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.
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2$\begingroup$ This book answers your question pretty well. I was going to put together an answer based on it and other papers I found, but suddenly lost the will to. $\endgroup$– canadianerCommented Apr 6, 2018 at 0:24
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2$\begingroup$ Many organisms are transported across the channel by ships, as hull fouling or in ballast tanks (which bypasses the salinity barrier of Lake Gatun). The book mentions that oil tankers routinely dump tens of thousands of tons of ballast water from the Caribbean into ports in Alaska. $\endgroup$– canadianerCommented Apr 6, 2018 at 0:25
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1$\begingroup$ Water treatment is performed in onshore facilities by US and some other countries. But the open ocean and many other countries have no requirements. $\endgroup$– blacksmith37Commented Apr 6, 2018 at 2:32
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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 :)
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4$\begingroup$ It doesn't seem unreasonable, to me, that at least some marine life could pass through the locks with ships. $\endgroup$ Commented Apr 5, 2018 at 17:04
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2$\begingroup$ Also, IIRC the canal and locks are fresh water, so it doesn't seem likely that most marine organisms could survive a passage. $\endgroup$– jamesqfCommented Apr 5, 2018 at 19:23
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$\begingroup$ I agree with @canadianer. It often takes surprisingly few members of an invasive species to completely change an ecosystem, since none of the original organisms have evolved to resist it from taking over. And if there have been 10 million lock cycles, then quite a few organisms could have passed through even if most cycles don't transfer any organisms. $\endgroup$– tparkerCommented Apr 5, 2018 at 19:29