Obviously, geographical separation is what – OK one of the things – that leads to speciation. The oceans are a lot less geographically separated than the land masses. So, we get rather wider distribution of sea species than we do land ones.
Sorta, at least.
I can imagine that a barrier in the sea is the Equator. Not so much currents and stuff but heat. Down deep even that doesn’t matter of course. But I could imagine at least that we get N Hemisphere fishies and S Hemisphere ones.
At which point, how true is this?
Take, say, cod. I know there’s a Pacific cod as well as an Atlantic one. And they’re both up in northern waters as they don’t like it hot. But, and here’s the thing, do we have closely related species which are S hemisphere? Or other, cognate species? Or something entirely different occupying the niche? Or does the niche not exist?
No particular reason, just a question that crossed the synapses.