The diving bell spider: reversed sexual size dimorphism

By Sunjeet Minhas

The diving bell spider (Argyroneta aquatica) is incredibly unique as it is the only known spider to spend its entire life under water (Seymour & Hetz, 2011). Its incredibility, however, does not stop here, as it is also one of the few spider species where males are larger than females (Schütz & Taborsky, 2011). It is therefore often considered whether this spider’s rare sexual size dimorphism may be an adaptation to surviving in an even rarer living ecology.

For starters, the diving bell spider’s name comes from the bell shaped structure it builds, which enables it to live underwater. You see, instead of having gills, these spiders contain the common book lungs found in terrestrial arachnids. So, in order to respire underwater, these creatures build (by wrapping their silk threads around aquatic vegetation) underwater housings to which air can be sequestered (Pedersen & Colmer, 2011). Furthermore, specialized setae on the spider’s abdomen help them trap air from the water’s surface, allowing them to deliver the surface air to their underwater retreats. Thankfully, in the process of doing so, these spiders can respire outside of their retreats by using the many tracheal spiracles found on their skin, specifically adapted to access the air trapped on their abdomens (Pedersen & Colmer, 2011).

Also, to complement the building of their retreats, these spiders have shown the ability to monitor the gas composition in their diving bells in order to maintain proper respiration conditions (Schütz et al. 2007). Further research has also demonstrated that the diving bell is self-sustaining as O2 from the surrounding water moves into the air bubble by osmosis while CO2 moves out, thus allowing for spiders to survive within their bells for more than a day without having to manually restock their retreats with fresh air (Seymour & Hetz, 2011). In addition to allowing these spiders to breathe underwater, the diving bell is also known to provide shelter necessary to molt, copulate, raise offspring, and consume and digest prey (Schütz et al. 2007).

Sexually dimorphic diving bell spider with smaller female (on left) and larger male
(on right). Photo by Norbert Schuller (Baupi). Used under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Now, besides its unusual ecology, the diving bell spider mainly differs from traditional spiders because it exhibits reversed sexual size dimorphism. Males (7.8-18.7 mm) can even be so much larger than females (7.8-13.1 mm) that reversed sexual cannibalism events have been observed (Schütz & Taborsky, 2003; Schütz & Taborsky, 2011)! Furthermore, Schütz and Taborsky, (2003) suggested that ecological parameters might naturally select for larger males. They found that larger body size correlated with larger “diving legs”, which may improve movement underwater. This is especially important for males, which are more mobile than females. Males would spend the majority of their days actively hunting outside their retreats and seeking mates. Females, however, spent the majority of their days within their respective air bells, thus reducing any potential benefit of  having larger diving legs (and thus larger bodies) to better facilitate movement. Females would only hunt prey straying close to their retreats and, in terms of reproduction, would have males come to them: males build diving bells near the female’s, which then allows them to bite into and thus enter their mate’s bell.

Further evidence to support ecological factors contributing to reversed sexual size dimorphism includes the findings that out of the four different types of silk threads produced by the diving bell spider (walking, air bell, anchor, and cocoon threads), males used walking threads most often (indicating high mobility) while females used air bell and anchor threads most often (threads to maintain diving bells) (Bakker et al. 2006). This finding along with additional reports showing that females use 30% of surface air to restock their diving bells while males only use 13%, may suggest that females take extra care in maintaining their bells. (Schütz & Taborsky, 2003). All might be done in an act to prepare for raising future brood, and thus may present females with additional costs associated with underwater living. This could therefore constrain female size (Schütz & Taborsky, 2003).

To conclude, the diving bell spider is not only miraculous in that it is the only known spider to live entirely underwater, but is also unique in that it shows a feature quite rare in spiders: reversed sexual size dimorphism. Ultimately, and to my surprise, further investigation into these unique characteristics, has given me substantial evidence to believe that ecology may play a vital role in developing this rare dimorphism.

References

Bakker, D. D., Baetens, K., Nimmen, E. V., Gellynck, K., Mertens, J., Langenhove, L. V., and Kiekens, P. (2006). Description of the structure of different silk threads produced by the water spider Argyroneta aquatica (Clerck, 1757) (Araneae : Cybaeidae). Belgian Journal of Zoology 136: 137-143.

Pedersen, O. and Colmer, T. D. (2011). Physical gills prevent drowning of many wetland insects, spiders and plants. The Journal of Experimental Biology 215: 705-709.

Schütz, D. and Taborsky, M. (2003). Adaptations to an aquatic life may be responsible for the reversed sexual size dimorphism in the water spider, Argyroneta aquatica. Evolutionary Ecology Research 5: 105-117.

Schütz, D. and Taborsky, M. (2005). Mate choice and sexual conflict in the size dimorphic water spider Argyroneta aquatica  (Araneae, Argyronetidae). The Journal of Arachnology 33: 767–775.

Schütz, D., Taborsky, M., and Drapela, T. (2007). Air bells of water spiders are an extended phenotype modified in response to gas composition. Journal of Experimental Zoology Part A: Ecological Genetics and Physiology 307: 549-555.

Seymour, R. S. and Hetz, S. K. (2011). The diving bell and the spider: the physical gill of Argyroneta aquatica. The Journal of Experimental Biology 214: 2175-2181.