Crayfish possess gills, a feature shared with other aquatic crustaceans like shrimp and lobsters. These freshwater organisms rely on these specialized organs to extract dissolved oxygen from the water. Unlike the prominent gills of a fish, a crayfish’s respiratory structures are concealed beneath the hard, protective outer shell, or carapace, covering the cephalothorax (the fused head and mid-body section). This design provides protection and creates a unique environment for respiration.
Anatomy and Placement of Gills
The gills are housed within a pair of specialized spaces called the branchial chambers, located on either side of the cephalothorax. The carapace forms a protective dome over these chambers, shielding the delicate respiratory tissues from damage. The outer shell is not fully fused along the sides, allowing water to enter the chamber from the ventral edge.
The gills are firmly attached to the body wall and the base segments of the walking legs. They are feather-like structures, consisting of numerous filaments that create a large surface area for gas exchange. Movement of the walking legs subtly helps circulate water within the branchial chamber.
The Mechanics of Respiration
Effective aquatic respiration requires a constant, unidirectional flow of water over the gill surfaces. The crayfish achieves this current using a specialized appendage called the scaphognathite, or gill bailer. This structure, a modified mouthpart, is located at the front of the branchial chamber.
The scaphognathite beats rapidly, creating negative pressure that draws water into the rear opening of the chamber. This action forces the water forward, over the gill filaments, and out through an exit near the head. The beating frequency is influenced by the surrounding oxygen concentration; in low-oxygen environments, the crayfish beats the bailer faster.
As water passes over the filaments, dissolved oxygen diffuses across the membranes into the circulatory fluid, called hemolymph. This process is driven by the concentration gradient. Simultaneously, carbon dioxide diffuses from the hemolymph back into the water and is expelled from the chamber.
Adaptations for Out-of-Water Survival
Although crayfish are fully aquatic, they have a limited, temporary ability to survive outside of water. The gills function only as long as their membranes remain wet, which prevents them from collapsing and allows gas exchange. The carapace is instrumental in this survival, sealing the branchial chamber and retaining a small reservoir of water around the gills.
When the gills are kept moist, the crayfish can extract oxygen from the retained water and directly from the surrounding air, making them facultative air-breathers. This adaptation allows for short terrestrial excursions, such as migrating or searching for food. This mechanism is temporary; if the moisture evaporates and the gills dry out, gas exchange ceases, leading to respiratory failure.