Finback whale (Balaenoptera physalus) baleen. Baleen is a series of stiff, flexible strands of material that hang from the upper jaw of Mysticeti (baleen whales) and is used like a sieve to filter their food from the sea water. Commonly filtered food include krill, plankton, and other small organisms. The inside of the baleen is edged with hairy plates that filter krill. Baleen is made of keratin and grows throughout a whale's life.