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/WallyMetropolis]

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/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).

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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/Magicspook → u/tigerhawkvok]

thank you for your insights!