Question: Are sharks mammals? If not, then what are they, anyway?

Answer: No, sharks are not mammals. They’re not amphibians or reptiles either. They’re also not their own group. They are, in fact, fishes.

But they’re a different (and fascinating) subgroup of fishes from the tuna or goldfish you may be more familiar with! Most fishes are Osteichthyes “bony fishes,” which means their skeletons are made of bone, just like ours, or those of other mammals, or birds, or amphibians and reptiles. In contrast, sharks and their relatives, the skates, rays, and chimeras, are Chondrichthyes, which have skeletons made out of cartilage-the same stuff our ears and nose are made out of. (Learn more about shark cartilage in a past Ask A Marine Biologist Do Sharks Have Bones? | Sport Diver ). You may have also heard the term Elasmobranch, which correctly describes sharks, skates, and rays, but not chimeras.

So what’s the difference between a shark and the other chondrichthyan fishes, the rays, skates, and chimeras? As most divers know, rays like stingrays and manta rays have a much flatter body than sharks with wider “wings”, but there are many kinds of rays. Sawfish are rays, and so are guitarfish (one species of which is, confusingly, sometimes called a “shark ray.”)

Related Reading: Are All Sharks Scavengers?

Skates are also flattened and have ray-like bodies at first glance, but are different from rays in several ways. Skates have a visible dorsal fin, while this feature is usually not found (or is so small that you can barely see it) in rays. Rays give live birth while skates lay eggs (which are often called “Mermaid’s Purses” when they wash up on the beach). And skates have shorter, stubbier tails than the longer, thinner tails found in most rays.

Chimeras look kind of like small sharks and, to use a term that’s going to get me in trouble with scientific colleagues, look a little more primitive than sharks. They’re sometimes called “ratfish” (which is a terrible name for something so awesome) and sometimes called “ghost sharks” (which is a term that I like a lot more).

Related Reading: What is the Biggest Threat to Sharks in the Next Decade? What Can We Do to Stop it?

Oh, and “fishes” is indeed a valid term. One skipjack tuna is a fish. Two skipjack tuna are fish. But one skipjack tuna and one albacore tuna, or any combination of multiple species, are fishes.

*Ask a Marine Biologist is a monthly column where Dr. David Shiffman answers your questions about the underwater world. Topics are chosen from reader-submitted queries as well as data from common internet searches. If you have a question you’d like answered in a future Ask a Marine Biologist column, or if you have a question about the answer given in this column, email Shiffman at WhySharksMatter@gmail.com with subject line “Ask a marine biologist.”

Dr. David Shiffman is a marine conservation biologist specializing in the ecology and conservation of sharks. An award-winning public science educator, David has spoken to thousands of people around the world about marine biology and conservation and has bylines with the Washington Post, Scientific American, New Scientist, Gizmodo and more. Follow him on @WhySharksMatter on Twitter, Facebook and Instagram, where he’s always happy to answer any questions about sharks.*

The views expressed in this article are those of David Shiffman, and not necessarily the views Scuba Diving magazine.