- Related Topics:
- Hoplocarida
- Phyllocarida
- Eumalacostraca
- Eucarida
- Aeschronectida
External features
The chitinous exoskeleton, or cuticle, covering the body and limbs of malacostracans is divided into segments interconnected by strong, flexible membranes, allowing for articulation at the joints. The cuticle is usually soft and thin in small, wormlike, generally subterranean species, in parasitic species, or in the respiratory surfaces of free-living species. In large, heavy, mostly carnivorous species, the cuticle is highly mineralized or impregnated with calcium salts. Such an exoskeleton provides considerable mechanical leverage and protection to the owner.
Malacostracans, like all arthropods, increase in size by molting. They shed the old cuticle, expand in size, and secrete a new cuticle that subsequently hardens. This process may require several days for completion (in some hard-shelled bottom dwellers). The animals remain in sheltered locations until the new exoskeleton is hardened.
Internal features
The nervous system and sensory organs
The malacostracan central nervous system consists, in primitive forms, of a ventral nerve cord and ganglia within each body segment. The supraesophageal ganglion innervates the eyes, antennules, and antennae, and the subesophageal ganglion innervates the mouthparts of the head region. In amphipods and anomuran decapods the ganglia of abdominal segments are variously fused. In brachyuran decapods the abdominal and thoracic ganglia are fused into a single central thoracic ganglionic centre.
Nearly all surface-dwelling members have pigmented eyes, but these are usually reduced or totally lost in underground and deep-sea species. Crustacean eyes are compound (as in insects) and may be composed of thousands of individual facets, or ommatidia. The compound eyes of most malacostracans and their advanced larval stages are located on a movable stalk. The overall image is formed by combining the images from many individual ommatidia. The compound eye is especially sensitive to movement and has a wide field of vision, often more than 180°. This is an enormous advantage to large, predatory malacostracans.
The eyes of smaller, mainly benthic, nonpredatory malacostracans, such as amphipods, hemicarideans, and isopods, are located on small lobes or flat on the sides of the head. Except in predatory or nocturnal amphipods, the eyes are small and consist of only a few facets. Light that may strike a large patch of facets is concentrated on one ommatidium. Such eyes provide poor visual acuity. Compound eyes can discriminate colour, initiating changes within skin cells to match the colour of the substratum.
Olfactory hairs, or esthetascs, are used to locate food and recognize other crustaceans and their sexual states. Tactile setae occur generally over the external surfaces and appendages, especially of the antennae, food-gathering limbs, and mouthparts. Tactile hairs are present in the statocysts (organelles of balance) located, for example, in the first peduncular segment of the antennules in amphipods and the superorder Eucarida.
Some decapods and amphipods are sensitive to pressure change. Minute pit sensory organs of the general body surface are suspected receptors. Many decapods and amphipods produce sound by striking (percussion) or rasping (stridulating) or by internal mechanisms. Organs of sound reception include, in brachyurans, the chordotonal organs on the hinges of walking legs. Highly specialized sound and vibration receptors include the antennal calceoli of amphipods, the individual microstructure of which consists of receiving elements arranged serially and attached to the antennal segment by a slender stalk. In more-advanced groups the basal elements are expanded into a cuplike receptacle, and the stalk is distally expanded into a bulla, or resonator. In highly advanced predatory amphipods two types of calceoli are found: one type is used to detect mates (found in males only), and the other is used to detect prey (found in both sexes).
Digestion and nutrition
The digestive tract of malacostracans consists of a mouth; an esophagus; a two-chambered foregut; a midgut with outpocketings called digestive glands, or hepatopancreas; and a hindgut, or rectum. The large anterior foregut, or cardiac stomach, occupies much of the posterior aspect of the head and the anterior thoracic body cavity. A constriction separates it from the smaller, more ventral, pyloric stomach that lies in the posterior part of the thorax. Lining the inside of the greatly folded and muscular stomach walls, especially the pyloric portion, are groups or rows of stiff bristles, teeth, and filtering setae known as the gastric mill. The mill is strongly and complexly developed in large decapods, which ingest food quickly and in coarse chunks. The filtering setae are prominent in malacostracans that ingest fine materials or masticate their food thoroughly with the mouthparts. The macerated and partly digested food slowly works its way through the filtering system of the pyloric stomach into the ceca, or pouches, of the hepatopancreas. There enzyme production and the storage and absorption of food takes place. The digestive secretions depend on the species and diet and include cellulase and chitinase. In stomatopods the cardiac stomach is large enough to hold the remains of large prey; it opens directly from the mouth without an intervening esophagus. The midgut, or main intestine, may either extend throughout the abdomen, as in lobsters, or be very short, as in crabs. Fecal material is voided through the anus from the short rectum.
Excretion
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. Antennal nephridial glands are present in the adult stages of eucaridans, mysidaceans, and amphipods and in the larval stages of stomatopods and hemicarideans. The antennal glands of amphipods are enlarged in freshwater forms but are small in terrestrial species. Maxillary nephridial glands are typical of adult stomatopods, syncaridans, hemicaridans, and isopods. Adult leptostracans have both types of glands. Nephrocytes are present at the bases of thoracic legs and elsewhere in the body of mainly primitive groups. Bathynellaceans have a unique uropodal gland. The sternal gills of amphipods are osmoregulators.
Respiration
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.
