Look for insects and small fish in the water below the boardwalk. Two species of small, inch-long mosquitofish feed on insect and mosquito larvae. The Big Bend Mosquitofish, Gambusia gaigei, is an endangered species that inhabits this warm fresh-water spring, and is found nowhere else in the entire world. This rare fish has the smallest range of any vertebrate and its survival is threatened by the Western Moquitofish, Gambusia affinis. It is believed the Western Mosquitofish entered this pond courtesy of periodic flooding of the Rio Grande. The National Park Service provides protected breeding ponds to ensure the survival of the Big Bend Mosquitofish.