Pools like Poza Azul, one of the most photographed of the pozas in Cuatro Ciénegas, seem lifted out of the Caribbean. The water is crystal clear. Black and yellow tropical fish dart about just below the surface and feed by poking around in the white sand lining the bottom of the pools. And just left of the pool’s center is a yawning cave, surrounded by a treacherous shelf of coral.

Yet things are not what they seem. Those fish are Minckley fish, a species found only in Cuatro Ciénegas—the black ones are male and the yellow female. The “sand” is composed not of crumbled rock but of tiny white snails, another endemic species, and, to put it delicately, their excrement. The coral is stromatolite, clusters of living cyanobacteria. The stromatolite in Poza Azul is estimated to be several thousand years old. And the water is hot—in this pool, just under 90 degrees.

Life teems in and around the pozas. Look closely, and you may spot a snake or one of the aquatic tortoises native to the region. Each poza has its own unique character. Some have reeds, others have lily pads, some are cloudy, some clear. Rice biologist and statistician Janet Siefert and her team postulate that these individual characteristics are dictated by the pools’ smallest residents and the chemicals on which they can feed. Where cyanobacteria are abundant and the pools are warm, the water is clear. Cloudy-blue pools are cooler and deeper; sulfur-rich water often enters these pools from the bottom, and the entries are fringed with black cyanobacteria species that assimilate the sulfur. Reddish pools may harbor colonies of bacillus, another microbial resident.

Previous Page | Home Page