The Goldfields region of Western Australia is covered in arid shrublands, where mallee eucalypt and acacia bushes reach into the sky in their lowly fashion, and the yellow afternoon sunlight spills through bluebushes, illuminating the red sandy soil. Vibrant wildflowers, including bush tomatoes, mulla mallas and daisies, spring from the ground, showing off iridescent purples, yellows, and whites. Eremophila bushes provide watchtowers for dusty White-fronted Honeyeaters and a drink of sugary nectar from the abundant purple flowers. However, this area was not always this dry and arid. Once rivers flowed through the region, filled with numerous aquatic animals, however, as Australia became more arid over millions of years these surface waters began to dry out. But now, below the red sandy surface, water still resides and some of the ancestors of those surface dwelling aquatic inhabitants remain.
A scientist wanders along the sandy surface looking for a PVC pipe sticking out of the ground. These pipes line boreholes drilled into the water filled aquifers formed by porous calcrete rock. The scientist has half a fishing rod with a reel, and a small plankton net attached to the line which is dropped down into the borehole and the water below. Down in the water, numerous small creatures zip about, swimming, or crawling along the pale creamy coloured calcrete rock. There are numerous types of animals, including various crustaceans like amphipods, copepods and charismatic haloniscus, as well as tiny diving beetles. In this aquifer, the Sturt Meadows aquifer, there are three species of diving beetle, Paroster macrosturtensis, Paroster mesosturtensis and Paroster microsturtensis. The largest, P. macrosturtensis, is only 4.2 mm long. These little beetles look odd compared to their surface dwelling cousins and have many traits typical of subterranean animals including reduced or no eyes, a lack of pigmentation, long hair-like sensory structures, and they have lost their ability to fly.
The scientist is looking for these beetles because their presence poses an interesting question. How do these beetles breathe in these underground waters when they may not be able to find air? Most surface dwelling diving beetles go to the surface to collect a bubble of air under their wing covers (elytra), which they take underwater to supply oxygen for their dive. However, the surface beetles need to return to the surface when the oxygen runs too low in the bubble. The subterranean beetles may not have access to air because they may get stuck in crevices or cracks underwater and there may not be an air-water interface over much of the aquifer because of water infiltrating into the soil and stone above the aquifer.
The subterranean diving beetles overcome this problem by using cutaneous respiration, breathing through the “skin”. Oxygen diffuses through the water to the beetles’ surface, and then through the hard cuticle either into the gas-filled respiratory system, the tracheal system, or directly through and into the tissues and cells to where the oxygen is needed. The subterranean beetles, P. macrosturtensis and P. mesosturtensis and a species from a different aquifer, Limbodessus palmulaoides, also differ from some submergence tolerant surface dwelling diving beetles, in that they do not have microscopic structures on their surfaces like pores or hair like-structures to help oxygen diffuse into their bodies.
There is, however, a trade-off for these subterranean beetles between body size and being able to successfully use cutaneous respiration. Western Australia has the most diverse assemblage of subterranean diving beetles in the world with more than 100 described species from approximately 50 isolated aquifers, but none of the beetles are greater than 5 mm long. As the beetles become larger their body surface area relative to oxygen demand decreases, and their cuticle becomes thicker and more resistant to oxygen diffusion. This makes it more difficult for the larger beetles to successfully utilise their environment by limiting the amount of energy that could be used for activities like finding food and mating.
This blog post is based on research that Roger Seymour, Steve Cooper and I conducted during my PhD and is now published in the Journal of Experimental Biology. See here for the full research paper: https://doi.org/10.1242/jeb.196659