Conquering the Ocean Depths Over Three Geological Eras

The deep sea is the largest habitat on Earth, but it is the least accessible and comprehensible. This apparently harsh environment is inhabited by Crustacea of diverse evolutionary lineages that have, to various degrees, evolved uniquely specialized morphologies and lifestyles. Following a century of debate about the antiquity of the deep-sea fauna, studies of Crustacea reveal that the faunas of the deep and shallow oceans have been continuously and repeatedly exchanged, probably since the Mid-Paleozoic. Deep-sea colonization and subsequent diversification has occurred across many crustacean lineages, during several periods, and may still be underway. Despite a commonly held view that shallow–deep phylogenetic relationships are unidirectional, there is also evidence for evolutionary emergence from the abyss into shallower zones. As a result, the present-day fauna represents an amalgamation of clades of various ages. Environmental factors such as pressure, temperature, and energy supply differ substantially between shallow- and deep-water layers, creating gradients that pose important ramifications for crustacean physiology. Consequently, depth-range expansions require adaptations, which may lead to peculiar phenomena such as gigantism and dwarfism, as well as diverse crustacean radiations.