Less than a tenth of one percent of the water on Earth is freshwater lakes and rivers – yet that
teeny tiny percentage of Earth’s water contains about half of all the world’s fish species!
Ocean water, which accounts for a whopping 99.6% of Earth's water, contains the other half.
You might think the vast ocean has comparatively few species because a lot of it is pretty empty,
which it is: lots of the ocean is too hot or cold or dark or nutrient-poor
for much to live there. But even if you ignore all that mostly uninhabited ocean water,
there's still over 1,000 times more habitable ocean water than freshwater available.
In fact, there are far more individual fish in the ocean than in lakes and rivers;
but not more species. So why does freshwater have comparatively so many species of fish?
Scientists have dubbed this the “freshwater paradox.”
For a long time, the top candidate for a solution was the fact that isolation leads to new species.
And of course, lakes and rivers are far more disconnected and isolating than the ocean. Both
because they're not literally connected, but also because their courses and connections change more
quickly over time, connecting and disconnecting populations of individuals over millions of years,
potentially leading to the evolution of new species. So, based on isolation, you’d expect
freshwater to develop new fish species more often than the ocean, leading to more fish
species in freshwater, but a recent study found that both rivers and the ocean develop new species
at similar rates, bursting the bubble on the idea that isolation alone is the answer to the paradox.
And isolation ISN’T the only thing that can lead to new species – so can competition, weird mating
rituals, a variety of ecological niches, and a stable environment. As it turns out, lakes have
the perfect combination of all those things and pump out new species up to five times faster than
rivers or the ocean. That’s it then! Lakes must simply churn out so many new fish species that
they pad the freshwater numbers. *Sigh* except there’s a tiny problem with that. Even though new
species arise more quickly in lakes, the current total number of species in lakes isn’t that
high - there are way more fish species in rivers! The explanation of the freshwater paradox, then,
may lie with something else: extinctions. When we look at the fossil record, we see that there have
been more ocean fish species than freshwater fish species throughout Earth's history,
but many were lost to extinction or transitioned into rivers to survive. Meanwhile, freshwater
fish in the fossil record were less affected by extinction events, which would explain why
there are just as many freshwater fish species. Except, as always, it’s complicated. Fossils
form more easily on the soft, undisturbed ocean floor than busy places like rivers. So, it could
be that there are more fossilized ocean-fish species not because there were more ocean fish,
but because more ocean fish became fossils. Gaah!! So then how do we explain the “freshwater
paradox”? For now, we don’t know! It could be some combination of isolation,
competition, extinction, and so on - it could just be an odd coincidence, or it could be a
natural phenomenon or process we haven’t yet figured out. We’ll just have to wait and sea.