Dendrobranchiata is a suborder of decapods, commonly known as prawns (though this may be ambiguous). There are 540 extant species in seven families, and a fossil record extending back to the Devonian. They differ from related animals, such as Caridea and Stenopodidea, by the branching form of the gills (hence their scientific name Dendrobrachiata) and by the fact that they do not brood their eggs, but release them directly into the water. They may reach a length of over and a mass of , and are widely fished and farmed for human consumption.
Nomenclature
While Dendrobranchiata and Caridea belong to different suborders of Decapoda, they are very similar in appearance, and in many contexts such as commercial farming and fisheries, they are both often referred to as "shrimp" and "prawn" interchangeably. In the United Kingdom, Australia and some other Commonwealth nations, the word "prawn" is used almost exclusively, while the opposite is the case in North America. The term "prawn" is also loosely used to describe any large shrimp, especially those that come 15 (or fewer) to the pound (such as "king prawns", yet sometimes known as "jumbo shrimp").
Due to their superficial similarity, Dendrobranchiata and Caridea were historically grouped together, though it is currently understood that they are distantly related. The classification of these groups is explained below.
Description Together with other swimming Decapoda, Dendrobranchiata show the "caridoid facies", or shrimp-like form.Tavares & Martin, 2010, p. 100 The body is typically robust, and can be divided into a cephalothorax (fusion of the head or cephalon and thorax) and a pleon (abdomen). The body is generally slightly flattened side-to-side. The largest species, Penaeus monodon, can reach a mass of and a length of .Dall, 1990, pp. 3–4
Head
The most conspicuous appendages arising from the head are the antennae; the first pair are biramous (having two flagella), except in Luciferidae, and are relatively small.Tavares & Martin, 2010, p. 106 The second pair can be 2–3 times the length of the body and are always uniramous (having a single flagellum). The mouthparts comprise pairs of mandibles, maxillules and maxillae, arising from the head, and three pairs of maxillipeds, arising from the thorax.Tavares & Martin, 2010, pp. 106–108 A pair of stalked eyes points forwards from the head.Tavares & Martin, 2010, p. 102
Thorax The carapace originates from the thorax to cover the cephalothorax, and extends forwards between the eyes into a rostrum. This is only as long as the stalked eyes in Benthesicymidae, Luciferidae and Sergestidae, but considerably longer in Aristeidae.
As well as the three pairs of maxillipeds, the thorax also bears five pairs of pereiopods, or walking legs; the first three of these end in small chelae (pincers).Tavares & Martin, 2010, pp. 108–110 The last two pereiopods are absent in Luciferidae and Acetes, but much longer than the preceding pereiopods in Hymenopenaeus and Xiphopenaeus.Tavares & Martin, 2010, p. 110
The thoracic appendages carry gills, which are protected beneath the carapace.Tavares & Martin, 2010, pp. 103–105 The gills are typically branched, and so resemble trees, lending the group its scientific name, Dendrobranchiata, from the Greek words (, tree) and (, gills).Tavares & Martin, 2010, p. 103
Pleon The pleon, or abdomen, is similar in length to the cephalothorax.Tavares & Martin, 2010, p. 105 It has six segments, the first five bearing lamellar pleopods, and the last one bearing uropods.Tavares & Martin, 2010, pp. 110–111 The pleopods are biramous, except in Sicyoniidae, where they are uniramous. The uropods and telson collectively form the tail fan; the uropods are not divided by a diaeresis, as they are in many other decapods.Tavares & Martin, 2010, p. 111 The telson is pointed and is usually armed with four pairs of setae or spines.
Internal anatomy Most of the musculature of a prawn is used for bending the pleon, and almost all the space in the pleon is filled by muscle.Tavares & Martin, 2010, p. 113 More than 17 muscles operate each of the pleopods, and a further 16 power the tail fan in the rapid backward movement of the caridoid escape reaction.Tavares & Martin, 2010, pp. 113–114 These muscles, collectively, are the meat for which prawns are commercially fished and farmed.Kanduri & Eckhardt, 2002, p. 42
The nervous system of prawns comprises a dorsal brain, and a ventral nerve cord, connected by two commissures around the oesophagus.Tavares & Martin, 2010, p. 114 The chief sensory inputs are visual input from the eyes, chemoreceptors on the antennae and in the mouth, and mechanoreceptors on the antennae and elsewhere.Tavares & Martin, 2010, pp. 116–118
The digestive system comprises a foregut, a midgut and a hindgut, and is situated dorsally.Tavares & Martin, 2010, p. 118 The foregut begins at the mouth, passes through the oesophagus, and opens into a sac that contains the grinding apparatus of the gastric mill. The hepatopancreas feeds into the midgut, where digestive enzymes are released, and nutrients taken up. The hindgut forms faecal pellets, which are then passed out through the muscular anus.Tavares & Martin, 2010, pp. 118–119
The circulatory system is based around a compact, triangular heart, which pumps blood into three main arteries.Tavares & Martin, 2010, p. 120 Excretion is carried out through the gills, and by specialised glands located at the base of the antennae, and is mostly in the form of ammonia.Tavares & Martin, 2010, pp. 120–121
Life cycle
Prawns may be divided into two groups based on the structure of the female genitalia, known as thelycum: those with an open thelycum and those with a closed thelycum.Tavares & Martin, 2010, p. 125 In the open-thelycum species, mating takes place towards the end of the moulting cycle, and usually at sunset.Tavares & Martin, 2010, pp. 125–126 In closed-thelycum species, mating takes place shortly after moulting, when the exoskeleton is still soft, and usually occurs in the night.Tavares & Martin, 2010, p. 126 Courtship and mating may take up to 3 hours in Penaeus monodon, while in Farfantepenaeus paulensis, mating lasts just 4–5 seconds. Spawning may occur several times during the moulting cycle, and usually occurs at night.Tavares & Martin, 2010, p. 127
With the exception of Luciferidae, the eggs of prawns are shed directly into the water, rather than being brooded.Tavares & Martin, 2010, p. 130 The eggs hatch into nauplius larvae, which are followed by zoea larvae (initially protozoea, and later mysis) and then a postlarva, before reaching adulthood. The changes between moults are gradual, and so the development is anamorphic rather than metamorphic.
Uniquely among the Decapoda, the nauplii of Dendrobranchiata are free-swimming. There are five to eight naupliar stages.Tavares & Martin, 2010, p. 131 The earlier stages have three pairs of appendages that are used for locomotion – two pairs of antennae and the mandibles. Later stages also have rudiments of other mouthparts, but the nauplius is unable to feed, and only lasts 24 to 68 hours. The body ends at a two-lobed telson, and the beginnings of a carapace emerge at this stage.
There are typically five or six zoea stages in Dendrobranchiata, divided into protozoea and mysis. In the protozoea larvae, the antennae are still used for locomotion, but the mandibles become specialised for mastication. All the thoracic somites (body segments) have formed, and a carapace is present, covering part of the thorax. It is smooth in the family Penaeidae, but bears many spines in the family Solenoceridae. The pleon (abdomen) is unsegmented in the first protozoea, and ends in a bilobed telson, which may be used for cleaning other appendages, or for steering. By the second protozoea, segmentation appears on the pleon, and by the third protozoea, which may also be called the metazoea, the uropods have appeared.
By the mysis stages, the pereiopods (thoracic appendages) start to be used instead of the antennae for locomotion.Tavares & Martin, 2010, p. 133 The larva swims backwards, with its tail upwards, spinning slowly as it goes. The carapace covers most of the segments of the thorax, and claws appear on the first three pereiopods. By the last mysis stage, the beginnings of pleopods have appeared on the first five segments of the abdomen.
The post-larva or juvenile stage is characterised by the use of the pleopods for locomotion.Tavares & Martin, 2010, p. 134 The claws become functional, but the gills are still rudimentary. The telson is narrower and only retains traces of its two-lobed development. Through a series of gradual changes over following moults, the animal takes on its adult form.
Systematics Dendrobranchiata were traditionally grouped together with Caridean shrimp as "Natantia" (the swimming decapoda), as opposed to the Reptantia (the walking decapods). In 1888, Charles Spence Bate recognised the differences in gill morphology, and separated Natantia into Dendrobranchiata, Phyllobranchiata and Trichobranchiata.Tavares & Martin, 2010, p. 99 Recent analyses using cladistics and molecular phylogenetics recognise Dendrobranchiata as the sister group to all other Decapoda, collectively called Pleocyemata.Tavares & Martin, 2010, p. 137
The cladogram below shows Dendrobranchiata's placement within the larger order Decapoda, from analysis by Wolfe et al., 2019.
Before 2010, the earliest known fossil prawns come from rocks in Madagascar of Permo-Triassic age, .Crean, 2004Schram et al., 2000 In 2010, however, the discovery of Aciculopoda from Famennian–stage rocks in Oklahoma extended the group's fossil record back to .Feldmann & Schweitzer, 2010 The best known fossil prawns are from the Jurassic Solnhofen limestones from Germany.
Living prawns are divided among seven families, five in the superfamily Penaeoidea, and two in the Sergestoidea, although molecular evidence disagrees with some aspects of the current classifications.Ma et al., 2009 Collectively, these include 540 extant species, and nearly 100 exclusively fossil species.De Grave et al., 2009 A further two families are known only from fossils.
The cladogram below shows Dendrobranchiata's internal relationships of extant families (excluding Solenoceridae):
Diversity
Dendrobranchiata comprises the following superfamilies and families: Superfamily Penaeoidea Rafinesque-Schmaltz, 1815 Aciculopodidae Feldmann & Schweitzer, 2010 – a single Famennian species, Aciculopoda mapesi Aegeridae Burkenroad, 1963 – two Mesozoic genera: Aeger and Acanthochirana Tavares & Martin, 2010, p. 151 Aristeidae Wood-Mason, 1891 – 26 extant species in 9 genera, and one fossil genus Tavares & Martin, 2010, p. 152 Benthesicymidae Wood-Mason, 1891 – 41 species in 4 genera Carpopenaeidae Garassino, 1994 – two Cretaceous species of Carpopenaeus Tavares & Martin, 2010, pp. 152–153 Penaeidae Rafinesque-Schmaltz, 1815 – 216 extant species in 26 genera, and several extinct genera, mostly Mesozoic Tavares & Martin, 2010, p. 153 Sicyoniidae Ortmann, 1898 – 43 species of Sicyonia Tavares & Martin, 2010, p. 154 Solenoceridae Wood-Mason, 1891 – 81 species in 9 genera Tavares & Martin, 2010, p. 155 Superfamily Sergestoidea Dana, 1852 Luciferidae De Haan, 1849 – 7 species in 2 genera Sergestidae Dana, 1852 – 90 extant species in six genera, and two extinct monotypic genera Tavares & Martin, 2010, p. 156
Distribution The biodiversity of Dendrobranchiata decreases markedly at increasing latitudes; most species are only found in a region between 40° north and 40° south.Tavares & Martin, 2010, p. 145 Some species may occur at higher latitudes. For instance, Bentheogennema borealis is abundant at 57° north in the Pacific Ocean, while collections of Gennadas kempi have been made as far south as 61° south in the Antarctic Ocean.
Ecology and behaviour