Crustacean Anatomy: Shrimp, Crayfish and Crabs
Aquarium crustaceans are armoured arthropods that moult to grow and carry copper-based blood. Learn the body plan, appendages, gills and why they are metal-sensitive.
Aquarium crustaceans (dwarf shrimp, crayfish and crabs) are decapods, a group of arthropods with a hard external skeleton and jointed limbs. Their body plan, the way they grow, and the chemistry of their blood all explain why they need stable water and are unusually sensitive to copper and other metals.
Body plan
The decapod body is built from segments grouped into two main parts: the cephalothorax, where the head and thorax are fused and covered by a shield called the carapace, and the abdomen (pleon) behind it. The carapace protects the internal organs and the gills. The whole body is enclosed in a chitinous exoskeleton.
Appendages
- Antennules and antennae: sensory feelers for touch and chemical detection
- Mouthparts: mandibles, maxillae and maxillipeds that handle and process food
- Pereiopods: the ten walking legs that give decapods their name; those bearing a claw are called chelipeds
- Pleopods (swimmerets): abdominal limbs used for swimming and, in females, for brooding eggs (a 'berried' female carries eggs under the abdomen)
- Tail fan: the telson plus a pair of uropods, used for steering and the rapid escape flip
The exoskeleton and moulting
Because the exoskeleton is rigid and cannot expand, a crustacean must shed it to grow, a process called moulting (ecdysis). It periodically casts off the old shell and forms a larger one. Right after moulting the new shell is soft and the animal is vulnerable, so it hides until the shell hardens. Building a new exoskeleton draws on minerals from the water, which is one reason stable, mineral-adequate water matters so much for shrimp and crayfish.
Respiration, circulation and copper blood
Crustaceans breathe through gills tucked under the carapace and have an open circulatory system, in which a heart pumps haemolymph (their blood) into an open body cavity rather than through a closed loop of vessels. Their oxygen-carrying pigment is haemocyanin, which is copper-based rather than the iron-based haemoglobin of vertebrates; oxygenated haemocyanin gives the blood a bluish tinge instead of red. They also have compound eyes, often mounted on movable stalks that give a wide field of view.