Evolution and Plastron Respiration:
Advantages and Limitations
When most people think of animals that are capable of aquatic respiration, they think of animals with gills. However, arthropods that utilize the physical gill or plastron have evolved a completely different method of gas exchange underwater.
First, I will explore some possible evolutionary advantages of the plastron. One of these is the ability to fulfill a unique ecological niche. Plastron breathers are able to live in habitats that are highly variable in dissolved oxygen levels and may undergo long periods of anaerobic conditions. Due to the higher solubility of CO2 and lower solubility of O2 in water, insects and spiders with plastron respiration are able to survive such conditions, whereas animals that undergo direct gas exchange with gills most likely could not survive in such conditions (Heckman 1986).
Another great advantage to plastron respiration and to the use of a compressible physical gill is that they have the option of replacing their air supply at the water’s surface if necessary (Heckman 1986). This is because with respiration through the use of gas gills, gas exchange does not take place directly between the water and the animal’s surface, but through the gas bubble. Animals with gills, however, do not have the option of using the surface air as a possible gas supply.
For these reasons, animals that can utilize a physical gill or plastron have the option of living in anaerobic waters without being harmed by high CO2, H2S, NH3, or other substances that would be a danger to animals with direct gas exchange with the water. Such anaerobic habitats often contain high amounts of vegetation and detritus, which the insects can feed on. This type of habitat can also provide protection from many potential predators, because the habitat provides many places for concealment, and many predators cannot survive in the oxygen deprived conditions. Thus, insects that can utilize this method of respiration can exploit this niche that animals with gills cannot (Heckman 1986).
There are some limitations to aquatic respiration of insects and spiders. One rather obvious limitation for those animals that utilize a compressible gill is the limited life of the gas gill. In an investigation of the diving bell spider (Argyroneta aquatica), Seymour and Hetz (2011) found this to be the only limit to this spider’s gas gill function.
Image provided courtesy of Seymour and Hetz (2011)
Record from the air on the abdomen of a 103 mg spider away from the diving bell.
This figure from their research shows the activity of a diving bell spider. Part B shows the air on the abdomen of the spider. This sharp increases in oxygen represent trips to the surface, and they are followed by a quick decline in oxygen, necessitating frequent trips to the surface (Seymour and Hetz 2011). Animals with gills that exchange gas directly with the water do not have this limitation. However, true plastron breathers do not need to replenish their gas gill and therefore do not have this limitation either.
The adaptation of the physical gill is also limited to animals with low metabolic rates. In other words, an endothermic animal such as a human could never survive with a physical gill, even one of a large enough volume, due to our high metabolic needs (Flynn and Bush 2008).
Finally, one should consider the effects of the energetic requirements for the growth of the hairs necessary to maintain the plastron (Flynn and Bush 2008). This energetic constrain places one more limitation on the evolutionary advantages of plastron respiration.
Image provided courtesy of Rovner 1987
Field-collected, debris-covered brood nest of Dysdera crocata just removed from the water
Another interesting evolutionary implication of the study of the physical gill concerns the evolution of silk nests, as investigated in a study by Rovner (1987). I have discussed the advantages of the physical gill for aquatic insects and spiders, but in areas where flooding is likely, it appears that the creating of a different type of physical gill, silk nests, has been selected for in terrestrial species of spiders. Rovner investigated the use of silk nests by the species Dysdera crocata and Ariadna bicolor to survive periods of flooding. In this experiment, Rovner found that the dissolved oxygen level of the water surrounding the nests in closed systems decreased, indicating that the gas bubble maintained by the nest acted as a physical gill. Rovner argues that flooding is therefore a strong factor that could have caused an evolutionary advantage to arise in the production of silk nests in spiders and other arthropods. Rovner also claims that the fact that these silken nests can act as physical gills for days, while compressible physical gills often only last for hours, the nest is a more effective gill than the setae of aquatic spiders (Rovner 1987).
|How do physical gills and plastrons work?|
|How is research of this topic performed?|
|Benefits and Limitations|
|Biomimicry: The plastron’s Contribution to Technology|
|Other Superhydrophobic Surfaces in Biology|
|Some Aquatic Insects and Spiders|
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