Re: have any animals evolved directly from water to air, or air to H2O?

Hi there admirer of flying fish,

Well, there's all kinds of examples I could cite here, but let's start 
with flying fish. As you know, they glide briefly above the water but 
remain fully aquatic. To become primarily airborne, they would have to 
become air-breathers in some way. This is a major evolutionary step and 
has happened via land which requires exactly the same modifications - land 
and air-dwellers both breathe air! So, if there was strong selective 
pressure to leave water due to predation or some other factor, who's to 
say it couldn't happen eventually? Such things are always possible.

So, what about other animals? Well, cetaceans (whales, dolphins etc) 
evolved from land mammals as did the seals, although they still require 
atmospheric air. So this route is possible although the difference between 
air and water-based breathing systems remains as it involves major 
structures. However, there are intermediate examples visible today;

1. Mudskipper - a type of fish which can use its fins to hop across short 
expanses of mud. Possibly how early amphibians developed.
2. Lungfish - tropical fish that remain encased in mud during the dry 
season below temporary pools, then re-emerge after the rains, having 
survived using air sacs.
3. Diving beetle - along with some other invertebrates, this can take a 
bubble of air as a supply whilst submerged - a sort of aqualung.

There are other examples which can be found in zoology books/websites - 
they may cover the physiology of such animals as well as penguins, seals, 
whales and other diving animals which can hold their breath impressively. 
Unfortunately I can't think of a single specific book covering this area, 
so just be imaginative!

Yours

Dr David Hubble,
University of Nottingham, UK