Fish are interesting because they are such a wonderful example of adaptive radiation (32,000 species and counting). The African ciclids are famous as being sort of Darwin's finches with scales, but I'm thinking of marine fishes at the moment. There are three living classes and two extinct ones, by the most parsimonious methods of evaluation. They gave rise to some of the mightiest life forms on Earth. My favorite fish is one that is, unfortunately, no longer with us, the orca-sized predator Dunkleosteus, he of the imposing armor plating and teeth (ok, biting plates) the size of small traffic cones. It also includes two creatures once thought to be30m long, though we now know they were smaller. One is the queen of sharks, megalodon, which lives on now only in bad movies and TV species. The other is the filter-feeding Leedsichthys problematicus, a Jurassic species with huge pectoral fins among other interesting features. This animal may or may not have been bigger than the modern filter-feeding whale shark, but it seems secure in its place as the largest bony fish ever to live. (Indeed, paleontologists still debate why and how a harmless fish in a sea of huge marine reptilian carnivores got that big at all.)
Fishes I wrote about in my first book as new included the eelike Lamprogrammus shcherbachevi, which was identified in
1993 from four specimens caught between 1972 and
1991 in the
Atlantic, Indian, and Pacific oceans. It is a rare mid-water and deep-water species: it deserves a re-mention because despite being reasonably large (two meters), it remains so obscure that some online descriptions of it include no species-specific illustrations. (As a sometime cryptozoological researcher, it strikes me it would make a serviceable sea serpent if it was 15m instead of 2. Hmmm, how do we know it isn't? I'll get back to that in a minute.)
New species keep piling up. We have the hopbeard plunderfish, which deserves to be mentioned on its common name alone, a chubby brown-splotched denizen of near freezing Antarctic waters, with catfish-like barbels. Another new species is the psychedelic frogfish, which has a great name and deserves it. Histiophryne psychedelica "bounces" off the seafloor with thrust of leglike fins and boosts its hop with water expelled from the gills. All that and it looks like something from the movie Yellow Submarine, only stranger. Another recent find, the striated frogfish, boasts changing coloration, a natural "fishing pole," and fins and other projections seemingly stuck on it at random. The whole frogfish family gets weirder from there.
Anyway, I said these were random notes, but here's the point I was going to get to. The fishes are an ancient group containing most of the vertebrate species on Earth, but we still don't know many things, including how many types there are. Ask the scientists in the Census of Marine Life, who are still examining hundreds of potential new species. We also know that the larger species have been reduced, in some areas more than 90%, by overfishing and pollution, and we don't know how the chain of life will shake out even if we tightly regulate fishing and rely more on farmed fish (A strategy with problems of its own, but that's beside the point for the moment.)
I think it possible, and indeed likely, that the fishes hide something else from us still: an eel or eel-like fish, at least 10 long, responsible for countless reports of giant eels and "sea serpents." Keep looking, scientists....