Underground aquifers in the Western Australian goldfields are a hotbed of biodiversity for underwater beetles. However, these beetles don't have the opportunity to replicate the approach of above-ground diving beetles, who capture bubbles of air at the surface and use them as scuba tanks. For the first time, a study has explained how they thrive in such an apparently inhospitable environment.
The aquifers these beetles inhabit are almost another world, sitting within a layer of porous rocks, but usually cut off from the surface by soil and other rocks that prevent the passage of air or water. University of Adelaide PhD student Karl Jones told IFLScience we only know the little we do about what lives there because of deep boreholes dug to supply the surface with water during the long periods of drought.
Subterranean diving beetles exist on other continents, Jones added, but the complexity of the Western Australian aquifers, which often become cut off from each other, encourages species diversification. We know of around 100 species from 50 aquifers and have probably barely scratched the surface. “It can be a really bustling metropolis down there,” Jones said in a statement.
So how do they do it? Observing the beetles in their natural habitat would be close to impossible, so Jones had to capture some, and even that was a challenge. “We used the bottom half of a fishing rod with a small plankton net attached to the line. We dropped the net into the aquifer water, dragged it up and down a bit and then pulled it up with the animals collected in a small vial at the bottom', he said. “The local farmers must have thought we were crazy.”
Like other aquatic insects, diving beetles use cutaneous respiration, breathing through their skin. Like fish, they collect the oxygen dissolved in water, but squeezing oxygen through their skin is hard, and harder still for larger beetles with thicker skin.
Some other aquatic insects have created modifications, hair or feather-like features that expand their surface areas and contain tracheal tubes to collect more oxygen. Paroster macrosturtensis, P. mesosturtensis, and Limbodessus palmulaoides, the three species Jones collected have not.
Instead, Jones reports in the Journal of Experimental Biology, they rely on their small size – producing a high surface area to volume ratio – and low metabolic rate to survive. They need these things because, besides the lack of reliable access to a layer of air above the waterline, the aquifers have concentrations of dissolved oxygen that vary by at least 40-50 percent.
Jones told IFLScience little is known about the beetles' diet, and they may feed on detritus from above. In captivity, they appeared to appreciate the fresh worms and tiny crustacea they were given.