Physiology of Finfish and Shellfish (2+1)

2.3.7. Shellfishes - Crustaceans

In many higher crustaceans (decapods), the excretory glands are located in the head. They are called antennal glands or maxillary glands, depending on whether they open at the base of the antennae or at the maxillae. The two glands are probably never functional together in the same animal, though one may replace the other in the course of development. In the Decapoda, where the antennal gland alone is well-developed in the adult, the maxillary gland sometimes precedes it in the larva. If the tubule adjacent to the excretory pore is green, the gland is called a green gland or nephridia . Crustaceans excrete through nephridia or green glands. Nephridia are invertebrate organs which occur in pairs and function similar to kidneys. They remove metabolic wastes from an animal's body. They serve 2 functions, osmoregulatory and excretory. They are present in many different invertebrate lines. There are two basic types, metanephridia and protonephridia. A metanephridium ('meta' = after)(pl. metanephridia) is a type of excretory gland or nephridium found in many types of invertebrates such as Annelids , Arthropods and Molluscs . It typically consists of a ciliated funnel opening into the body cavity or coelom connected to a duct which may be variously glandularized, folded or expanded (vesiculate) and which typically opens to the organism 's exterior. These ciliated tubules pump water carrying surplus ions , metabolic waste , toxins from food , and useless hormones out of the organism through openings known as nephrostomes . This waste is passed out of the body at the nephridiopore . The primary urine produced by filtration of blood (or a similar functioning fluid) is modified into secondary urine through selective reabsorption by the cells lining the metanephridium.

Most insects and spiders have a excretory system called Malpigian tubes which collects liquid wastes in the gut. Wastes are crystallized and then excreted to reduce weight for flight. Malacostracans excrete waste fluids mainly through the ducts of the nephridial glands, which are present in the body segments of the second antennae and the maxillae. The ducts open on the basal segments of those head appendages.

There are two different types of arthropod excretory systems. In aquatic arthropods, the end-product of biochemical reactions that metabolise nitrogen is ammonia , which is so toxic that it needs to be diluted as much as possible with water. The ammonia is then eliminated via any permeable membrane, mainly through the gills. All crustaceans use this system, and its high consumption of water may be responsible for the relative lack of success of crustaceans as land animals. Various groups of terrestrial arthropods have independently developed a different system: the end-product of nitrogen metabolism is uric acid , which can be excreted as dry material; Malpighian tubules filter the uric acid and other nitrogenous waste out of the blood in the hemocoel, and dump these materials into the hindgut, from which they are expelled as feces . Most aquatic arthropods and some terrestrial ones also have organs called nephridia ("little kidneys"), which extract other wastes for excretion as urine .