Crustaceans span a tenth of a millimetre to spider-crab giants
Crustaceans are mandibulate arthropods traditionally grouped in subphylum Crustacea—mostly aquatic forms from shrimp and crabs to barnacles and woodlice. About 67,000 described species stretch from 0.1 mm Stygotantulus stocki to Japanese spider crabs with 3.8 m leg spans.
The name Crustacea goes back to the Renaissance naturalists Pierre Belon and Guillaume Rondelet, though Linnaeus ignored it and lumped the animals among his wingless 'Aptera', and Morten Thrane Brünnich's work was the first nomenclaturally valid use. Nearly 67,000 species have been described, possibly only a tenth to a hundredth of the true total. Most swim free in the sea or fresh water, but woodlice and sandhoppers live on land, barnacles stay fixed in place, and fish lice and tongue worms are parasites. Fisheries and farms take more than 7.9 million tons a year.
A crustacean body has three regions, head, thorax and abdomen, and the first two often fuse into a cephalothorax under one carapace. Growth requires shedding the hard exoskeleton. Each segment can carry a pair of limbs: head segments bear two pairs of antennae plus mandibles and maxillae, and thoracic ones carry walking legs and feeding legs. Most limbs fork into two branches, though the first antennae usually do not.
Blood moves through an open circulation, pushed by a heart near the back into a body cavity. Malacostracans such as crabs and shrimp carry oxygen with haemocyanin, while copepods, ostracods, barnacles and branchiopods rely on haemoglobins, and the gut often contains a grinding 'gastric mill'. Most species have separate sexes, but barnacles, remipedes and Cephalocarida include hermaphrodites, and male copepods of the species T. californicus favour algae-fed females over yeast-fed ones.
The earliest larva, the nauplius, has three pairs of head limbs and a single eye, and later stages include the zoea, a name coined when naturalists thought it was a separate animal. Some swimming larvae hide their dark eyes under a crystalline isoxanthopterin layer tinted like the surrounding water. Christmas Island red crabs mate on land and return to the sea to release eggs, whereas woodlice lay theirs on damp ground. Kinship with insects is unsettled, with DNA suggesting the hexapods nest inside a paraphyletic crustacean group.
Source: Crustacean
