Crustaceans are mainly a group of free-living aquatic arthropods that belong to the subphylum Crustacea. This subphylum includes prawns, crabs, lobsters, shrimp, krill, barnacles, and seed shrimp, among others. Crustaceans are recognized by their biramous (two-part) appendages and unique larval forms, such as the nauplius larva of those belonging to their classes Copepoda and Branchiopoda.
Although most are aquatic, some, like woodlice and sandhoppers, are terrestrial, whereas fish lice and tongue worms are parasitic and remain attached to their hosts.
The 67,000 characterized species range from 0.1 mm (0.004 inches) in body length, as found in the smallest crustacean Stygotantulus stocki, to the Japanese Spider Crabs, the largest in the world, measuring up to 12.5 feet (3.81 meters) from the tip of one front claw to the other.
A crustacean’s body is segmented and divided into three broad parts: the head or cephalon, the thorax or pereon, and the abdomen or pleon. The head and the thorax are often fused to form the cephalothorax, encased by a single covering or carapace. The entire body is encased in a hard exoskeleton, which molts as the animal grows.
Each body segment is called a somite and bears a pair of appendages. Each somite is encased in a shell that consists of a dorsal tergum, a ventral sternum, and a lateral pleuron. In some cases, different segments of the exoskeleton are fused together. The head segments have modified appendages like the sensory antennae and the two mouthparts, the maxillae and mandibles. The thoracic segments bear the walking legs (pereiopods) and the feeding legs (maxillipeds). Although most crustacean classes bear no limbs in the abdominal segment, those belonging to class Malacostraca and Remipedia possess abdominal appendages. Malacostracans bear pleopods or special swimming legs in their abdomen, culminating in a telson, which contains the anus.
Each appendage is typically divided into two parts (biramous), except the first pair of antennae, which is generally unbranched (uniramous). In malacostracans, the antennules are even found to be triramous, with three distinct parts.
They have an open circulatory system, with blood pumped into the hemocoel through channels called sinuses that lack a definitive wall. The heart, if present, lies dorsally within a sac called the pericardium. In contrast, crustaceans belonging to the classes Copepoda and Ostracopoda lack a heart.
Small crustaceans, such as copepods, have no specialized respiratory organs and thus breathe through their entire integument. Many malacostracans and fairy shrimps possess gills formed by modification of their appendages. Similarly, a few land crabs that undergo aerial respiration have lung-like branchial chambers with a rich supply of blood vessels to facilitate gaseous exchange.
In most crustaceans, the digestive tract, or gut, is simple and straight; however, those belonging to the order Anomopoda have coiled guts. Their foregut is often modified into a complex, chitinized structure called the gastric mill, which consists of a series of calcified plates or ossicles serving as a grinding apparatus.
The midgut is divided into one or more diverticula (pouch-like structures), followed by a relatively simple, ciliated hindgut.
Crustaceans have two types of excretory organs, the antennal gland and the maxillary gland, which share the same basic structure. These organs possess an end sac, followed by a long, convoluted duct that sometimes expands into a bladder before opening to the exterior.
While aquatic forms excrete their nitrogenous waste in the form of ammonia (ammonotelic), terrestrials excrete uric acid (uricotelic).
The nervous system of crustaceans comprises a brain, or supraesophageal ganglion, connected to a long ventral nerve cord. A subesophageal ganglion is present under the esophagus, formed by the fusion of ganglia from the mandibular and maxillary segments.
They have compound eyes composed of multiple visual units called ommatidia, capable of forming mosaic images. In contrast, the nauplius larva has a single median eye comprising three or four simple units innervated by a median nerve from the forebrain.
Crustacea is universally considered a subphylum under the clade Mandibulata. Although the exact phylogenetic relationship between the subphylum and other taxa is unknown, recent DNA sequence studies suggest that it is paraphyletic, with Hexapoda (insects and entognathans) placed as a subgroup of Crustacea under the larger Pancrustacea clade.
The traditional scheme of classification based on morphological features suggested four to six classes under the subphylum Crustacea. Bowman and Abele (1982) identified 652 extant families and 38 orders and organized them into six classes: Branchiopoda, Remipedia, Cephalocarida, Maxillopoda, Ostracoda, and Malacostraca. However, in 2001, Martin and Davis revised the classification into 849 extant families in 42 orders.
Under the current classification, the 67,000 described species (1/10 to 1/100 of all existing crustacean species) are divided into ten to twelve classes, including the former maxillopod subclasses Thecostraca, Tantulocarida, Mystacocarida, Copepoda, and Branchiura.
The classes Cephalocarida, Branchiopoda, and Remipedia are considered to be more closely related to the hexapods than any other crustaceans.
Although crustaceans primarily inhabit marine or freshwater habitats, a few, like woodlice, terrestrial crabs, and hermit crabs, live on land. Sea lice, fish lice, whale lice, and tongue worms are parasitic and constantly remain attached to the bodies of their hosts.
Since the formation of the Suez Canal, around 100 crustacean species from the Red Sea and the Indo-Pacific region have become invasive to the East Mediterranean waters. Similarly, the Chinese mitten crab and the Asian shore crab are also invasive to Southeast Asia.
Crustaceans are omnivores that feed on other invertebrates, like worms, squids, starfish, and snails, apart from plant matter like algae.
Although most crustaceans have separate sexes and reproduce sexually, barnacles and remipedes are hermaphroditic (with different sex organs in males and females). In some branchiopods, isopods, and ostracods, reproduction occurs asexually through parthenogenesis, where the females produce viable eggs without being fertilized by males.
In sexually reproducing species, the fertilized eggs are released into water or held onto the parent’s bodies until they are ready to hatch. Crustaceans from the orders Isopoda, Notostraca, and Anostraca often carry their eggs in an external brood pouch derived from the carapace and thoracic appendages. Similarly, krill and most members of the order Leptostraca hold their eggs between their thoracic limbs, while copepods bear them in special thin-walled sacs.
The eggs hatch into multiple larval forms, beginning with the characteristic nauplius larva. This form is characterized by a single naupliar eye and three pairs of appendages that emerge from the head. In most crustaceans, the naupliar stage is followed by the zoea larva, which swims with the help of its thoracic appendages. The zoea is the first larval stage in many members of the order Decapoda that witness accelerated growth. Based on the species, the zoea is followed either by the mysis or megalopa stages.
A thin layer of crystalline isoxanthopterin covers the swimming larvae’s eyes. This compound gives their eyes the same color as the surrounding water, while the tiny holes in the layer allow light to pass to the retina.
As the larva matures, the layer migrates to a new position behind the retina, where it functions to scatter light, increasing the intensity of light that can enter the eye, as found in many nocturnal creatures.
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