Species at home in fresh and seawater

I was pondering this recently; can’t remember why, but are there any aquatic species that are equally at home and healthy in either fresh water or seawater?

:smack: I remember now. As a secondary question, what are the conditions like at a boundary such as a river mouth?

There are some that move from one to the other, and back, as part of their life cycle – Salmon being the best-known example, but there are others. I believe, though, that there are physiological changes that take place in preparation for the transition, so it’s not an everyday thing.

I don’t know offhand of any that regularly move back and forth, but I’m sure there are some.

I assume birds (e.g. ducks) and mammals don’t count?

Fish that can live in either fresh or saltwater are called “anadromous”, and a couple examples that I can think of off the top of my head are:

Striped bass (Morone saxatilis), which is found along the Atlantic coast and in the Gulf of Mexico as far West as Louisiana, as well as where they’ve been stocked in freshwater lakes, such as Lake Texoma, where they grow to monstrous size.


Redfish/Red Drum (Sciaenops ocellatus) are primarily a Atlantic/Gulf species, but have been stocked in a few lakes also.


Lakes with Red Drum:


swordtails, a common fish for home aquariums, live in brackish water. Brackish water is saltier than fresh, but less salty than saltwater. It occurs where rivers empty into the sea and whatnot.*

*I do not claim to be a fish expert. This information was given to me a few years ago by a pet shop when I bought said fish. Knowledge may have changed by now, but my fish never seemed to mind the couple tablespoons of salt I added to their water

This was answered, indirectly, by Pullet. I will add that brackish waters will extend quite a bit from the sea, the further away the less salty. The ocean tides also affect rivers for the same distance, resulting in their brackishness.

There is (or, at least, was) a species of dolphin that lives in the Yangtze river (name escapes me at the moment), though I don’t think it would be “at home” in the ocean. I’m not sure if there are any other freshwater cetaceans. Otters will swim up estuaries into fresh water; but otters can get around on land pretty well too, so I guess they’re not a true aquatic mammal.

Interestingly, when I was in New Zealand on a boat tour of Doubtful Sound, I was able to see pods of Bottlenose and Dusky Dolphins (the latter being small and very acrobatic!) that frequented heads of side-arms of the Sound. Powerful cascades feed into the fiord from the headwalls of such branches, as well as a torrent of water flowing out of Lake Manapouri via the tailraces of the Manapouri Power Station. Couple that with all the rain and other intermittent fresh water funneling into the Sound from virtually every direction, and the effect is that the water in Doubtful sound is more fresh than salty. We were told by the boat captain that, by rights, the Dolphins shouldn’t be interested in such a place, but these critters were, as well as anyone knew, year-round residents, and happy ones at that.

Lipotes vexillifer. It is a freshwater species, and doesn’t venture out into the open ocean. It’s also quite endangered.

There’s also the finless porpoise (Neophocaena phocaenoides), which inhabits both freshwater and saltwater regons (specifically, it also inhabits the Yangtze River, as well as the adjacent coastal areas). And the Ganges River dolphin (Platanista gangetica) and the Indus River dolphin (Platanista minor). And the Irrawaddy dolphin (Orcaella brevirostris) is another cetacean that lives in both fresh and saltwater environments. And, in South America, there is the Pink river dolphin (Inia geoffrensis).

Alligators and crocodiles can happily live in fresh or brackish water. From this site, regarding the American croc’s habitat: "Both freshwater (including river, lakes and reservoirs) and brackish coastal habitats (including tidal estuaries, coastal lagoons and mangrove swamps). A large population is present in Lago Enriquillo (Dominican Republic), a landlocked hyper-saline lake. Crocodiles in these conditions osmoregulate primarily by drinking available freshwater. Possibly the most unusual location is a population which occupies the brackish water cooling canals at the Turkey Point nuclear power plant in Florida. "

Same site says this about the American Alligator: "Primarily freshwater swamps and marshes, but also in rivers, lakes and smaller bodies of water. They can tolerate a reasonable degree of salinity for short periods of time, being occasionally found in brackish water around mangrove swamps, although they lack the buccal salt-secreting glands present in crocodiles. "

Sea Lampreys

Bull Shark
(Carcharhinus leucas)

Cool cites, DF. I’m quite the lover of Dolphins, though I don’t know very much about them. Thanks!

Better or worse than the dreaded candiru?

You tell me.

river shark. Better.

I’ve read that some mollies (Poecilia species) can live in full seawater (they’re a common freshwater aquarium species, though they like a bit of salt; they’re livebearers, in the same family as the above-mentioned swordtails and close relatives of guppies).

Well, IIRC, the Bull Shark holds the world record for biting humans. They’re very aggressive.

The Candiru, as you probably know, will get stuck in your dingus with it’s backward projecting spines, requiring an operation to remove it.

So, is it better to endure the intense pain of the Candiru, or have large chunks of your person excised by a Bull Shark, probably requiring a premature and expensive funeral?

Personally, I’d go with the Candiru, but to each his own.

Of course, there’s always the possibility that after being invaded by a Candiru, you might be immediately eaten by a Bull Shark. That would be hilarious. (If it were you, that is).

Note that, strictly speaking, anadromous refers specifically to saltwater fish that return to freshwater to breed, such as salmon. The opposite term, “catadromous” refers to fish that spend their lives in freshwater and then go into the ocean to reproduce. The American eel, for instance, migrates into the Sargasso Sea to breed at the end of its life: Home - Gulf of Maine Research Institute

As does the European eel.

Guppies! It’s a great way to feed your lionfish… what you do is buy a trio of guppies and slowly acclimate them to a salinity matching your saltwater tank…

this is easily done, by the way, you just add salt slowly over weeks until the specific gravity is in the neighborhood of 1.023, then they can be moved to your full-salt tank where they’ll breed like mad and keep your predatory fish well fed :slight_smile: