Crustacean - Classification

Classification

The name "crustacean" dates from the earliest works to describe the animals, including those of Pierre Belon and Guillaume Rondelet, but the name was not used by some later authors, including Carl Linnaeus, who included crustaceans among the "Aptera" in his Systema Naturae. The earliest nomenclaturally valid work to use the name "Crustacea" was Morten Thrane Brünnich's Zoologiæ Fundamenta in 1772, although he also included chelicerates in the group.

The subphylum Crustacea comprises almost 67,000 described species, although the number of undescribed species may be 10–100 times higher. Although most crustaceans are small, their morphology varies greatly and they include both the largest arthropod in the world – the Japanese spider crab with a leg span of 12 feet (3.7 m) – and the smallest – the 0.1 mm (0.004 in) long Stygotantulus stocki. Despite their diversity of form, crustaceans are united by the special larval form known as the nauplius.

The exact relationships of the Crustacea to other taxa are not completely settled as of April 2012. Studies based on morphology led to the Pancrustacea hypothesis, in which Crustacea and Hexapoda (insects and allies) are sister groups. More recent studies using DNA sequences suggest that Crustacea is paraphyletic, with the hexapods nested within a larger Pancrustacea clade.

Although the classification of crustaceans has been quite variable, the system used by Martin and Davis largely supersedes earlier works. Mystacocarida and Branchiura, here treated as part of Maxillopoda, are sometimes treated as their own classes. Six classes are usually recognised: