar example is the woodlouse (a terrestrial isopod), which has evolved a unique structure called a pleopodal lung in its abdominal appendages. In species like the common rough woodlouse, the front two pairs of abdominal flaps develop air-breathing structures complete with tiny openings (spiracles) that let air in, while the rear flaps retain the ancestral gill form. Those rear gills still function, but primarily for regulating water and salt balance rather than for breathing.
This dual system, lungs in front and gills in back, reflects an evolutionary transition caught in progress. The lungs develop during the animal’s growth after hatching, forming through cell migration and new exoskeleton construction at each molt. Woodlice still need moisture to survive, partly because the gill-bearing rear appendages work best when wet, but their pleopodal lungs allow genuine air breathing rather than relying on a film of water over a gill surface.
Land crabs take a different approach. Many species retain functional gills inside their branchial chambers but keep them moist with stored water, periodically refreshing it. Some, like the coconut crab, have gone further, developing spongy tissue lining the inner wall of their gill chamber that functions much like a lung. These crabs can drown if submerged too long because their modified breathing apparatus is optimized for air, not water.
Temperature, Salinity, and Breathing Stress
Because crustaceans are cold-blooded, their metabolic rate rises with water temperature, and so does their oxygen demand. Studies on estuarine crabs show that temperature significantly increases oxygen consumption at every salinity level, with metabolic rates roughly doubling for every 10°C increase in some cases. This creates a dangerous squeeze in warm water: the animal needs more oxygen at the same time that warmer water holds less of it.
Salinity adds another layer of stress. At moderate temperatures, salt concentration doesn’t change breathing rates much. But at high temperatures combined with extreme salinities (either very high or very low), oxygen consumption can spike dramatically. Small crabs are hit hardest. In one study on estuarine crabs, small individuals at 30°C and low salinity nearly doubled their oxygen consumption compared to those in normal seawater. Their higher surface-area-to-volume ratio, which helps with oxygen uptake, also means they lose or gain water and salts faster, forcing their bodies to work harder to maintain balance.
Molting and Temporary Breathing Difficulty
Crustaceans periodically shed their exoskeleton to grow, and molting places real strain on their respiratory system. The process of breaking out of the old shell and expanding a soft new one is physically demanding, and the soft post-molt shell may allow the gills and branchial chamber to function less efficiently until the new exoskeleton hardens.
Respiration rates shift across the molt cycle. In fiddler crabs, post-molt individuals exposed to high salinity showed a threefold increase in oxygen consumption compared to those at low salinity, reflecting the combined energy cost of hardening a new shell and managing salt balance. Under normal conditions, though, respiration doesn’t spike dramatically after molting, suggesting that the stress becomes dangerous mainly when it overlaps with other environmental challenges. This is why crabs in aquaculture or in fluctuating estuarine habitats are most vulnerable right after a molt.