Most large malacostracans respire through gills, which develop as vascularized outgrowths of the first segment of the thoracic legs (epipodal gills). The gills of decapods are in a branchial chamber beneath the carapace, and oxygenated water is funneled through them. The lining of the chamber itself may be soft and vascularized for respiration, as in mysids, thermosbaenaceans, hemicarideans, and peneid shrimps. Land crabs have larger and more vascularized branchial chambers than do aquatic crabs. Land crabs also possess specialized chambers for keeping the gills moist.

The epipodal gills in syncarids and euphausiids are unprotected, since a carapace is either lacking or does not cover the leg bases. In amphipods the gills are usually simple sacs or plates, which in the course of evolution have migrated to the inner side of the legs. The gills are fanned and oxygenated by the pleopods in the ventral tunnel formed by the coxal plates. In stomatopods and isopods gill-like outgrowths of the pleopods or invaginated pseudotracheae (in terrestrial isopods) are the main organs of respiration.

Gases diffuse across the respiratory surface. Since the chitinous material of the body wall is relatively impermeable, special mechanisms have evolved to boost oxygen uptake. These include increased surface area (dendritic, foliate, pleated, or “double” gills), rich vascularization of respiratory surfaces, ventilating mechanisms (current-directing exopods and baler plates of the maxillae and maxillipeds), and presence in the blood of special respiratory pigments such as hemocyanin (which contains copper).

Blood vascular system

Malacostracans have a more complex open circulatory system than do other crustaceans. The single-chambered heart is enclosed in a pericardial sinus and is located dorsally, above the gut. It is elongate and tubular with several holes (ostia) for return flow in primitive forms (orders Leptostraca and Stomatopoda and the superorder Syncarida), but it is short and boxlike with one to two ostia and located in the thorax in advanced forms (decapods). The blood, or hemolymph, is pumped to the head through an aorta and to the gills and locomotor appendages through lateral and ventral arteries. Veins are lacking, and the blood returns to the heart via a series of sinuses.

Endocrine system and hormones

The major neuroendocrine control centre of malacostracans is the X-organ–sinus-gland complex, which lies in the eyestalk or in an equivalent part of the head in which the eyes are sessile. This complex regulates maturation, dispersal of pigments in the eye and for body colour change, and some metabolic processes, including molting. The female’s ovaries, the male’s reproductive glands, the pericardial organs, and the maxillary Y-organs of decapods also produce hormones that function in the molt and reproductive cycles.

Defense and aggression

Malacostracans must compete for food, shelter, space, and mates. Hermit crabs fight over shells to occupy, stomatopods and alpheid shrimps fight over shelters, and terrestrial crabs and tube-building amphipods contest burrows and domiciles. Males of many species grow enlarged and embellished appendages at maturity for use in fighting and winning mates. Fights to determine status range from highly ritualized displays to death struggles. In decapods the most aggressive fighters are aquatic species, which are well armed, meet infrequently, and compete only occasionally over patchy, ephemeral resources, including females. Terrestrial species, which are more prone to injury, more social, and less limited by availability of resources, exhibit more complex, formalized interactions. Male fiddler crabs attract females by waving the enlarged claw and sending sound signals. The signals establish the identity and intent of the sender. Male ghost crabs build sand pyramids to attract females. Numerous shrimps and some amphipods snap the movable finger of the enlarged claw against the hand as part of threat displays and courtship signals. Many stomatopods have a colour-coded, species-specific eyespot on the claws, which is displayed during posturing. More aggressive species have brighter eyespots. Stomatopods that fight with the same or closely related species reduce the force of their blows or engage in ritualized combat. Relatively docile species are more aggressive when facing more bellicose neighbours. An elaborate set of courtship signals is needed by the male stomatopod to prevent the female from attacking him.