Crustacean

• 1. Introduction

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  A Japanese spider crab stretches its legs to a span of 3.8 metres and tips the scales at 20 kilograms. At the other end of the spectrum sits Stygotantulus stocki, just 0.1 millimetres of body, a creature you would need a microscope to find. Both belong to the same sprawling group: the crustaceans, mandibulate arthropods whose name comes from the Latin crustacea, meaning those with shells or crusted ones. Roughly 67,000 species have been described so far, and they crowd the oceans the way insects crowd the land. Some of them you have eaten. Shrimp, crabs, lobsters, and crayfish all belong here. So do barnacles, krill, woodlice, and a parasitic worm that fixes itself headfirst to the tongue of a fish. How does one group hold a giant crab and a near-invisible speck? Why do scientists now say insects are crustaceans too? And what does a moulting shell have to do with all of it?

• 2. The Crusted Body

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  Segments build a crustacean. They cluster into three regions: the cephalon or head, the pereon or thorax, and the pleon or abdomen. The head and thorax can fuse into a cephalothorax, sometimes hidden under a single large carapace. A hard exoskeleton shields the whole animal, and that armour is also a problem. To grow, a crustacean must shed its shell entirely and form a new one, a process called moulting.

  Biramous limbs set crustaceans apart from insects, myriapods, and chelicerates. The word means two-parted, a limb split into two branches. This even includes the second pair of antennae, though the first pair is usually uniramous, a single branch. In the class Malacostraca the antennules break that rule and may be biramous or even triramous. Whether the two-parted limb evolved within crustaceans or was lost in every other group remains unclear, and trilobites carried biramous appendages too.

  Appendages, in fact, may be the secret to the group's success. Each body segment, or somite, can bear a pair of them. On the head sit two pairs of antennae, the mandibles, and the maxillae. The thoracic segments carry legs that may specialise as pereiopods for walking or maxillipeds for feeding. In malacostracans the abdomen bears pleopods and ends in a telson, which holds the anus and is often flanked by uropods that form a tail fan.

  Inside, the heart pumps blood into an open body cavity called the haemocoel. Malacostraca carry oxygen using haemocyanin, while copepods, ostracods, barnacles, and branchiopods use haemoglobins. A gizzard-like gastric mill grinds food along a straight gut, structures near the antennae do the work of kidneys, and a brain forms from ganglia close to those same antennae.

• 3. Land, Sea, And Hosts

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  Marine crustaceans are as ubiquitous in the oceans as insects are on land. Most live free and motile in marine or freshwater, but a few groups crawled ashore: terrestrial crabs, terrestrial hermit crabs, and woodlice. Others abandoned movement altogether. Adult barnacles cement themselves headfirst to a surface and cannot move on their own.

  Parasites take the strangest paths. Sea lice, fish lice, whale lice, tongue worms, and Cymothoa exigua all attach to a host, and all may be called crustacean lice. Some branchiurans tolerate sudden swings in salinity and will even switch from a marine host to a non-marine one. This adaptability lets certain species travel far and settle where they do not belong.

  Invaders ride that flexibility. The Chinese mitten crab, Eriocheir sinensis, and the Asian shore crab, Hemigrapsus sanguineus, both rank as significant invasive species. Since the Suez Canal opened, close to 100 species of crustaceans from the Red Sea and the Indo-Pacific have established themselves in the eastern Mediterranean sub-basin, often with significant impact on local ecosystems. In Antarctic waters, by contrast, krill form the bottom and most important layer of the food chain.

• 4. Making More Crustaceans

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  Sex is not fixed in this group. Most crustaceans have separate sexes and reproduce sexually, but a small number are hermaphrodites, including barnacles, remipedes, and Cephalocarida, and some change sex during their lives. A recent study found that male Tigriopus californicus choose mates by diet, preferring females fed on algae over those fed on yeast. Parthenogenesis, in which a female produces viable eggs without any male, runs through many branchiopods, some ostracods, some isopods, and the Marmorkrebs crayfish.

  Eggs reveal a catalogue of holding strategies. Some crustaceans simply release fertilised eggs into the water column. Most decapods instead carry the eggs attached to their pleopods. Peracarids, notostracans, anostracans, and many isopods build a brood pouch from the carapace and thoracic limbs. Female Branchiura skip pouches entirely and glue their eggs in rows to rocks and other objects, while some copepods seal theirs in thin-walled sacs and others trail them in long tangled strings.

  Reproduction on land carries its own demands. Many decapod mothers hold their eggs until they hatch into free-swimming larvae. The Christmas Island red crab and other terrestrial crustaceans mate seasonally and return to the sea to release their eggs. Woodlice lay theirs on land, though only in damp conditions, and in many decapods the first pair of pleopods, sometimes the second, is reshaped in the male for transferring sperm.

• 5. The Larval Disguise

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  The nauplius is the earliest and most characteristic larval form, and it unites the entire group despite their wild diversity of shape. It carries three pairs of appendages, all sprouting from the young animal's head, and a single naupliar eye. From there the stages branch out depending on the group.

  The zoea earned its name through a mistake. Naturalists once believed it was a separate species and named it accordingly. It follows the nauplius and comes before the post-larva, and it swims with its thoracic appendages, unlike the nauplius, which uses cephalic appendages, and the megalopa, which uses abdominal ones. Spikes often line its carapace, perhaps helping these tiny organisms hold a steady direction. In many decapods, development is so accelerated that the zoea is the first larval stage, sometimes followed by a mysis stage, sometimes by a megalopa.

  The eyes of swimming larvae hide a clever trick. In several forms, the otherwise black eyes are covered by a thin layer of crystalline isoxanthopterin, tinting them the exact colour of the surrounding water for camouflage against predators. Tiny holes in that layer still let light reach the retina. As the larva matures, the layer migrates behind the retina and becomes a backscattering mirror, brightening the light passing through the eyes the way it does in many nocturnal animals.

• 6. Reading The Genome

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  Penaeus monodon, the black tiger shrimp, became the subject of basic research into whether DNA repair can shield crustaceans from damage. When researchers studied breaks in the double strand of its DNA, they found repair was carried out mostly by accurate homologous recombinational repair. A second, less accurate process called microhomology-mediated end joining also patched such breaks.

  Ultraviolet light revealed a wider response in another species. After irradiating the intertidal copepod Tigriopus japonicus, researchers analysed how its DNA repair and damage-response genes behaved. The study showed increased expression of proteins tied to four distinct mechanisms: non-homologous end joining, homologous recombination, base excision repair, and DNA mismatch repair.

• 7. Naming The Shelled

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  Pierre Belon and Guillaume Rondelet used the name crustacean in some of the earliest works to describe these animals. Carl Linnaeus later dropped it, filing crustaceans among the Aptera in his Systema Naturae. The first nomenclaturally valid use of Crustacea came in 1772, in Morten Thrane Brünnich's Zoologiae Fundamenta, though he also folded chelicerates into the group.

  The boundaries kept shifting as evidence mounted. Studies of morphology produced the Pancrustacea hypothesis, making Crustacea and Hexapoda sister groups. Later work with DNA sequences argued instead that Crustacea is paraphyletic, with the hexapods nested inside a larger Pancrustacea clade. Three classes, Cephalocarida, Branchiopoda, and Remipedia, turn out to be more closely related to the hexapods than to other crustaceans. By that reasoning any hexapod sits closer to a multicrustacean than an oligostracan does.

  Counting the classes has been just as unsettled. Bowman and Abele recognised 652 extant families and 38 orders in six classes in 1982. Martin and Davis updated that in 2001, keeping six classes but raising the tally to 849 extant families across 42 orders. Both retained Maxillopoda despite evidence it was not monophyletic. Phylogenetic studies later confirmed its polyphyly, and recent classifications recognise ten to twelve classes, elevating former maxillopod subclasses such as Thecostraca, Tantulocarida, Mystacocarida, Copepoda, Branchiura, and Pentastomida.

• 8. Stone And Plate

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  The Cambrian holds most of the major crustacean groups before its close: the Branchiopoda, the Maxillopoda including barnacles and tongue worms, and the Malacostraca. Some debate surrounds whether Cambrian animals assigned to Ostracoda are truly ostracods, a group that would otherwise begin in the Ordovician. Cephalocarida have no fossil record at all. Remipedia, first described from the fossil Tesnusocaris goldichi, do not appear until the Carboniferous, the same period when fossil crustaceans become abundant.

  The groups arrive on a staggered schedule. Cumacea, Isopoda, and the first true mantis shrimp are all known from the Carboniferous. Prawns and polychelids appear in the Triassic, then shrimp and crabs in the Jurassic. The fossil burrow Ophiomorpha is attributed to ghost shrimps and the burrow Camborygma to crayfishes, while the Permian-Triassic deposits of Nurra preserve the oldest fluvial burrows of both, dating to the Roadian. The great radiation came in the Cretaceous, especially among crabs, possibly driven by the adaptive radiation of bony fish, their main predators, and the first true lobsters appear there too.

  From fossil to fishery, the harvest is enormous. More than 7.9 million tons of crustaceans are taken each year for human food, and nearly 10,700,000 tons were harvested in 2007. Shrimp and prawns make up over 60 percent by weight, and nearly 80 percent is produced in Asia, with China alone producing nearly half the world's total. Krill go largely uncaught, only 118,000 tons, despite holding one of the greatest biomasses on the planet, a reminder that the most abundant crustaceans are the ones humans notice least.