Water-to-land transitions

Abstract

Arthropods are, by a considerable margin, the most species-rich group of animals alive today and have long been a major component of the Earth's biodiversity. Exact counts of the total number of species are not easy to come by, but Zhang (2011) offered a recent summary. Together the ca. 1,023,559 described living species of hexapods, 11,885 myriapods and 110,615 arachnids-most of which live on land-massively outnumber the ca. 66,914 recorded crustaceans, 1,322 sea spiders and the four species of horseshoe crab. Put bluntly, in terms of raw species numbers, the primarily terrestrial lineages (Hexapoda, Myriapoda, Arachnida) outnumber the primarily aquatic ones ('Crustacea', Pycnogonida, Xiphosura) by a factor of almost seventeen to one. In fairness, there is a degree of bias in these figures.