Adaptation to life in fresh water by decapod crustaceans: Evolutionary challenges in the early life-history stages

Abstract

The Decapoda evolved in the Palaeozoic as a marine group and have since then shown limited radiation in limnic and terrestrial environments. About 80 % of the extant decapod species still live in the sea. The colonization of non-marine environments required physiological adaptations such as hyper-osmoregulate (for fresh water) and hypo-osmoregulate (for terrestrial and hypersaline habitats). Osmoregulatory abilities are strong in juvenile and adult freshwater decapods, but are typically absent in larval stages. Diadromous breeding migrations to estuaries or the sea are part of an adaptive strategy that allows adult life in non-marine habitats in combination with a conservation of the ancestral life-history pattern (extended planktonic larval development in salt water). Phylogenetically old groups of hololimnetic decapods (crayfish, aeglids, primary freshwater crabs) have completely eliminated the larval phase, showing direct development and brood care. Recent colonisers generally show intermediate patterns with an abbreviated and partially or fully lecithotrophic larval phase. Macroevolutionary patterns support possible colonization routes from the sea to fresh water and land. It is suggested that ecologically transitional habitats such as brackish mangrove swamps, salt marshes, seasonally inundated wetlands, and anchialine caves have been the principal entrance portals, rather than direct invasion through estuaries and rivers.