Why do we see more anadromous species (spawning in freshwater, while living the duration of their lives in saltwater, only returning to freshwater environments to spawn, e.g. salmonids or lamprey) compared to catadromous species (spawning in saltwater, while living the duration of their lives in freshwater, only returning to saltwater environments to spawn, e.g. american eels or some mullets)? The difference in these diadromous fish migration has ultimately been described as a paradox.
Geographically, you will find the highest quantity of aquatic species in tropical regions, containing the highest amount of diversity. Anadromous species are more prevalent within temperate latitudes, while finding catadromous species are more common in the tropics. There have been numerous hypotheses of the differences in food availability in ocean and freshwater habitats. Oceans often produce higher quantities of food compared to freshwater habitats in temperate latitudes where anadromous species predominate.
Vega and Wiens (2012) assessed actinopterygian (class of ray-finned fish) richness, which contains 96% of all fish species found today. They found that richness was similar between both freshwater and saltwater clades. That marine actinopterygians derived from a freshwater ancestor! Diversify from this ancestor only started 110 million years ago. Through evolution percomorpha (spiny ray finned fish) now makes up 40% of bony fish. Another group, ostariophysi evolved from freshwater to saltwater and now makes up 70% of freshwater species. They indicate that some possible extinction might have caused the low marine species richness. After these mass extinctions, freshwater actinopterygians were then able to re-occupy and repopulated saltwater environments. Lack of marine diversity could be one explanation to why there is low diversity within marine environments.
It’s easy to think how easily a freshwater species could diversify compared to a saltwater species. Freshwater habitats could be subjected to geographical barriers, take for instance the formation of the Andes and the Amazon River basin. The rise of the Andes enclosed a small bay. Overtime the bay became less saline, allowing for species to evolve. The bay would periodically be flooded with saltwater, introducing new species and more diversity.
Vega, G. C., & Wiens, J. J. (2012). Why are there so few fish in the sea? Proceedings of the Royal Society B: Biological Sciences, 279 (1737), 2323-2329.
Gross, M. R., Coleman, R. M., & McDowall, R. M. (1988). Aquatic productivity and the evolution of diadromous fish migration. Science(Washington), 239 (4845), 1291-1293.