Hippopotamus
There is no doubt that hippopotamus are aquatic mammals, and they spend most of their lives rolling in African rivers and various waterways - in fact, although they are very similar to pigs, their close relatives are actually whales and dolphins! However, they won't really swim because the dense structure of their bones and the weight of their huge body often make them sink.
So whenever hippos are in deep water (as literally), they have to push themselves off the bottom of the water to float out of the water to breathe and then suddenly sink. So, they are not actually swimming, but walking at the bottom while underwater - because their limb muscles are designed to provide strong propulsion in the water, not to swim
keeps breathing for an hour or more.
Many kinds of crabs.
Although crabs are also aquatic organisms, among all the 4,500 species known, only a few have swimming ability. In fact, many of them, such as hermit crab (see the picture above) , because they cannot breathe underwater and therefore cannot swim.
So if immersed in water for more than 30 minutes, they are at risk of drowning, mainly because their gills have adapted to breathing oxygen from the air on the shore;
and other species can survive underwater for a long time by using the same respiratory system as fish.
- To be precise, it draws small oxygen molecules from water through their gill surfaces and enters the blood.
So, since most crabs are not adapted to swimming, they turn to walking on the seabed (see above and below) . They are usually able to walk sideways and bend the second joint of each leg
if needed, because they move much faster in this way.
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