I love Twitter! Since I’ve started using it, I’ve learned more about a wider variety of scientific studies than I ever did before. It’s a great source of science news! This story absolutely exploded on Twitter a couple of weeks ago. I can see why it was so popular though: it’s a super cool story! Aquatic spiders + special webs for holding air underwater = SCUBA spiders! This story has it all. It also happens to be closely aligned to my own area of expertise, so it’s time for another From the Literature!
This adorable little guy is the diving bell spider, Argyroneta aquatica:
This spider is one of the only spiders in the world that spends its entire life underwater, from egg to adult. Like its aquatic insect relatives, the diving bell spider has a number of adaptations that help it survive in water. Its respiratory adaptations are especially interesting. See that sheen along the abdomen of the spider in the photo? That is an air bubble held in place by specialized hairs that trap air against the body when the spider surfaces. Underwater, the spider can use that air store to breathe. In addition to carrying the air film, this species does something truly spectacular: they build special silk webs underwater, fill them with air that they carry from the surface, and then use the air stored in the silk balloon (the diving bell) to breathe! However, neither the air film carried by the spiders on their bodies nor the air contained within the diving bell are enough to completely satisfy the oxygen requirements of the spiders, so they depend on occasional surface trips to survive.
Scientists have known that these spiders use the diving bell as a sort of SCUBA tank for a while, but many questions remained. No one knew what the oxygen conditions inside the bubble are, how much gas exchange occurs between the air inside the diving bell and the water outside, or how effectively the diving bell acts as a physical gill for the spiders. (Please see my post Better Breathing Underwater in Aquatic Insects for a complete description of how physical gills work, why oxygen flows from water into an underwater air bubble, and why these air bubbles do not last forever.) These unanswered questions drove researchers Roger Seymour and Stefan Hetz to look into the oxygen dynamics of the diving bell more thoroughly. In the process, they discovered some exciting things about both the spiders and the unique air containers they build!
The researchers did several different things for their study. First, they measured the volume of empty diving bells and used specialized oxygen detecting equipment to measure the oxygen levels both inside the bells and in the water surrounding them. This information allowed the researchers to use mathematical equations to determine the potential rate of flow of oxygen from the water into the air bubble. They also measured the oxygen inside the bubble while the spider was inside to calculate the rate of flow of oxygen from the diving bell to the spider, the spider’s oxygen consumption. Using these two pieces of information, they could then show how effectively the diving bell acted as a physical gill, that is how long a spider could stay submerged when using a diving bell. The researchers also measured the oxygen levels within the diving bells when spiders voluntarily returned to the surface to reveal the oxygen level at which the respiratory requirements of the spiders were no longer met by the diving bell and they were compelled to replenish it with fresh air from the surface. Finally, the pair measured the metabolic rates of the spiders directly using a respirometer, a machine that measures the amount of carbon dioxide released and/or oxygen absorbed by a biological organism.
Based on the result of all these measurements, Seymour and Hetz revealed several interesting things about the spiders and their diving bells. First, they showed that the larger a diving bell, the more effectively it acted as a physical gill and the longer the spider could remain underwater. Not only do larger bells contain more oxygen to begin with, but the flow from the water into the bell as the spider consumes oxygen is greater in larger bells than smaller. Also, the larger spiders, those with greater oxygen requirements as measured with the respirometer, produced larger bells. The authors further showed that diving bells easily provided the entire oxygen requirement of inactive spiders in warm, stagnant water (i.e., water with low dissolved oxygen) for over a day. If the spiders moved around, built or cared for cocoons containing eggs (cocoons are stored inside the diving bells!), ate prey that they captured, etc, then they had to return to the surface more often. If they remained mostly still, the diving bell more than adequately provided their respiratory oxygen requirements for long periods of time. Clearly the bells are acting as highly effective physical gills for the spiders!
Seymour and Hetz also demonstrated that the spiders stay within the diving bells until the oxygen drops d to 5-20% of the original level. At this point, the bell apparently provides insufficient oxygen for the spiders and they return to the surface to collect air to replenish the diving bell. The spiders also appeared to enlarge their bells if their oxygen demands increased or if the dissolved oxygen levels in the water dropped.
A few important implications are suggested by the results of this study. The observations and measurements reported in the study were made in rather unfavorable conditions: warm, still water with low dissolved oxygen. If diving bells are enough to meet the respiratory needs of the spiders for over a day under poor conditions like these, then spiders may be able to stay underwater nearly indefinitely in cooler and/or flowing waters. This is important for several reasons. If you happen to be an aquatic spider, you represent a nutritious meal to other aquatic animals such as fish, amphibians, and large insects. Minimizing your trips to the surface, and thereby minimizing the attention you draw to yourself, is desirable. It likely requires a lot of energy to make trips to the surface and back as well, so staying underwater as long as possible may help the spiders conserve energy.
The spiders are also apparently able to respond to their environmental conditions and adjust the properties of their diving bells to match! Seymour and Hetz observed their spiders enlarging bells under several conditions. Spiders that captured prey enlarged their bells and added air to them before they started eating. Spiders with cocoons also enlarged the bells as the broods inside developed, so the parents may be able to compensate for the increasing oxygen demands of their offspring as they develop by modifying the bell. Apart from demonstrating how effectively the diving bell acts as a physical gill, I think the most exciting result from this study is that it reveals how these spiders intentionally modify their environment in response to their changing needs.
In summary, the diving bell does act as a physical gill for diving bell spiders as scientists have long proposed. These bells allow the spiders to stay underwater for a very long time, and the spiders can adjust the bells to match their oxygen requirements and the dissolved oxygen levels of the water. However, even under the most favorable conditions, the air contained within the diving bell will eventually need to be replenished, so the spiders will always depend on air from the surface and must have access to the surface to survive.
Super cool, right? Aquatic spiders are amazing enough on their own, but spiders that build little air balloons to breathe underwater are infinitely more interesting! Because I think it’s helpful to see it, I’ll end this post with a YouTube video (not my own) of a diving bell spider building a diving bell. (I recommend turning the sound off – it’s got obnoxious music). Enjoy!
Seymour RS, & Hetz SK (2011). The diving bell and the spider: the physical gill of Argyroneta aquatica. The Journal of experimental biology, 214 (Pt 13), 2175-81 PMID: 21653811
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