Shrimp, as decapod crustaceans, breathe underwater using specialized respiratory organs called gills. While functioning similarly to those in fish, shrimp gills are housed differently. These structures are not exposed to the open water but are protected and maintained by the shrimp’s exoskeleton, allowing them to efficiently extract dissolved oxygen from the surrounding water.
Location and Structure of Shrimp Gills
The gills are sequestered in a protective space known as the branchial chamber, located on either side of the shrimp’s cephalothorax (the fused head and midsection). This chamber is covered and shielded by a lateral extension of the carapace called the branchiostegite, or gill cover. The gills are delicate, feathery, or bushy structures, typically attached to the bases of the walking legs (pereopods) and sometimes the feeding appendages (maxillipeds).
The physical design maximizes the surface area available for gas exchange. Each gill consists of a central axis from which numerous thin, lateral filaments or lamellae project. These structures are coated in a thin cuticle and are highly vascularized with blood channels, allowing for the rapid passage of gases across the membrane.
The Mechanism of Gas Exchange
Breathing involves the movement of oxygen from the water into the shrimp’s circulatory fluid, the hemolymph. Dissolved oxygen diffuses across the thin gill membranes into the hemolymph vessels within the filaments. Simultaneously, carbon dioxide diffuses out of the hemolymph and into the water flowing through the branchial chamber.
The efficiency of this transfer is significantly enhanced by a biological principle known as countercurrent exchange, a common strategy in aquatic organisms. In this system, the hemolymph flows through the gill filaments opposite to the flow of water across the gill surface. This maintains a continuous concentration gradient, ensuring the hemolymph always encounters water with a higher oxygen concentration, allowing for the absorption of up to 90% of the available oxygen. Once oxygenated, the hemolymph transports the gas throughout the body using hemocyanin. This copper-containing respiratory protein is dissolved directly in the hemolymph and turns a faint blue when bound to oxygen, delivering it to tissues for metabolic activity.
Protection of the Respiratory System
The branchiostegite protects the fragile gills from external trauma. This enclosed space requires a constant, active flow of fresh, oxygenated water to prevent oxygen depletion and the buildup of waste gases. Water is typically drawn into the gill chamber from the posterior and ventral openings.
The primary mechanism for generating this current is the rapid, rhythmic movement of a specialized appendage called the scaphognathite, or gill bailer. The paddle-like scaphognathite, which is part of the second maxilla, is located at the anterior end of the chamber and actively pushes water out. This expulsion creates a negative pressure that continually draws a fresh stream of water over the gills from the rear, ensuring a steady supply of oxygen. Other appendages, such as specialized epipods and exopods, are equipped with fine setae that act as cleaning brushes, sweeping over the gill surfaces to remove fouling particles and maintain respiratory function.