Life in Total Darkness–Investigating Underwater Cave Ecosystems

Release Date:

For more than 30 years, scientists have known that remarkably complex ecosystems thrive within underwater coastal caves, habitats that naturally contain no light and very little food or oxygen. Yet, almost nothing is known about the ecology of these systems.

This article is part of the October-November 2018 issue of the Sound Waves newsletter

Photo showing the cave passage and diver, with green tint from the water and strong shadows from the light source.

Cave passage and diver (Bil Philips, cave explorer) in Ox Bel Ha Cave System of the northeastern Yucatan Peninsula. Photo credit: HP Hartmann.

How does life exist in total darkness, in a habitat with little oxygen or food?

For more than 30 years, scientists have known that remarkably complex ecosystems thrive within underwater coastal caves, habitats that naturally contain no light and very little food or oxygen. Yet, almost nothing is known about the ecology of these systems.

John Pohlman of the USGS Woods Hole Coastal and Marine Science Center and David Brankovits, a post-doctoral scholar with the USGS and the Woods Hole Oceanographic Institution (WHOI), are providing answers to these questions. The two are using sophisticated cave diving techniques and sampling devices created at the USGS to learn more about what types of life exist in underwater caves along limestone and volcanic coastlines. With this information, they are developing an understanding of the processes that allow life to flourish within the submerged darkness.

Four men stand together, smiling for the camera, three on left are suited up for scuba diving.

Ox Bel Ha Cave Project Field Team Members (left to right) David Brankovits (USGS/WHOI), Jake Emmert (Moody Gardens), John Pohlman (USGS), and Francisco Bautista De La Cruz (Speleotech). Photo credit: Jacob Pohlman.

Man kneels at a laptop computer balanced on a rock in a forested area.

Deploying equipment in a cenote (entrance of the cave). Photo credit: John Pohlman, USGS.

So far, their research has been conducted along the Yucatan Peninsula of Mexico, where over 1,000 km (about 621 miles) of cave passages within the limestone coastline have been mapped. By analyzing the chemical and isotopic content of water samples and animals collected from within the caves, they have shown that dissolved methane gas and other dissolved organic materials that trickle in from the jungle floor are an important component of the cave-adapted animal’s diet. These dissolved materials though, are not directly accessible to the animals living there—the microbes in particular—unless they mix with oxygen. John and David have shown that oxygen enters the system from beneath, with the seawater, and from above through sinkholes that connect to the caves. Once the dissolved materials have mixed with oxygen, bacteria are able to grow. In turn, higher-level organisms, like crustaceans, feed on the bacteria, which form the basis of the food web for the Yucatan caves.

Diagram that shows the components of a coastal cave and how organisms, methane, and oxygen move through the system.

A summary diagram of their findings from the Ox Bel Ha Cave System of the northeastern Yucatan Peninsula.

Diver in an underwater cave.

David Brankovits collecting water samples in Molnar Janos Cave in Budapest, Hungary. Photo credit: Zsolt Sasdi.

Recently, John and David had the unique opportunity to conduct a similar study in the Atlantida Tunnel, the world’s longest submerged basaltic lava tube that extends beneath the coastline of the Island of Lanzarote in the Canary Islands. Using the same approach as in the Yucatan Peninsula, they collected water samples and animals that they are now analyzing for their stable isotopic composition to construct a model of the food web for lava-tube caves. However, because the Canary Islands are dry and lack dense tropical vegetation found in the Yucatan, they hypothesize the microbial loop in the Atlantida Tunnel is supported by material that originates in the ocean and washes into the lava tube. The study is being conducted in collaboration with Lanzarote and Chinijo Islands Unesco Global Geopark and Dr. Alejandro Martínez García (Water Research Institute IRSA-CNR).

These are very delicate ecosystems; slight changes to the environment can have devastating consequences to the animals inhabiting the area. Thus, as John puts it, “the cave-adapted animals are the metaphorical canaries in the coal mine for the condition of water quality.” In other words, these sensitive life forms serve as early warning indicators for habitat degradation caused by pollution, land-use change, and the effects of climate change. Similar environments are found within the Florida platform and the Hawaiian Islands, where sea-level rise and human activity are impacting the condition of the groundwater. John and David hope to take what they have learned from the pristine settings in Yucatan and the Canary Islands and apply these principles to U.S. coastal margins to understand how to protect and preserve these unusual, but globally distributed, ecosystems.

Diver holds a small plastic tube.

David Brankovits collecting cave-adapted animals for the study in the Yucatan Peninsula.

A diver jumps into water from a low, rocky cliff, and another diver is already in the water off to the side.

David Brankovits and Tom Iliffe entering a cenote (entrance of a cave) in the Yucatán Peninsula. Photo credit: Sergio Benitez.

Interested in learning more? View the YouTube video “This Cryptic Underwater Maze Holds Life That Survives on Methane.” Also, their research from the Yucatan Peninsula was recently published in the journal Nature Communications.

Related Content