ABSTRACT
Barnacles (Cirripedia) are almost exclusively a marine taxon. Within two of the three suborders that constitute the Cirripedia, i.e., Thoracica (the sessile and pedunculated barnacles) and Acrothoracica (the burrowing barnacles), no truly freshwater-dwelling species has ever been reported. In the third suborder, Rhizocephala (parasitic barnacles), four species are known to thrive in semiterrestrial, brackish waters, or fully freshwater habitats, as parasites of crabs from the corresponding habitats. In this chapter, we present molecular data that indicate a single marine-to-freshwater transition event in a lineage that is either part of the genus Heterosaccus or closely related to it. Morphological characters of the settling larval stage, the cypris, further support this evolutionary scenario.
