BioEd Online

The diving-bell spider Argyroneta aquatica is unique in its ability to live most of its life underwater. But how does it make sure it always has enough oxygen? Scientists have now discovered that the underwater web it uses to store air is more than just a static diving bell — it can function in a similar way to fish gills by obtaining oxygen from the water.

The spider collects air from the surface in tiny hairs on its body and releases it underneath webs that it spins between underwater plants, forming its own air-filled diving bell. It was previously thought that the spider had to constantly refresh the air by taking frequent trips back to the surface.

However, it has been known for almost a century that air bubbles carried by certain diving insects can actively take up oxygen dissolved in the water, thus supplying much more oxygen than the original bubble contained. This led Roger Seymour at the University of Adelaide in Australia and Stefan Hetz at the Humboldt University of Berlin to question whether the spiders' diving bells could act in a similar way, taking up dissolved oxygen so that the spiders wouldn't always need to refill them with fresh air — exchanging dissolved oxygen rather like an artificial fish gill.

Because the spiders consume oxygen from the diving bell and expel carbon dioxide that dissolves rapidly into water, the team proposed that the decrease in oxygen concentration in the bell would cause oxygen to be taken up from the water. However, whether it was sufficient to meet the metabolic needs of the spider was not clear, as previous studies had revealed that the spiders constantly added more air from the surface.

Pressure probe

To test the idea, Seymour and Hetz collected spiders from the wild and kept them in aquaria in the lab. They then used oxygen-sensitive fibre-optic probes to measure the partial pressure of oxygen both in the diving bell webs and in the water 5-10 millimetres away from them, taking measurements when the spider was present and again when it was absent. By monitoring the rate of change of oxygen inside the bell and the volume of gas inside it, the researchers were able to determine how much oxygen entered the bell from the water as well as how much the spider consumed.

Seymour and Hetz report in The Journal of Experimental Biology¹ that partial pressures of oxygen did differ between the outside water and the diving bells, and that this drove oxygen to slowly flow into the diving bells. "We were surprised to find that diffusion of oxygen was high enough to cover the breathing needs of the spiders when they were at rest," says Seymour.

They also found that spiders that were larger or engaged in energy-intensive activities, such as egg laying or feeding, made their diving bells bigger to allow more oxygen to transfer across, just as growing animals enlarge their own lungs and gills.

The spiders also showed a remarkable tolerance to low oxygen levels, allowing the oxygen level in the diving bells to drop to 10–20% of the level in the atmosphere before venturing to the surface to grab a bubble of fresh air. "Most insects are not happy below 4 or 5 kilopascals of oxygen," explains entomologist John Terblanche at Stellenbosch University in South Africa, "yet these spiders seem to be dropping to around 1 or 2 kilopascals without any obvious signs of distress."

Terblanche speculates that the spiders might have mechanisms at work in their bodies that allow them to use oxygen in a very efficient way. "This might be a particularly fruitful area for future research," he says.