Plankton is not the name of a plant or animal but more of a category for any drifting organism that inhabits the middle to upper levels of the ocean, namely the pelagic zone.
Most vets do spays, neuters, bone surgeries, spleen surgery, tumor removals, and intestinal surgeries. Heart surgery and more complicated surgeries are done by specialists that requires more schooling. If you want to be a vet, you have to have REALLY good grades and Vet school is MUCH harder to get into than human medical school.
In many cases, your veterinarian will suggest a referral to a specialist if the case is a difficult one. Some specialty practices only work with referrals, meaning cases sent in by veterinarians; other specialty practices will see new patients directly, no referral needed.