Malacostracans represent the second-largest class of crustaceans, Malacostraca, encompassing over 40,000 living species, including shrimps, krill, crabs, amphipods, isopods, and decapods, divided into 16 extant orders. These arthropods are distributed globally (except in Antarctica), occupying both marine and freshwater environments, with some species even adapted to terrestrial life. They feed on plankton, aquatic invertebrates, carcasses of large fishes and aquatic mammals, as well as bacteria and other microbes.
They display a wide variety of body forms, having a segmented body with three divisions: head, thorax, and abdomen, with each division having multiple segments and appendages.
Most of these animals are 0.3 to 1.1 in (1 to 3 cm) long. However, some free-living malacostracans of the orders Amphipoda, Isopoda, and Stomatopoda measure 0.8 to 1 in (25 to 30 cm) in length.
Their bodies are partitioned into three broad divisions or tagmata: head, thorax, and abdomen. A dorsal carapace may sometimes cover these tagmata.
The first part of the body, the head, contains five segments and two pairs of antennae. The first pair is usually biramous (two-part), while the second pair contains exopods or outer branches that flatten into antennal scales called scaphocerites.
The mouthparts consist of one pair each of mandibles, maxillules, and maxillae. The head also features compound eyes, which are typically mounted on movable stalks.
This region has eight segments, each bearing a pair of appendages. Generally, these appendages are biramous, with the endopods (inner branches) specialized for crawling or grasping. The endopod consists of seven segments: the coxa, basis, ischium, merus, carpus, propodus, and dactylus. In some species, the exopods (outer branches) are absent, making the thoracic appendages uniramous. In decapods, the dactylus and propodus articulate to form a functional claw. Additionally, up to three thoracic segments are often fused with the head, forming a cephalothorax, where their appendages are modified into accessory mouthparts known as maxillipeds.
The abdomen has six segments and a telson, with each segment, except the last, bearing a pair of biramous pleopods that are specialized appendages used for swimming, burrowing, gas exchange, or brooding. In males, the first and second pairs of pleopods are often modified into gonopods for reproductive purposes. The pleopods on the last segment usually develop into flat uropods, forming a tail fan that aids in locomotion.
In Leptostraca, the appendages on the telson are modified into spine-like structures known as caudal rami. This group also retains the ancestral condition of seven abdominal segments.
Like other crustaceans, their body is covered by a chitinous exoskeleton called the cuticle. To correspond with the body segments, the cuticle is also partitioned, though they are interconnected by strong, flexible membranes at joints.
In smaller subterranean species, the cuticle tends to be soft and thin, whereas in larger malacostracans, particularly those that are carnivorous, the cuticle becomes hard, calcified, and heavily mineralized.
Malacostracans have an open circulatory system, but its complexity varies from that of other crustacean groups. The dorsal heart is single-chambered, located above the gut, and is encased in a pericardial sinus.
In primitive forms, like mantis shrimps, the heart is tubular and elongated with multiple holes or ostia for return flow, while in relatively advanced forms, such as decapods, it is short and boxlike, having one or two ostia.
Most large malacostracans use their gills for gaseous exchange. These gills originate from the first segment of the thoracic legs and are hence called epipodal gills. The respiratory pigment found in this group is copper-based hemocyanin.
Because the outer surface of these crustaceans is largely impermeable and prevents gas exchange, their gills are specially adapted to enhance oxygen absorption. The gill surface is often modified into intricate structures such as dendritic, foliate, or pleated patterns to maximize surface area. It is also highly vascularized with blood vessels to ensure ventilation.
The digestive tract is straight and divided into a mouth, an esophagus, a two-chambered foregut or stomach, a midgut containing digestive glands called hepatopancreas, and a hindgut or rectum.
The anterior part of the foregut is large and is called the cardiac stomach, which is separated from the smaller, posterior pyloric stomach by a constriction. The cardiac stomach contains a gizzard-like gastric mill for grinding the ingested food particles. This structure has walls made of chitinous ridges, teeth, and calcareous ossicles.
The hepatopancreas in the midgut has pouches where digestive secretions are released, and chemical processing and absorption of the food occurs. The hindgut is the site for water reabsorption and formation of feces, followed by the anus at the base of the telson.
These crustaceans possess excretory organs called nephridial glands through which they expel fluid wastes. These glands are located in the basal segments of the second antennae and the maxillae.
While antennal nephridial glands are found in groups, like adult eucaridans, mysidaceans, and amphipods, as well as in larval stomatopods and hemicarideans, maxillary nephridial glands are common in stomatopods and isopods. In one particular group, the bathynellaceans and uropodal glands help with excretion.
In primitive forms, the central nervous system is characterized by a ventral nerve cord and ganglia within each body segment. Two main ganglia, the supraesophageal and the subesophageal, innervate the different organs and body parts. While the supraesophageal ganglion innervates the eyes, antennules, and antennae, the subesophageal ganglion innervates the mouthparts.
In amphipods and some decapods, like anomurans, the ganglia of abdominal segments are fused, whereas, in brachyuran decapods, the abdominal and thoracic ganglia are fused to form a central thoracic ganglionic center.
In 1802, French zoologist Pierre André Latreille coined the term Malacostraca for this class of crustaceans. The name has its roots in the Greek words malakós, meaning soft, and óstrakon, meaning shell. However, such taxonomy is misleading since these crustaceans become hard-shelled upon reaching adulthood.
Based on morphological and molecular evidence, class Malacostraca is universally considered a monophyletic group; however, the relationship within its orders remains uncertain because of their differences in mutation rates and evolutionary patterns.
According to the phylogenetic analysis of Richter & Scholtz (2002), malacostracans are classified under 3 subclasses and 16 extant orders.
The status of Phyllocarida, initially based on the origin of their limbs, was long debated. However, American biologists Trisha Spears and Lawrence Abele provided phylogenetic evidence that supports its designation as a subclass that had diverged from the main lineage.
Currently, malacostracans are abundant in almost all continents except Antarctica, where they once lived. Though mostly marine, they are also found in subterranean fresh waters.
Some isopods, like woodlice, and a few amphipods, such as sand fleas, are terrestrial. Additionally, terrestrial crayfish and hermit crabs, land crabs, and marsh crabs are decapods that live on land.
Malacostracans feed on any kind of organic matter available, both plants and animals.
Small to medium-sized malacostracans primarily feed on plankton or act as parasites on aquatic animals like fish, marine mammals, and reptiles. In contrast, larger species, such as mantis shrimp, are mainly carnivorous, preying on smaller invertebrates. They also scavenge, feeding on the remains of dead whales, seals, fish, and other large invertebrates.
Some malacostracans, like porcelain crabs, are filter feeders that consume bacteria and other microbes.
These crustaceans are swimmers that use the synchronized beating of their abdominal appendages to move efficiently. In the early larval stages, they rely on head appendages for movement, transitioning to thoracic appendages in the later stages. Certain groups, such as krill, decapods, and leptostracan shrimps, continue to use their thoracic legs for swimming as adults. Amphipods, on the other hand, utilize their tail fin for quick and forward movement in short bursts. Some malacostracans also walk or burrow into sediments. Decapods and stomatopods, for example, use the pincer-like tips of their legs to dig and clear away sediment.
They typically obtain food using their thoracic appendages; however, early free-swimming larvae and adults of some malacostracans, like amphipods and isopods, use their antennae and other head appendages to the same. Except for leptostracans, all of these crustaceans gather food using their maxillipeds and pass them on to the mouth for chewing.
Like all members of the animal kingdom, malacostracans, too, compete persistently for food, shelter, space, and mates. For instance, stomatopods, hermit crabs, and alpheid shrimps battle over shelter, while terrestrial crabs and tube-building amphipods fight over burrows to occupy.
Many shrimps and amphipods snap their enlarged claws to threaten their competitors.
Most malacostracans, except a few hermaphroditic isopods, are gonochoric, with separate male and female sexes. While the female gonopores (genital openings) are located on the sixth thoracic segment or appendages of their bodies, male gonopores are found on the eighth (sometimes seventh).
Malacostracans engage in a number of courtship behaviors to attract potential mates. For example, male fiddler crabs wave their enlarged claws and emit specific sounds to attract females, while ghost crabs build sand pyramids to exhibit their sexual fitness. A few other male stomatopods display the eyespots on their claws to appeal to the females.
In primitive species, males locate females in open water, usually timed with environmental cues such as temperature cycles, lunar phases, or resource availability. They take only a few seconds to copulate. However, in advanced, bottom-dwelling species, mating occurs on or within the sediments of the floor. The males often guard the females or carry them around (preamplexus), after which they copulate (amplexus) for hours.
In most malacostracans, fertilization is external, with females releasing their eggs from the gonopores into the surrounding water, where males fertilize them almost instantly. However, in some groups like isopods and stomatopods, males transfer sperm to females using their first and second pleopods, causing internal fertilization.
In most malacostracans, the fertilized eggs usually hatch into free-swimming, planktonic nauplius larvae. However, in some groups, like decapods, the eggs are brooded in the pleopods and hatch as late-stage larvae, such as zoeae and phyllosoma larvae. These larvae gradually swim or sink to the bottom and undergo multiple stages to develop into juveniles.
In some malacostracans, like isopods, embryos develop in thoracic brood pouches, bypassing the larval stages. These embryos generally hatch directly as immature adults.
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