The Underwater life of Argyroneta aquatica
by Joel Kaminski
BIOL/WATR 361, Spring 2015
Spiders are members of the phylum Arthropoda and make up the largest order in the Class Arachnida. Spiders are air breathing organisms which can be found almost anywhere in the world in many different habitats and are well known for the webbed structures they build using specialized silk they produce within their bodies. While many spiders live near water, there is only one that makes its living and will spend most of life underwater. Argyroneta aquatica, also known as the “silvery net” or “diving bell” spider is able to spend most of its life, up to two years, underwater despite having book lungs that can only breathe atmospheric oxygen (Seymore & Hetz, 2011). The key to the water spider’s success is its diving bell and the spider’s ability to create and maintain it.
The diving bell spider is found primary in Northern and Central Europe and some parts of Asia within slow moving streams, ponds, and lakes. Their name comes from the diving bell which they build to survive in underwater environments. The diving bell is essentially a large air bubble in the shape of a dome, with a silk web over the top and an opening at the bottom. It’s comparable to throwing a small net over a large round balloon, so the net holds it together, but there is still access from the bottom. The bell is anchored to a surface, usually a plant, and the spiders will use this and the surrounding area as their home site (Knight, 2011).
The diving bell is a structure unique to this spider which acts as an external physical gill. Physical gills are structural adaptions that are present on a few aquatic arthropods which allow them to stay submerged for long periods of time (Pederson et al., 2011). This adaption occurs mainly outside of the body on structures known as spiracles, which hold a thin layer of oxygen around the tracheal system and other parts of the body. This works similarly to plastrons on underwater insects, which are either hairs, scales or tiny openings in the body that collect oxygen so they can breathe underwater. These traits allow these species to breathe underwater as though they were breathing air on the surface. The diving bell spider has a similar adaption around its legs and abdomen which it uses it to build the diving bell. Its trachea is located within its abdomen, so the spider does not need to have its head within the bell to breathe. (Seymour & Hetz, 2011).
When building the diving bell, the water spider must first find a suitable place to build it, which is usually on underwater plant life. The spider then will go up to the surface to trap air, and will stick its hind legs and abdomen above the surface to “pull” the air down with it. Then the spider will weave a net and anchor it so it can stick the trapped air to it. Then it continues to replenish the bell with oxygen from the surface until it is large enough for the spider to use. As more oxygen is added to the bell, the spider must also add more silk to the bell to keep the air trapped inside. Once the bell is created, it can usually sustain itself, but the spider may have to surface occasionally to replenish the bell if it begins to lose oxygen. In more hypoxic waters, the spider will spend more time maintaining the bell than in waters with more oxygen (Pedersen & Colmer, 2011).
The bubble of air within the bell allows for gas exchange with the surrounding environment. The size of the bubble affects the rate of diffusion, which means that larger bells are able to exchange oxygen and carbon dioxide faster than smaller ones. Many times the bell’s size will depend on the size of the spider, the sex of the spider, the amount of oxygen present in the water, and many other factors. Currently, there is very little known about the gas concentrations inside the bell or the specific mechanisms of gas exchange through the walls (Woermann, 2010).
The diet of the diving bell spider consists of small aquatic invertebrates and small fish. These spiders are also poisonous and have a powerful bite which they use to kill prey. While not lethal to humans, the bite of the water spider can still be painful to those it bites. Around the area where the diving bell is located (or its home space) the spiders put up “safety nets” which alert them to nearby prey and predators. Using these safety nets, they can sense when prey is near and can quickly dash out to catch them. When the spider captures its prey, it will put it inside the diving bell. Then it will usually go to the surface to collect oxygen to put inside of the bell. This way it can consume its prey without the fear of running low on oxygen inside the bell (Seymour & Hetz, 2011)
Like most spiders, this species exhibits sexual dimorphism. However, in this case, the male is about 30% larger than the female, which is unusual for spiders. The size of the female may be due to the amount of time she must spend creating and maintaining the large bell, rather than actively hunting for prey. The female’s smaller size also limits her diving ability, so she must remain close to the surface, which also makes it easier to maintain the bell. The male on the other hand, has a smaller bell and spends less time maintaining it then the female. The male’s size also allows him to dive deeper in search of prey. Females usually have larger bells than males and spend more time in the bells as well, despite being smaller (Schutz & Taborsky, 2005).
Mating is different for the water spider than the other spiders which live on the land. A male will often seek out a female and will build its diving bell right next to hers. When he has finished his bell, he will create an opening between his and the females. Mating occurs within the bell once the male has created enough space for both spiders to fit. The female will then lay about 30-70 eggs in a cocoon within the bell and will constantly maintain the bell so the eggs can survive. Once the eggs hatch, the baby spiders will receive very little care from the parents and will move out to build their own bells shortly after birth (Schultz & Taborsky, 2005).
Spider silk is fascinating because of its unique chemical properties and wide array of uses. To humans, the thread may seem quite weak, however to insects, the silk is stronger than steel (Lewis, 1996). This silk that spiders create is actually a protein fiber made in the spider’s silk gland. The external part of the gland is called the spinneret and spiders have two to eight sets depending on the species. There are different types of silk that spiders use like tubiliform silk, which is used to protect its eggs and aciniform silk, which is used to secure captured prey. Spiders are able to use this silk in many different ways and are able to adjust its composition based on what the spider needs it for (Bakker et al., 2006).
For the Water spider, the silk it uses to make the bell varies depending on the environment that the spider is in. It can adjust the silk covering the diving bell so that it is strong enough to hold the air in place while keeping the pressure inside the bell constant, which is important for the diffusion of gasses. This silk is also different from the silk used to make the cocoon or catch prey. Studies have been done to look at the threads used to make the diving bell compared to the other threads used by other spiders that live on land, but so far there are still more questions about it than answers (Bakker et al., 2006).
The water spider is able occupy a different kind of niche than its brethren living on the land, but it still faces many problems. Scientists are still unsure about the mechanisms that brought the diving bell spider to make its living underwater rather than on land. In the water it is more difficult to use the web to catch prey like spiders do on land. Instead the water spider uses the silk lines it puts out for its safety nets to feel vibrations in the water to sense prey or nearby enemies and must take a more active role. Unlike other underwater arthropods, the diving bell spider is not well adapted to swimming or diving, which makes it more difficult to move around in the water. The spider is also reliant on the bell for its survival which means that it cannot venture very far away from it. (Knight, 2011)
Living underwater helps the spider to avoid its natural enemies like birds or large insects, but it finds other enemies underwater. Larger fish and frogs find this spider an easy target and will eat the spider if given the chance. The water spider, unable to leave its bell and being a poor swimmer underwater, is an easy meal for these animals. It must rely on its ability to hide within its surroundings in order to survive. In rare cases the spider may also attempt to defend itself from these enemies by biting them, but is rarely successful (Bhanoo, 2011).
Despite all this, there are also many advantages to living in the water. Not only does the spider have fewer predators in the water, but it is also able to hide itself better. Since these spiders are vicious hunters, they are able to catch and disable prey fairly quickly. There is plenty for the spiders to eat underwater and they are not picky about what they eat, so long as it can fit inside the bell. As long as the water spider stays close to and maintains the diving bell, breathing underwater is not a problem for it (Davies, 2011).
To conclude, the water spider is a fascinating species which is able to make its living underwater. Using the diving bell, which is a special structure unique to this spider, it is able to breathe underwater the same as it would on land. It is able to use the specialized openings on its body like plastrons in other insects to put oxygen into the bell for later use. Its ability to move around in the water and catch prey is also unique to this species and makes it a dangerous hunter in the water.
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