During the age of dinosaurs, many reptiles returned to the water, where their sharp teeth were able to latch onto the millions of fish that knew no other enemies. But these dinosaurs died out over the years, and today the most familiar aquatic reptiles are sea snakes and crocodiles (which you can fine on the Australia page), alligators (which you can find on the North America page), and sea turtles. The seaturtle. Sea turtles can face danger with sharks and saltwater crocodiles, but the biggest enemies of the future welfare of sea turtles are land dwellers. This is because the sea turtles must come on land to lay their eggs, and when they leave, many different predators raid the turtle nests and eat the eggs. And even if the little baby turtles are lucky enough to hatch, their trek back to the ocean is frought with enemies, including birds on the beach and large fish once they do make it to the ocean. However, if they can survive for a year or so, then the hard shell that they develop will protect them in most situations for the rest of their lives. One bird that returned to the ocean is the penguin (which can be seen on the Antarctica page), although many other birds live atop the water, such as ducks and geese (which can be seen in the Barn on North America). Today, however, it is the mammals that make up the greatest numbers of truly aquatic air-breathing animals. They all look somewhat like fish, but all must come up to get a breath of fresh air. Seals and killer whales (orcas) are among these air-breathers, but you can read about them in Antarctica. What is a walrus tusk made of? Ivory. The walrus looks like a huge sealion with large tusks. Because of their size, very few animals will mess with a walrus, including polar bears. About the only animal that will tangle with a large male walrus is another male walrus, hoping to keep a harem of female walruses by dint of its physical dominance. Although the orca is often miscast as a villainous animal in movies, the bottle-nosed dolphin is portrayed in the movies in a more friendly light, such as in "Flipper." Unlike some of the other animals we have discussed on this page, dolphins and whales spend their entire lives in the water. Indeed, whales are so large that if they come on land their heavy weight may prevent them from breathing, and they can die. There are two types of whales - those with regular mouths (like the whale in "Pinocchio") and the baleen whales. Baleen whales have seive-like structures (made of a bony substance called baleen) that they use like tiny fish nets. They take a big mouthful of water, and then force the water through the baleen. This leaves all the little animals on the inside, which the whale swallows. Most of these whales cannot eat anything larger than krill and other tiny sea animals. The blue whale. Oddly enough, however, the largest whales of all are the baleen whales. And the blue baleen whale is the largest animal that has ever lived on the planet Earth. Return to the Ocean.