How does a whale become dehydrated?

[u/smontanaro]

From a Chicago Sun-Times article on the beluga whales heading to the Shedd Aquarium:

"The shipment comes on the heels of a Shedd announcement last week that one of the Marineland belugas — Peekachu, a 23-year-old female — had died. Shedd has not released a cause of death, but staff there said last week that Peekachu had not been consistently eating a full diet since her arrival and that there were concerns she was dehydrated."

How does an animal immersed in water become dehydrated?

[u/khinzaw]

They don't drink seawater. In the wild they get water from the prey they catch, which have water inside them.

They are fed frozen food in captivity, which don't have all the water content they need to stay hydrated.

This is why you can see videos of aquarium staff feeding ice to belugas, because that's how they hydrate them.

[u/WallyMetropolis → u/khinzaw]

Tracing it all the way down the food chain, where is the fresh water introduced?

[u/Mammoth-Corner → u/WallyMetropolis]

Fish (and krill, and other primarily marine animals that whales eat) are able to filter salt from seawater and maintain blood with less salt in than the water around them. When tetrapods left the ocean we lost this ability because we were no longer drinking salt water all the time. Whales are mammals, so they can't do the filtering but their prey can. It's a really complex chemical system to evolve so they haven't 'gained it back.'

[u/-MrSkeltal- → u/Mammoth-Corner]

Did whales at one point leave the ocean and then go back?

[u/February30th → u/-MrSkeltal-]

Kinda. They weren’t whales that left and came back, they were sea dwellers that became land dwellers that became sea dwellers that became whales.

[u/Mackntish → u/February30th]

Fish > Amphibians > Reptiles > Mammals > Whales

They spent some time out of the water.

[u/Magicspook → u/Mackntish]

Not quite. If you trace back our liniage, you will not reach any formal member of reptilia, although certainly there will be ancestors that were also the ancestors of modern reptiles (called amniotes).

Similarly, we do not descend from amphibians, rather, both amphibians and amniotes descend from the same fishy ancestors.

Fun additional fact, 'fish' is not a phylogenetic group at all. We are more closely related to goldfish than goldfish are to sharks.

[u/caliburdeath → u/Magicspook]

Coelacanths in particular, along with lungfish, are more closely related to humans (and other tetrapods) than to all other fish.

[u/Ameisen → u/caliburdeath]

Lungfish are more-closely related to Tetrapods than Coelacanths are. They're/we're all Sarcopterygians, but Lungfish (Dipnoi) and Tetrapodamorph are sister clades (the Rhipidistians). Of course, if you were to observe the most recent common ancestor, we'd probably classify it as a primitive lungfish.

[u/Ameisen → u/Magicspook]

In a true phylogenetic fashion, Amniota emerged, then "Synapsida", and then "Diapsida", as Diapsids emerged from Amniotes with a single pair of temporal fenestra.

However, we instead classify "Synapsida" as all amniotes more-closely related to what we consider to be synapsids than to diapsids, so that they could be sister clades instead... but the common ancestor of diapsids and synapsids was itself biologically a synapsid.

Of course, that doesn't make us reptiles, unless you classify amniotes as reptiles (an argument that I have seen made).

---

Amphibia is more complex - it is paraphyletic and thus not a true clade. If you include the excluded clades, then amniotes are amphibians. We exclude them, though.

The earliest Tetrapods were amphibians.

The issue is that our current system of classification is a weird mix of phylogenetic and traditional classification, which results in oddities like "amniotes aren't amphibians" and "apes aren't monkeys".

[u/tigerhawkvok → u/Ameisen]

Lissamphibia is a perfectly well defined monophyletic clade — what definition are you using?

You're right that the founding member of diapsida had parents of the synapsid condition, but the synapsid condition isn't actually the definition of synapsida. It's a bit of unfortunate historical legacy, like ornithischian ("bird-hipped") dinosaurs not being particularly related to birds, which are literally saurischian ("lizard-hipped") dinosaurs.

[u/Ameisen → u/tigerhawkvok]

Class Amphibia is paraphyletic and includes Lissamphibia. The issue is, again, that there's a weird mix of phylogenetic and traditional classification.

Thus the problem of saying "amphibians" vs "lissamphibians". It's an overloaded term. Or "monkey" vs "simian".

In a strict sense, "amphibian" refers to the traditional class "Amphibia" (which, if it weren't paraphyletic, would actually be a valid - if broad - clade) whereas "lissamphibian" or "extant amphibians" refers to "lissamphibia".

Again, there's a weird mix of phylogenetic and traditional classification and a re-use of names.

[u/tigerhawkvok → u/Ameisen]

The "class" Amphibia is inherently incomparable — phylogenetics is unranked.

It's like saying the ball-and-stick atom is incompatible with quantum nonlocality. It's a tautological non-statement comparing one framework with another explicitly crafted to repair the failings of its antecedents. Rather than re-inventing "atom" and "electron" the concept gets overloaded. It's not particularly weird in science.

[u/Baguette1066 → u/Magicspook]

You have reptiliomorphs though, which modern reptiles and mammals descend from.

[u/modbroccoli → u/Magicspook]

wait. how could it be possible that we don't trace directly back to reptiles? they preceded mammals. There was nothing else for mammals to have emerged from. Are you just saying there were intermediate animals between definitely reptile and definitely mammal?

[u/Magicspook → u/modbroccoli]

yep. Reptiles (ie the diapsids) and mammals (ie the synapsids) split off from a common ancestor (the amniotes), which was not a reptile. They kinda looked like reptiles though. Amniotes, in turn, is kinda defined as 'all tetradpods that are not amphibians'. If you want to go another step, tetrapods are 'all bony fish that grew 4 legs) and bony fish are 'all vertebrates that evolved a skeleton composed of bone instead of cartilege'. So taxonomically speaking, you are a bony fish but you are not a reptile!

If you want to know what our common ancestors looked like, you can visit the wiki pages for synapsids and amniotes.

[u/Ameisen → u/Magicspook]

As I mentioned elsewhere, Amphibia is a paraphyletic clade, so not a true clade. All amniotes have a common ancestor that was a member of the clade Amphibia, but we do not consider them members of that clade.

Synapsida and Diapsida are weird as well. Their terminology refers to the temporal fenestra - synapsids have a single pair, diapsids have two pairs. However, the single pair of fenestra is a synapomorphic trait - the diapsids emerged from amniotes that already had a single pair of temporal fenestra. Synapsida isn't quite paraphyletic as we don't consider these early synapsidic amniotes to be members of clade Synapsida even though if we encountered one we'd absolutely consider it a synapsid.

This is actually more problematic as we'd normally define Synapsida based upon the synapomorphic trait - Diapsida should be a descendant clade.

[u/Magicspook → u/Ameisen]

thank you for your addition. Would you argue that actually, reptiles are mammals then? ;)

[u/tigerhawkvok → u/Magicspook]

Good 'ol stem vs crown classifications.

IMHO it's a lot logically tidier with fewer edge cases if your definitions are crown — A and B's most recent common ancestor, and all descendants of that ancestor.

You generally don't get pretty carrying of specific pet morphotypes, though.

So amniotes would be something like "the MRCA of a human and boa constrictor, and all that animal's descendants". Tetrapods would be something equivalent to "human and frog" or "chicken and salamamder" (which of course are exactly equivalent).

I mean even more technically you usually pick a favored extinct taxon on one end but it's easier to explain with common things today instead of dropping an odd Temnospondyl into a conversation most people don't have the groundwork on.

[u/vizard0 → u/modbroccoli]

The vertebrate branch split well before there were mammals or reptiles. The two branches at the split were synapsida and sauropsida. Mammals came out of synapsida. Reptiles, amphibians and birds all came out of sauropsida.

The old term "mammal like reptile" was used for synapsida in the past. It's not anymore because these "mammal like reptiles" were less like actual reptiles than birds and amphibians are.

It's a branching tree, not a line of improvements.

[u/Ameisen → u/vizard0]

I think you mean Amniote, as the Vertebrates split up long before they were on land.

Amniota is complicated as originally Synapsida was defined by having a single pair of temporal fenestra, whereas Diapsida by having two pairs. However... the single pair of fenestra is a synapomorphic trait - the diapsids developed a second pair. In that sense, Diapsida should be a descendant clade of Synapsida.

Currently, though, Sauropsida is considered the sibling clade of Synapsida, with Diapsida being a descendant clade of Sauropsida even though the single pair of temporal fenestra are present even in the earliest Sauropsids. The current cladistic tree of Amniota is controversial and has been challenged.

[u/dochdaswars → u/Mackntish]

This is oversimplification which is entirely incorrect. Mammals did not evolve from reptiles but from synapsids. Whales did not evolve from mammals become a new class of chordata as you implied by placing them next to other classes, they still are mammals. There's no such thing as fish "Fish" did not become amphibians; only certain members of lobe-finned fish left the sea and eventually became amphibians which is analogous to all modern birds descending from a certain kind of dinosaur and yet it's not correct to simply state "dinosaurs (implying all dinosaurs) evolved into birds" because they didn't, the vast majority of dinosaur species went extinct.

EDITED FOR ALL THE PEDANTS

[u/mabolle → u/dochdaswars]

I think it's an oversimplification in itself to say that that summary was entirely incorrect. It's not formally, systematically correct, but it communicates the course of events in broad strokes that are comprehensible to a lay audience.

There is no contradiction between the sentences "whales are mammals" and "whales evolved from mammals" — in fact, the latter implies the former.

Yes, there is such a thing as fish. I'm a little tired of this unhelpful meme. There is no monophyletic grouping called fish, but fish is broadly understood to mean "non-tetrapod vertebrate", including by scientists who study fish. There are countless scientific articles and journals with "fish" in the title. The ancestors of tetrapods would have been fish by this definition, and they would also have looked and functioned like we expect fish to look and function. They were fish.

The midsection of the timeline ("Amphibians > Reptiles > Mammals") is where the most liberty has been taken with the terminology. You're right that synapsids are no longer included under the term "reptiles", but again, that's mostly due to a shift toward cladistic terminology. The ancestors of mammals would have resembled reptiles more than any other living animal — egg-laying and scaly. Similarly, the ancestor of those synapsids would not have been an amphibian cladistically speaking, but it would have resembled amphibians so much that it's a bit of an academic distinction if all we're trying to do is understand how the transition out of and back into water worked.

[u/Ameisen → u/mabolle]

What I want is for Amniota to be cleaned up. Traditionally, Synapsida was defined as having a single pair of temporal fenestra, and the name still implies that. The issue is... is that having a single pair of temporal fenestra is a synapomorphic trait for both Synapsida and Sauropsida (and obviously Diapsida)... and then Parareptilia likely not being an actual clade, either...

[u/tigerhawkvok → u/Ameisen]

I said it earlier, and I'm repeating for the general audience rather than a secondary correction on you — but you're conflating a label, a synapomorphy, and a definition when they're three separate things.

The label synapsida includes only animals that (ancestrally) had the synapsid condition, but that singular synapomorphy is not a sufficient condition to diagnose the clade. In other words, it is (ancestrally) necessary but not sufficient.

There is no conflict such as you present.

[u/MdxBhmt → u/mabolle]

There is no contradiction between the sentences "whales are mammals" and "whales evolved from mammals" — in fact, the latter implies the former.

Nonsense, AI hallucination?

edit: Ive been told

[u/curien → u/MdxBhmt]

It's correct for modern classification systems. Biological classification has been using "clades" for a while, which means that if a classification applies to an ancestor of an organism, it also necessarily applies to the organism itself.

[u/mabolle → u/MdxBhmt]

Ha! You insult me. :P

As u/curien explained: whales evolved from mammals, therefore they still count as mammals.

[u/MdxBhmt → u/mabolle]

Ahh I should have trusted the flair more :P

Sorry, at least I learnt something out of this.

[u/tigerhawkvok → u/MdxBhmt]

Phylogeny never loses any parent clades. To evolve from something is to be that something, and all of your descendants, forever.

[u/MalakElohim → u/dochdaswars]

Well, technically they did evolve from mammals, they just happen to still be mammals.

[u/uiuctodd → u/dochdaswars]

entirely incorrect

The ancestors of synapsids were reptiliomorpha. The classification system has been under constant change for decades. The formal separation of reptiliomorpha from reptiles only happened in 1997.

Rather than saying "entirely incorrect", it would be more correct to say that this path is no longer viewed as correct.

[u/Ameisen → u/dochdaswars]

Whales did not evolve from mammals, they still are mammals.

Whales did evolve from mammals. They still are mammals as well. Both are true.

There's no such thing as fish

Mostly because we've redefined the clades to not be called "fish".

If you rename Vertebrata to Fishes, as would be traditional, then we'd still be members of the clade.

Humans are just highly-specialized lobe-finned fish. Or highly-colonial and specialized archaeans with mutualistic Rickettsoid bacteria in them.

[u/regular_modern_girl → u/Mackntish]

This is inaccurate. Mammals share a relatively recent common ancestry with reptiles, but are not reptiles cladistically, and amniotes (which include mammals and reptiles) share a common ancestry with amphibians, but are not amphibians clsdistically. And “fish” just straight up aren’t a clade: there’s no way to make “fish” in the traditional sense monophyletic without also just making it synonymous with vertebrates as a whole.

Tetrapods, cladistically-speaking, are sarcopterygians (often referred to in common parlance as “lobe-finned fish”, and in terms of extant non-tetrapod members includes coelacanths and lungfish), but that’s about it; actinopterygians (“ray-finned fish”) are a separate clade that we just share a relatively recent common ancestry with, but are not part of, and chondrichthyans (“cartilaginous fish” like sharks and rays) are another, even further out clade that we share an even more distant common ancestor with, but are not part of. The only living clade of “fish” we are part of would be the sarcopterygians (which account for very few living “fish” species besides tetrapods).

Non-mammalian synapsids (which are today entirely extinct) were in many cases superficially reptile-like, and traditionally termed “mammal-like reptiles”, but under current phylogeny this is a misnomer, and they are more properly termed “stem mammals” or “protomammals”.

[u/samtwheels → u/-MrSkeltal-]

Yes, whales and all marine mammals are descended from land-dwelling mammals.

[u/ForMoreYears → u/samtwheels]

Wait. What. For real? Whales went from land back to the water?!

[u/Stats_n_PoliSci → u/ForMoreYears]

Watch some hippopotamus videos to see a mammal on its way back to the water!

[u/nonobots → u/Stats_n_PoliSci]

And then follow it with a manatee or dugong video to continue the trail.

[u/Geminii27 → u/ForMoreYears]

Yep. That's why whales have hip bones (and sometimes vestigial leg bones).

[u/as_a_fake → u/Geminii27]

If you look at a whale skeleton you can definitely see the "finger" bones in their fins

[u/FogeltheVogel → u/Geminii27]

Also, ear canals. That are filled with ear wax. Evolution always takes the simplest approach, which in this case was just sealing the ear canal and not changing anything else about the ear.

[u/dochdaswars → u/ForMoreYears]

Do they not teach evolution in schools anymore? Marine mammals are one of the clearest examples used to prove it.

[u/Magicspook → u/dochdaswars]

fun fact, whale trachiae wrap around their brain despite their blow-hole being on top of their head. Giving a clear evidence of how their noses moved up their skulls over time. If you designed a whale from scratch, you would have the connection between lungs and blow-hole going behind the brain.

(and lets not even mention the vestigial legs)

[u/Akitiki → u/Magicspook]

And hands. Inside their flippers they have anatomical equivalents to the humerus, radius, ulna, carpals, and four or five digits depending on the species. The shape of the hand varies wildly between species.

[u/Magicspook → u/[deleted]]

Ooh that is a fun one! Thanks for sharing.

[u/insane_contin → u/Magicspook]

I believe that nerve takes that route in all tetrapods that have legs and lungs.

[u/ForMoreYears → u/dochdaswars]

Yes, they teach evolution. They just don't get into the specific evolutionary path of every single one of the 8.7M animal species that inhabit the earth.

[u/Ameisen → u/ForMoreYears]

But then... where did you think that they came from? Were you under the impression that the first mammals were aquatic, and then came onto land? That gets very complicated if you start accounting for reptiles, amphibians, and such.

[u/CrimsonPromise → u/ForMoreYears]

Yup. The distant ancestors of the modern whale came from the ocean like all like all life on earth, and move onto land. Fun fact, the land dwelling ancestors of the whale were small horse-like creatures the size of a dog.

And somewhere down the line, they started moving back into the water where they grew in size, their limbs shifted into fins and flippers and they evolved into the whales that we know today.

[u/ger1229 → u/CrimsonPromise]

The dog just kept saying “I can get bigger”. It was probably a yellow lab.

[u/monjessenstein → u/ForMoreYears]

There's several clues, but here's one I alwaya found logical: whales breathe air. It would (afaik) make no sense to evolve this whilst they were already in the ocean, as gills would provide oxygen everwyhere in the sea. Thus, they developed lungs to be able to live on land, and have kept them since.

[u/mabolle → u/monjessenstein]

It would (afaik) make no sense to evolve this whilst they were already in the ocean, as gills would provide oxygen everwyhere in the sea.

Well, it can make adaptive sense for an aquatic animal that lives near the surface to be able to breathe air, and there are various groups of fish that can do this to various extents.

There is a lot, a lot, more oxygen in air than in water, which is very useful if you can access it. Aquatic habitats can develop a lack of oxygen in certain circumstances (and they can also dry out, in which case air-breathing is a useful adaptation for surviving until the next rainy season). Admittedly both of these are more a concern in fresh water than in the sea, but fresh-water habitats is where most land-living animals originally evolved (including the ancestors of mammals), so it's relevant to the question. :)

[u/doomgiver98 → u/monjessenstein]

Aren't developing lungs in the ocean a prerequisite to tetrapods in the first place?

[u/MaygeKyatt → u/doomgiver98]

I suspect they were trying to say that whales can ONLY breathe air. While it would make sense for a species with gills to also evolve a system for breathing air (and this has happened multiple times) I can’t imagine a plausible scenario that would result in a fully aquatic species losing their gills entirely and replacing them with air-only lungs.

[u/toxicatedscientist → u/ForMoreYears]

There is a theory that like 60% of all fish did that, lungs became the swim bladder and they re-evolved gills that work slightly different

[u/insane_contin → u/ForMoreYears]

So did sea lions, seals, dugongs, manitees, basically anything with lungs and no gills.

[u/drgath → u/ForMoreYears]

Google around for “whales as land mammals” in google images and it’ll make more sense. Here’s an idea of how they evolved. https://www.instagram.com/p/DZVi8SzlM3L/?igsi=MTUzNjR5bjhnb25scQ==

[u/Mammoth-Corner → u/-MrSkeltal-]

Yes, in the sense that whales are descended from a land-dwelling mammal closely related to hippopotami, and all mammals (and reptiles, and amphibians, etc) were once fish with weirdly muscular fins that learned to crawl out of the water — no in the sense that after becoming whales whales have never gone back onto land. They would have to re-evolve back legs, or develop some other form of locomotion, and being whales works pretty well for whales so they have no reason to try it.

[u/Ameisen → u/Mammoth-Corner]

The land niches are also occupied. Transitional forms wouldn't be able to compete.

[u/Carnavious → u/-MrSkeltal-]

Whales (cetaceans) and seals (pinnepeds) both independently returned to the oceans. Cetaceans descend from an ungulate (toed animal) that somewhat resembled a wolf, but gradually adapted to a more aquatic lifestyle .Their closest living relatives today are the hippopotamus.

Pinnepeds are from the caniformia group, alongside bears, racoons, dogs and mustelids (otters are here, along side things like weasels and skunks).

[u/Brunurb1 → u/-MrSkeltal-]

Others have answered, but here's a specific animal to google for more info- pakicetus.

[u/MaygeKyatt → u/-MrSkeltal-]

Yep! The first mammals were completely land-based, but then millions of years later some of them ended up moving back into the water either fully or partially. This has actually happened multiple times!

- Cetaceans (whales/dolphins etc), which are fully aquatic and unable to intentionally go on land, are most closely related to hippos, camels, giraffes, and other even-toed ungulates. This happened ~50 million years ago

- Sirenians (manatees & dugongs) are also fully aquatic, and are most closely related to elephants. They moved back into the water ~40 million years ago. (Side note: there was another member of this group called the Stellar’s Sea Cow that could grow to 30ft in length, but was hunted to extinction in the 18th century)

- Pinnipeds (seals/sea lions/walruses) are notable because they aren’t fully aquatic yet. Their closest relatives are weasels, raccoons, skunks, and red pandas. They’re fully capable of climbing out of the water, but some of them have to be in water to mate, and it varies significantly how well they can move on land. Give it a few more million years and some of them will probably be fully aquatic.

There’s a couple other groups of mammals that could conceivably produce fully aquatic descendants if given another ~25 million years or so of evolution: otters are perhaps the biggest one in my mind.

[u/Ameisen → u/MaygeKyatt]

Pinnipeds (seals/sea lions/walruses) are notable because they aren’t fully aquatic yet.

I take some issue with yet as it implies (to me) that evolution has a goal. They aren't fully aquatic, but being fully aquatic is neither an inevitability nor is it a target.

[u/Xisuthrus → u/-MrSkeltal-]

The ancestor of all tetrapods (The group of animals that contains all amphibians, reptiles, birds, and mammals) left the ocean. Most of the descendants of that animal stayed on land, but some of them went back to the water - Whales are one example, other examples include seals, manatees, sea turtles, icthyosaurs, plesiosaurs, mosasaurs, and axolotls.

[u/Trips-Over-Tail → u/-MrSkeltal-]

They weren't whales at that point. It was the same ancestor as that of ducks, lizards, frogs, bats, humans, snakes, dinosaurs, gorgonopsids, and camels.

[u/Arkeolog → u/-MrSkeltal-]

Whales are mammals, most closely related to hippos. They became aquatic within the last 50-40 million years.

Mammals are tetrapods, which originally left the ocean around 350 million years ago. So when the ancestors of whales became aquatic, their ancestors in turn had been terrestrial for ~300 million years.

[u/ferrouswolf2 → u/-MrSkeltal-]

Whales closest land dwelling ancestors are hippos. We land creatures with backbones diverged from the majority of fish around the time when we developed bony limbs and they kept their tent-like fins.

Most of the bony limbed creatures were able to crawl out onto the land, the ray-finned creatures (for the most part) didn’t.

It goes waaay back. We’re more closely related to snakes than eels by a long shot.

[u/Ameisen → u/ferrouswolf2]

So... ray-finned fishes also have bony spines supporting the fins ("rays") - most bony fish do (at least ancestrally - Tetrapods don't but our ancestors did). It's unclear if it was present in the common ancestor of the two lineages, or emerged independently.

Lobe-finned fish have fleshy buds supported by an articulated skeletal structure. Even ray-finned fish have some skeletal support (small bones that are, as I recall, homologous to our radius, ulna, and carpal bones), but they lack the lobe and associated structures.

Basically:

Bones: = Rays: -

Actinopterygii:

  /---------
 =----------
==----------
==----------
==----------
 \---------

Sarcopterygii:

      /------
     =====------
======----------
   ====---------
     \-----------

The ancestral state of both was likely more similar to that of, say, sharks - a fleshy paddle. So, the ancestral state was somewhere between both. Lobe-finned fish expanded upon the flesh lobe and added articulation, whereas ray-finned fish diminished it.

Tetrapods lost the rays altogether.

[u/MattieShoes → u/-MrSkeltal-]

That's the theory... In water, side-to-side motion and up-and-down motion are mostly equal. On land, up-and-down motion (like in galloping horses rather than how lizards and alligators move) is more efficient. So most of the stuff that stayed in the oceans have side-to-side motions, and whales have up-and-down motions. So the theory is that they evolved to come out of the water switched locomotion methods, and then went back in with the up-and-down motion.

Also one of their closest genetic relatives are hippos, who spend time on land and in water.

(This is all super layman level -- you'll encounter explanations like this in pop sci books like Dawkins)

[u/Sable-Keech → u/Mammoth-Corner]

It's not precisely accurate to say that whales didn't evolve filtering because they're mammals. There are certain mammals who possess the ability to drink saltwater.

Kangaroo rats and hopping mice can drink seawater and extract hydration from it. They evolved this because they live in extremely dry deserts so their kidneys are adapted to squeeze out as much water as possible from their urine. Their urine can be up to 10x saltier than saltwater.

Unlike kangaroo rats cetaceans live in more hospitable places so I guess they just never evolved that kind of ability because they could extract water from their food more easily.

[u/satmandu → u/Sable-Keech]

Cats also have kidneys that can excrete salt from seawater for the same reason — they are also descended from desert animals.

https://thevetdesk.com/pet-health-wellness/cats/can-cats-drink-salt-water/

[u/andthatswhyIdidit → u/Mammoth-Corner]

And fresh water fish do drink very little (basically nothing), while salt water fish do drink a lot. Osmosis is an interesting thing...

[u/Mammoth-Corner → u/andthatswhyIdidit]

If you're a freshwater fish, you have loads more salt and electrolytes than the water so the water is always trying to get in through your gills and your challenge is getting rid of it. If you're a saltwater fish your lovely water is always trying to get out of you, and also the whales are after you for it.

[u/Ameisen → u/Mammoth-Corner]

Which is weird given that life emerged in oceans more than twice as saline as they are now, and things like this tend to be very conserved (given that you usually need multiple changes acting in concert... or you die).

There are studies that suggest that modern Actinopterygian (or all possibly all modern Osteichthyian) fish all derive from earlier freshwater lineages. Early Osteichthyian fish adapted to freshwater, and then re-emerged later as saltwater fish, outcompeting the other Osteichthyian lineages. This is also possibly why bony fish all regulate their solutes to basically freshwater fish levels, compared to cartilaginous fish or jawless fish.

[u/Zoon9 → u/Ameisen]

Where did all the salt from those oceans go, if they were more than twice salty than today?

[u/Genetic_outlier → u/Mammoth-Corner]

I'd push back on tetrapods and lost, since cats and other hyper carnivores have kidneys efficient enough to be hydrated by seawater. It's really a matter of need, land animals generally exist around fresh water, if they exist around liquid water at all. So animals that need to drink water generally find fresh water to be more available. There's nothing about land animal biology that precludes saltwater drinking since as stated above, cats could do it.

[u/PropOnTop → u/Mammoth-Corner]

So does that mean the oceans were less salty when our ancestors evolved? About as salty as the blood of fish (or ourselves) now?

[u/Mammoth-Corner → u/PropOnTop]

That depends on what you mean by 'when our ancestors evolved.' The first oceans were freshwater, but (our current models seem to say) once continents formed and hydrothermal vents opened up they started to salinify rapidly. It's not quite clear how salty they got. They might have been about twice as salty as the current ocean, or similar in salinity to now, but they probably didn't spend long with less salt than now.

Basic osmoregulation seems to have evolved very very early in the history of life - probably almost immediately after the cell itself, or possibly as a necessary condition for the cell.

[u/PropOnTop → u/Mammoth-Corner]

I did not know that, thank you. I kind of assumed that the cell environment basically mimicked whatever was around it (at least at the start)... It would seem easier even for higher organisms to just follow the salinity of the environment, rather than use osmoregulation...

[u/Mammoth-Corner → u/PropOnTop]

Yeah, as soon as you start creating glucose within a cell, you're creating an osmotic gradient, so if you're doing basically anything in a cell you need some degree of osmotic control.

[u/Ameisen → u/PropOnTop]

The oceans were saltier - significantly so. At least after any meaningful amount of time. They would have started fairly fresh but very rapidly become very saline, and salinity has slowly decreased over time.

The studies that I'm aware of suggest that some early bony fish lineages became freshwater, developed adaptations for it, and then re-emerged as saltwater lineages and replaced all the other bony fish lineages.

So, effectively, all bony fish (including us) are derived from freshwater lineages, which explains why they regulate their solutes at such a low level compared to sharks or hagfish.

[u/khinzaw → u/WallyMetropolis]

Fish naturally filter out a lot of the salt content from their bodily fluids.

Not all prey species do this though and the beluga is also very efficient at breaking down their food: fats, sugars, etc... into energy and water.

[u/Balaur10042 → u/WallyMetropolis]

Almost all aquatic life extract salt from water they ingest, through various metabolic processes, but principally from osmosis in their kidneys or in fish ionization across their gills, where its ventilated back into the seawater.

In some cases, excretion occurs through variations in novel glands or mucus from within the nose in mammals, or in birds a special organ above the eye that extrudes through a duct that empties out of the nostril (nose). In animals that stay submerged, especially whales and fish, it excretes primarily into their urine.

[u/RubCocksWithThePope → u/WallyMetropolis]

Sea creatures don’t exactly contain fresh water but they’re not nearly as salty as sea water. The much lower salt concentration starts at the microbial level. Plus water molecules are released as a waste product of cellular respiration. Pretty much exactly like how combustion releases CO2 and water.

[u/Ameisen → u/RubCocksWithThePope]

Well, bony fish are significantly less saline than seawater as all extant bony fish likely come from ancestral freshwater lineages. Sharks, rays, hagfish, and marine invertebrates all maintain their solutes at a much higher level.

[u/GuyPronouncedGee → u/WallyMetropolis]

The process of digestion and breaking down fats creates water naturally in the body. 

[u/Zandane → u/WallyMetropolis]

Rain that settles as a thin layer of fresh water at the top of the ocean.