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.
Demand for cultivating and farming of oysters has been increasing around the world due to growing food demand as well as practicing of sustainability. Potential threats to oyster aquaculture practices are numerous. These potential threats can be caused by disease “Dermo/MSX disease,” climate change, ocean acidification, pollution/eutrophication, and also invasive species.
Potential diseases to oyster can firstly arise through parasites, leading to Dermo or MSX disease. These two diseases have caused massive mortality to the Atlantic shellfish fishery. This pathogen enters through the blood stream of oysters after being filtered through their gills, becoming abundant throughout their tissue. The oyster will become stressed, reduce its growth rate, begin to produce non-fertile spat, and will decrease the integrity of its shell.
Rising ocean temperatures produce a potential threat to oyster aquaculture. Rising ocean temperatures allow for increased ocean acidification, which has the potential of impacting the integrity of oyster shells and in turn aquaculture practices. Rising temperatures coupled with inland run off of fertilizers and heavy metals allow for new and hazardous organisms to thrive. Microscopic planktonic dinoflagellates are known to thrive in eutrophication environments and can be linked to red tides and brown tides, both toxic and non-toxic blooms that at high levels cause death to marine organisms. Shellfish digest and filter out these dinoflagellates, which contain neurotoxins. Red tide formations are usually non-native to a naturally occurring system. The recognition of red tides causes paralytic shellfish poisoning (PSP). Another influential algal bloom beginning to occur is brown tides. The occurrence of the brown tides causes densities to get so numerous that shellfish are unable to filter. Shellfish then become unable to filter particles leaving these sessile organisms to starve.
Oyster cultivating and farming plays a major role in benefiting the ecosystem and economy. Although, beneficial to ecosystem services and economic gain, potential threats cause integrity to this industry. This industry can be susceptible to disease, climate change, and pollution. Biotic and Abiotic factors both have the potential of influencing this dynamic cultivating practice.