Decapods are ten-footed crustaceans of the order Decapoda, which include crabs, lobsters, prawns, shrimp, and hermit crabs. The order contains nearly 15,000 extant species divided into 2,700 genera and some 3,300 extinct species.
These animals first appeared in the Ordovician Period (455 million years ago) and gradually inhabited marine habitats, especially in warm, shallow tropical waters. However, they also live in terrestrial and freshwater environments, feeding on dead and decaying organic matter.
The giant crab (Macrocheira kaempferi) holds the title of the largest decapod and arthropod, with their body spanning 15 in (37 cm) and a weight of approximately 40 lb (18 kg). In contrast, shrimplike macrurous species measure just 0.5 in (1 cm), while some crablike brachyurous species can grow as long as 12 ft (4 m).
Like all crustaceans, their segmented bodies are divided into three parts: head (cephalon), thorax (pereon), and abdomen (pleon). The hindmost segment, the telson, follows the abdomen and bears the anus.
A decapod may possess up to 38 appendages, one pair per segment; however, only ten appendages in the thoracic region are considered true legs.
This region has five pairs of appendages, including the sensory antennae, antennules, and the mouthparts maxillae and mandibles. The distal portions of the mandible or maxilla bear sensory structures called palps. If present, the compound eyes, typically unstalked, are also located in this region.
Together with the thorax, this region is referred to as the cephalothorax. It is covered by a carapace, which projects in front of the eyes as a rostrum.
The thoracic region contains five pairs of walking legs, known as pereiopods, which give decapods their name. In addition, there are three pairs of maxillipeds, specialized appendages used for feeding.
The reproductive organs are located in the third pereiopod in females and the fifth pereiopod in males.
This region contains five pairs of appendages called pleopods. They are the primary swimming legs but are also used for brooding eggs and sweeping prey.
The first one or two pairs of these legs are sometimes modified into gonopods for the males to transfer their gametes to the females. Similarly, in many decapods, like fiddler crabs, one pair of pleopods called chelipeds contains enlarged pincers or chelae. These chelipeds are used, especially by males, during defense.
The telson bears a pair of biramous appendages called uropods that form a fan-like structure called the tail fan. This structure helps in steering the animal forward while swimming.
These animals derive their name from the Greek words deca, meaning ten, and pod, meaning foot, referring to their ten true appendages.
Their classification is based on the anatomy of their gills and legs, as well as their larval development. Depending on these criteria, these crustaceans are broadly divided into two suborders: Dendrobranchiata and Pleocyemata. While the first group comprises prawns, including members colloquially called ‘shrimps,’ the latter includes ‘true shrimp.’ All members of Pleocyemata, excluding the families Stenopodidea and Caridea, are grouped under a clade Reptantia, including isopods that walk rather than swim.
Currently, the two suborders contain about 15,000 extant and 3,300 extinct species under around 2,700 genera. Of these, nearly half are crabs, while shrimps constitute around 3,000 species. Hermit crabs, porcelain crabs, squat lobsters, and other members of the infraorder Anomura constitute around 2500 species.
Molecular clock analyses suggest that these crustaceans first appeared around 455 million years ago in the Ordovician Period. The group Dendrobranchiata, comprising prawns, was the first to diverge from other decapod lineages. To follow, members of the suborder Pleocyemata then diverged from the broad group Reptantia, which includes lobsters and crabs.
During the Jurassic and Cretaceous Periods, with the rise of coral reefs, the species diversity of decapods increased drastically.
Though decapods originated in the Ordovician, the oldest fossils from the genus Palaeopalaemon date back to the Late Devonian Period (around 382 to 372 million years ago).
Decapods mostly live in marine habitats, particularly in warm, shallow tropical waters. Only about 10% of known decapod species inhabit freshwater and terrestrial environments. However, terrestrial decapods migrate for a brief period to the seas for spawning.
Although these crustaceans are typically scavengers, feeding on dead animals lying on the ocean floor, they are also omnivores, consuming both plant and animal matter. Shrimps, lobsters, and some crabs eat worms, mollusks, echinoderms, and other crustaceans.
Though decapods are gonochoric in general and have separate male and female sexes, a few groups, like some true shrimps, are simultaneous hermaphrodites having both male and female reproductive organs.
These animals copulate depending on which stage of their life cycle they are in. For example, male decapods copulate only after their exoskeletons have hardened, whereas some females can copulate only when their shells are soft after a single molt. In most decapods, fertilization is external, where the sperm and ova are released in the surrounding water; however, in some, like true crabs, internal fertilization also occurs.
The fertilized eggs are typically attached to the abdominal appendages of the females, who carry them around till they are ready to hatch. These eggs may hatch into any of the four types of larvae: nauplius, protozoea, zoea, and postlarva, based on their mode of locomotion. However, they typically hatch into zoea larvae, which undergo further development into juveniles.
They are preyed upon by cephalopods, fish, water birds, aquatic mammals, other crustaceans, and other decapods.
Some shrimps, like those in the family Oplophoridae, possess special light organs called photophores, which help them recognize their conspecifics and camouflage against their surroundings through counterillumination.
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