A chance encounter with killer crustaceans led Paul Wood to delve into cave ecosystems, where he found an unlikely predator at the top of the food chain

The freshwater shrimps we found deep in the subterranean streamway of Peak Cavern looked strange. Above ground, where they thrive in brooks, the creatures usually appear plump, pink and robust, with a distinctive black eye. I was very familiar with the species, having studied them as part of my work on the invertebrate communities that live in the relative safety and tranquillity of English chalk streams. But below ground, the specimens we observed were strikingly different. From their white bodies hung long hairy legs and antennae. They were also very thin and on first inspection appeared to have white eyes. I was intrigued and decided to scoop up four living individuals in a sample carrier for analysis under the microscope.

In the laboratory the next day, I opened the container to find that just one shrimp remained. It was slightly fatter than before. I carefully checked the equipment - there was no way any could have escaped. Freshwater shrimps usually feed by shredding leaf material and other particles of decaying organic matter within streams. Although I knew that many invertebrates were capable of preying on other organisms, including their own kind, I had not expected to be confronted by a voracious cannibal. That little creature provided me with a glimpse of the remarkable aquatic food web that existed deep below the English countryside.

As a geographer and freshwater ecologist, I have always been fascinated by the ability of plants and animals to survive in even the most extreme environments. I have wondered how they survive severe droughts that desiccate their habitat and where they seek refuge during floods.

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Nevertheless, the last place I thought I would end up undertaking research was in a cave.

It began by accident. Soon after joining Huddersfield University, I met John Gunn, professor of geography and environmental sciences and a keen caver. It was something of a tradition in the department to join John for a caving trip, and I was curious. On my way out the door, on a whim, I picked up my pond net. Since then, backed by the British Ecological Society and the Natural Environment Research Council, I have been back every month for the past five years.

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British caves are not noted for their biological diversity - they are, if anything, impoverished compared with other European countries. However, during that first visit we discovered a small diving beetle that just happened to have a high conservation status. Furthermore, it was a carnivorous beetle. This suggested that there must be a range of other organisms living in the cave for it to feed on.

Caves and most other subterranean habitats are harsh environments that most animals find difficult to thrive in because of the lack of suitable energy and food resources. Without light, there is an absence of the green plants that form the base of most food chains. Those organisms that do live down there have to exploit the very limited resources available to stay alive.

As a result, many of the true cave organisms are highly adapted to their environment. They frequently withstand periods of starvation and often lack eyes and body pigmentation. There are relatively few of these known in the British Isles, and none has been recorded in the Peak-Speedwell system. The vast majority of the animals that have been found in British caves have been swept in accidentally via flowing or infiltrating water passing through the overlying soil and rock. Most of them perish underground, but a few are able to survive, reproduce and even maintain viable populations.

In the two years following our encounter with the cannibal shrimp, we developed a greater understanding of the aquatic food web in the caves.

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Almost all of the organisms we found were able to utilise a range of scant resources. The beetle recorded on our first visit appeared to feed primarily on the dead and decaying bodies of worms carried into the cave after heavy rains. And there at the head of the food chain, the top predator and consumer of almost everything that was edible, was the humble shrimp.

We now know how the artificial illumination of the outer passages of the show caves open to the public allows algae to develop, providing an oasis of potential food for organisms within the cave. We also know the devastating influence that pollution has on the organisms in the passageways and how affected systems slowly recover. But we still know relatively little about the secret lives of the animals living below ground. Caves now fascinate me. But I will never be able to view a freshwater shrimp the same way again.

Paul Wood is a lecturer in physical geography at Loughborough University.

His work on cave biology will be presented at the British Cave Research Association cave science symposium at Bristol University tomorrow.

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