Some fish, like sharks, don’t posses an air bladder to help keep them afloat and must either swim continually or rest on the bottom.  

There are over 25,000 identified species of fish on the earth, but there may be over 15,000 fish species that have not yet been identified.

March 14, 2012 | Categories: From the Gremlin

There are over 25,000 identified species of fish on the earth, but there may be over 15,000 fish species that have not yet been identified.