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Young World
WORLD OF SCIENCE
Fish? Not quite
DR. T.V. PADMA
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Did you know that every animal that lives underwater is not a fish?
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Not every animal that lives underwater is a fish, either. Which of these six sea creatures are fish, and which ones aren’t? Starfish, jellyfish, sea horse, shark, whale, dugong.Interestingly, the two animals in this list having the word ‘fish’ as part of their name, are actually not fish at all. Jellyfish are coelenterates, not fish. Coelenterates are invertebrates (animals without backbones). They often have sti
nging cells. So if you see a jellyfish washed up on the beach, don’t touch it.
Sea stars
It’s also a misnomer to call a starfish ‘starfish’. It would be more accurate to call them sea stars. Starfish or sea-stars are echinoderms. Echinoderms are a special group of animals that have no back-bones and show five-sided symmetry. What’s five-sided symmetry? Think of cutting a seastar into five parts such that each part is similar in shape to the other — can you do it? Yes.
Some other interesting facts about sea-stars: they can regrow their ‘limbs’. They also have interesting table manners. When seastars eat, they wrap their ‘arms’ (or ‘feet’ if you’d rather) around an animal such as a clam, for instance. Of course, when a seastar does this, the clam immediately clams up and closes its shell tightly. The seastar pulls and pulls at the two shells and prises the clam apart. Even if the clam doesn’t open all the way but just a little, it’s usually enough. The seastar then sticks its stomach out (yes, it actually puts its stomach out into the soft part of the clam) and digests it. Think that’s strange? To a seastar, it must seem a strange concept to put food into the stomach instead of sending the stomach out to where the food is!
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