Can a frog swim on its back, or it can use only the classic frog style? Does any (not specifically marine) animal exist with many different swimming styles, just like humans?
Don't know whether it was ever investigated, but I'm pretty sure, that frogs can in principle swim upside down (e.g. when escaping from predator), but they normally do not do it.
One example of alternative swimming styles in animals is provided by some munnopsidid isopod crustaceans. Several genera are pelagic and have very elongated forelegs equipped with rows of setae and paddle-like hindlegs (see Marshall & Diebel 1995 and Giver 1998):
- the forelegs are used to "walk" in the water column
- the same legs can be stretched out for passive parachuting
- the hindlegs are used for backward swimming
Below is a photo of a parachuting specimen. And here is a very nice video of a backward swimming specimen.
Foto by Karen Osborn from here
Yes, creatures can swim different ways.
If you drop a frog into water while it is in the ventral up position, it will indeed swim upside down to escape before righting itself, which occurs by rotating while swimming. If the water is too shallow, it will flop and flip before jumping away.
Swimming on the surface ventral side up is kind of silly for a frog since it's eyes would be looking into the water, and it wouldn't see predators or prey on the surface. But I doubt if it is impossible. It's certainly not instinctual. To see an aquatic animal belly-up usually means something bad.
Otters can float belly up and even swim that way:
While snorkeling, I have seen many fish (especially parrot fish) swimming completely sideways in order to keep an eye on me while I hover over them.
Male orcas will swim upside down to mate, and will float upside down when semen is collected. Cetaceans have been taught to swim on their sides and expose one flipper to give the appearance of waving. In the documentary Blackfish, you can see Tilicum doing just this thing, circling the enclosure on his side waving for two or three laps before killing one of his trainers. Other orca videos show Tilicum swimming upside down to show off their white ventral surface for the crowd. And dolphins, of course, can swim backwards.
Dogs will do the dog paddle on the surface of the water, but use a slightly different swimming technique to dive.
I wish I could provide videos but I can't find them.