A Comparative Study of Static Bodies in Mysid Crustaceans: Evolutionary Implications of Crystallographic Characteristics.

We studied the mineral composition of statoliths in 154 species belonging to 55 genera of Mysidae. Fluorite (CaF2) was found in 86% of Recent species, vaterite (CaCO3) in 9%, and no crystalline component in 5%. Seven samples of fossil statoliths from Upper Miocene deposits were exclusively calcite (CaCO3). Vaterite has the peak of occurrence in fresh water, fluorite in the photic zone of marine waters, and organic statoliths in oceanic deep waters. With respect to population numbers in the different aquatic biota, vaterite prevails in freshwater species and fluorite is dominant among species in all brackish to marine environments. The occurrence of CaCO3 in fresh to brackish waters coincides with fossil records and biogeographical observations. The Ponto-Caspian region is the center of abundance for Recent CaCO3-precipitating species. The rich brackish to freshwater fauna in this region probably has its roots in the brackish Paratethys, where a rich fossil material of calcareous mysid statoliths is known from Upper Miocene sediments. Morphological and scarce palaeontological evidence suggests that the earliest (Carboniferous to Jurassic) Mysidacea were mainly oceanic shrimps without statocysts; these were followed by (bentho) pelagic animals with nonmineralic organic statoliths. With the colonization of coastal to littoral areas by benthopelagic to benthic forms, mineralic statoliths were formed by precipitation of fluorite. Among the modern Mysidae, a special development occurred (in the Miocene) in the Ponto-Caspian region where CaCO3 statoliths appeared in brackish to freshwater forms. As in vertebrates, the patterns of mineral composition of static bodies in the Mysidae reflect both anatomical and ecophysiological differences.