Friday, July 15, 2011

Beachcombing, Part I: Shark Teeth

I love the beach. I love the way the blue sky meets the turquoise water at the horizon. I love the hiss of the waves against the sandy shore. I love the smell as I drive down the highway and get closer to the shoreline. I love walking the beach when the sky is boiling and slate gray, just before a storm. I love walking the beach in the fog in the evening, when everything sounds muffled and close by. I love the crisp blue sky and angry surf after a hurricane passes through. To me, there is nothing like being oceanside, any time of day or year.

My mom came to visit us from her home in Florida, bringing treasures for our youngest son, as always. Among her treasures was a bottle full of hundreds of teeth she found while beachcombing during a recent trip to the ocean. We set about to study them and learn what kind of animal left them behind. We looked for a Outdoor Hour Challenge about beachcombing or sharks, but didn't find one, nor did we find much in the Handbook of Nature Study. So we invented our own activities! We immediately narrowed our choices down to one or more elasmobranchs, the group of bony fishes that includes sharks, rays, skates and sawfish. But which one?


Shark Teeth:

Shark teeth vary in size and shape, depending on the size of the shark and the type of food it eats. They grow a continuous supply of teeth -- every time a shark loses a tooth, a new one grows in. One row of teeth has another row behind it, and the new tooth just rotates into the empty space. A shark can use up to 20,000 teeth in their lifetime.



The teeth we sorted were all pointed and sharp, meaning they were designed to catch and tear meat. All elasmobranchs are carnivores, meaning they eat meat. Some kinds (most sharks, skates, sawfish) eat fish of various sizes. Others (rays, some sharks) have little peg-like teeth, because they sift tiny creatures out of the sea water for their food. So the teeth were not likely from rays.

We consulted the website from the Florida Museum of Natural History to see if we could find pictures showing the difference between shark and skate teeth. To the left is a collection of shark teeth that looks a lot like the ones we have in our bottle.

We also found a drawing of skate teeth. Skates' teeth are triangular, and look pretty much the same no matter where they come from in the skate's mouth. They have multiple rows of teeth, like a shark, on both the top and bottom jaw. Check out the photo, below...
After looking at various illustrations and photographs, we decided that most, or all, of the teeth in our collection were shark teeth.

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If you are planning a field trip to the beach with a group of kids, there is a nice guide for planning beachcombing trips from Australia that you can use to organize your trip, based on the different wash zones on the seashore. 

If you are studying environmental science, the Marine Debris Program in Japan outlines the scientific procedures for documenting the kinds of debris that you find on a beach -- a great project for high school age beachcombers. The Australian government also produces a nifty checklist that you can use to categorize the kinds of refuse that wash up on the shore -- this would be fun project for a middle or high school class or a homeschool co-op.

The Carnegie Library of Pittsburgh has compiled a terrific list of field guides, video guides, nature study books and other resources on beachcombing and shorewalking -- grab a copy of Rachel Carson's The Edge of the Sea for a more ecology-oriented field guide, or A Field Guide to the Atlantic Shore, a Peterson guide that includes plants, animals and shells that you may find on your hike.