Sawfish Outreach and Education

HOW LONG DO SAWFISH LIVE?

Lesson Summary:
This lessson compares shark centra to tree rings. Shark centra are used instead of sawfish centra since sawfish are endangered. However, the principle is the same across the elasmobranches which include sharks, rays, skates, and sawfish. By determining that shark centra do record annual growth lines similar to what is observed in trees, conclusions can be made about the age of the shark from which the centrum in the “Sawfish In Peril” teaching kit came from.

Vocabulary:
Growth, vertebrae, centra, centrum, elasmobranches, cartilaginous

Background Information:
Shark centra (similar to our vertebrae) are useful in determining the age of elasmobranches which includes sharks, rays, skates, and sawfish. Members of this subclass of cartilaginous fish record annual growth rings corresponding with the seasons similar to the growth rings found in trees. Dark bands indicate seasons of slower growth rates typically during the winter months while lighter bands represent faster growth rates during the summer months in both elasmobranches and tree rings. By counting these bands, we can determine the approximate age of the shark at the time of their death. This method has been tested in different species of sharks, rays, and skates. However data is currently lacking for sawfish primarily due to their endangered status.

Materials:
Copies of the activity sheet
Pencils

Procedure:
Begin this activity by passing around the shark centra and tree section that is included in your “Sawfish In Peril” teaching kit. Ask students if they notice any similarities between the two specimens. Also be sure to ask if they know what the rings represent. Give the students their activity sheets and ask them to determine the age of the tree section, then the ages of the different centra drawings by counting the number of annual growth lines. Discuss how the wider growth rings represent more growth per year than the narrower growth rings and how much faster one shark grew than the other when comparing the centra drawings on the activity sheet.

Discussion Questions:
How are shark/sawfish centra similar to trees? Are the lines in shark centra laid down annually? How can growth rings in the centra be used to compare growth rates with other sharks?

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