Before the Panama Canal opened in 1914, its two ends were only 77 km apart over land, but the closest sea route was 32,000 km long (around the entirety of South America) and passed through much colder waters near Cape Horn. Therefore, I suspect that the Atlantic and Pacific marine ecosystems were more or less completely isolated from each other and contained very different species before the Canal opened. It also seems plausible that once the canal opened and a 77-km marine route opened up, these two ecosystems very quickly came into contact and may have reached a new, drastically different equilibrium. Is this correct? I could imagine the ecological effects being equivalent to the sudden creation of a 77-km bridge connecting the ecosystems of, say, the United States and Kenya.
The panama canal is not just a long contiguous connection between the two oceans as you might think. It operates under a system of locks of varying height because the surrounding region is actually fairly mountainous, so a ship has to enter a lock, let it fill until equilibration with the next lock whilst the boat rises, the barriers open, the boat moves into the next section and the process repeats. It is fairly unlikely much wildlife would be able to pass through this system thus not causing the vast ecological effects you might imagine. https://www.youtube.com/watch?v=hoQ7RHyG-EA this link visualises it for you :)