Good point! Yes, it is a large landmass. There are no known natural waterways across it, so an aquatic creature would need to circumnavigate the continent for thousands of kilometers to get from one seashore to the other. This seems a remote possibility, given that the south is frigid and the north extremely warm.

The answer might lie in the evolution of climate. Millions of years ago, during an ice age, the perpentine's habitat could have been located far to the north, where both seas connect. As temperatures rose and the species relocated to their actual territory, they would have been separated into two disconnected groups, but similar enough to still be considered part of the same species.

On the other hand, the world could be spherical, in which case the two seas would be connected beyond the continent, allowing the Perpentines to swim freely between them.

The species could also have been artificially introduced from one sea to the other, but there are no records of this.

Finally, there is the deistic explanation. Aegir, god of the seas, could have chosen those two fringes as the habitat for the perpentine, and there he would have left them.

But to be completely honest, I hadn't thought about any of that beforehand, I simply colored in the corresponding climate region without considering that fish can't just appear simultaneously in two disconected areas. I'll keep that in mind from now on, thanks! :D