National Geographic : 1995 Oct
Survivors and Their Aquatic Turfs * Unique in its ways, the Coahuilan box turtle-like this pair in Laguna de los Burros-spends much of its time underwater. While other box turtles evolved as land creatures, the Coahuilan re mained aquatic, or may have returned to the water from land as local desertification occurred. Omnivorous, it feeds on plants and small fish-anything it can catch. A spring pool embraces Mexico's Desert Aquarium two worlds: one above and one below the waterline. Surrounded by cattails, most pools are alive at the surface with Mexican ducks and migratory waterfowl. Under water their character is dictated largely by water temperature. Fed by a cool spring, Laguna del Hundido is often green with stonewort algae, which in turn swarms with endemic shrimp. An inch-long female grass shrimp (above left) carries eggs in her abdominal pouch, through which devel oping embryos are visible. Pugnacious and posturing, a male Coahuilan pupfish bites a rival's anal fin (left). During breeding season males will not tolerate one another's presence. Hardy survivors, little more than an inch long, Coahuilan pupfish can withstand espe cially high temperatures and salinity. During rare spells of rain, or when variation in a spring's outflow causes local flooding, they swim into the temporary waters, only to be stranded in hot, saline cracks and pools. Awaiting rescue, a few live to fight another day.