Hidden within the porous limestone of the Yucatan Peninsula is a vast subterranean world where an ecosystem, millions of years in the making, has been found to be dependent upon a genus of cave shrimps.
Separating the Gulf of Mexico and the Caribbean Sea, the Yucatan is a flat expanse covering some 181,000km² and the site of the Chicxulub impact crater formed from an asteroid collision 66 million years ago.
The karst landscape, formed through the dissolution of soluble rock, is largely free from visible water courses, although there is plenty to find beneath the surface.
The water in these underground caves and flooded sinkholes, known as cenotes, is a mixture of fresh and salt, and partially tidal, but the absence of sunlight restricts any plant life, normally the base of a food chain. Instead, as organic matter decomposes in the forest above, it seeps through the limestone and arrives underground where it forms methane. Bacteria known as methanotrophs then form large ‘mats’ on the rock-face, which, in turn, is consumed by several species of Typhlatya shrimps.
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The anchialine systems of the Yucatan have long fascinated people. The Mayan people would use the cenotes, which number more than 6,000, as water wells and also believed them to be gateways to Xibalba, the Mayan underworld. In recent history, the area has become popular for freediving, with the some of the cenotes 100m deep.
For marine biologist Fernando Álvarez, the sinkholes offered fascination. “My first impression of these incredibly beautiful places,” he noted, “was that I had to work there to find out how that rich crustacean fauna had evolved in these exceptionally large cave systems.”
Alongside Brenda Durán, also of the National Autonomous University of Mexico, Álvarez used stable isotope analysis of the carbon and nitrogen within the shrimps’ body tissue to work out the crustaceans’ diet. Their study, recently published in Subterranean Biology, found that different species of Typhlatya shrimp had evolved to find their own dietary niche within the underground system.
“What we see now is that Typhlatya shrimps are a key component of the anchialine trophic web,” Álvarez explained. By converting microbial growth into animal tissue, the shrimps provide vital nutrients for larger subterranean predators feed on. Because of this they are considered a 'keystone species' – an animal, plant or fungi that plays a central role in the structure of an ecosystem.
Worryingly, a rapid urbanisation of the Yucatan Peninsula is placing the ecology of the cave systems under threat. The delicate balance, that has remained intact since the time of dinosaurs, is dependent upon the breakdown of organic matter on the surface, and deforestation will remove that vital component.
Álvarez counsels, “We are losing the vertical integrity that these anchialine caves need to function; any changes occurring on the surface within the caves’ area will inevitably affect them.”
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