An all-conquering ecological journey: from the sea, calanoid copepods mastered brackish, fresh, and athalassic saline waters

Abstract

Although it is commonly acknowledged that calanoid copepods inhabiting fresh water evolved from marine ancestors via the brackish water of estuaries, it is less well appreciated that a restricted number of species with freshwater affinities have conquered athalassic saline waters. The global importance of the latter habitat has been under-estimated and, with climate change and human population growth, it is expanding at the expense of fresh waters. Considering Australia, South America and the Holarctic, at least seven halobiontic calanoid species occur in athalassic saline waters (the situation in Africa is not visited). In Australian inland-water Centropagidae, there is a high degree of congruence between the ecological trend in habitat occupancy (marine through brackish and fresh to athalassic saline waters) and the assumed evolutionary trend towards reduction in the setation and segmentation of swimming legs. The validity of the inference by Adamowicz et al. (Biological Journal of the Linnean Society of London 90: 279–292, 2007a) that a hypothesis of oligomerization was not supported as the mode of evolution of South American non-marine Centropagidae is criticized for reasons of inadequate character sampling. The phylogeny, biogeography and osmo-regulatory physiology of Southern Hemisphere inland-water centropagids are reviewed in some detail. Calanoids have mastered a significant portion of the total salinity range for athalassic saline waters (3 to 300+ g l−1) but, unlike brine shrimps, they have not evolved a mechanism for hypo-osmotic regulation and do not tolerate saturated or near-saturated brines.