You all know that I have a soft spot in my heart for all things related to aquatic insect respiration. I’ve written several blog posts about the topic in the past. I was thus very excited to come across a new paper a month ago, a commentary on physical gills in aquatic invertebrates and plants by Ole Pedersen and Timothy Colmer. It was the first time I’d ever considered the possibility that plants might have hit upon the same means of underwater respiration as insects. Mind blown! So, I’d like to share the paper with you all too, just in case any of you find it as fascinating as I do. (One can dream, right?)
Freshwater insects, spiders, and plants all have one thing in common: they are adapted for life on land and depend on respiratory systems that were intended for use in air. Oxygen is much less abundant in water than in air and moves very slowly through water, so any organism built for living on land that wants to move to an aquatic habitat has to adapt to the available oxygen of their new watery home. Insects have evolved a variety of means of compensating for the relatively low oxygen levels in water, many of which I highlighted in another blog post. These include snorkels (such as those on giant water bugs and water scorpions), scuba tank style air stores (also in the giant water bugs, among other true bugs and many aquatic beetles), physical gills (many true bugs and diving bell spiders), plastrons (in a very limited number of aquatic insects), and gills (damselflies, mayflies, hellgrammites, etc). According to Pedersen and Colmer, nearly all of these animals must return to the surface at some time to refresh their air supply because the respiratory needs of the animal is greater than the ability of the respiratory surface to supply oxygen.
However, gas films such as physical gills and plastrons significantly increase the length of time an organism can remain submerged. These air films are so important that Pedersen and Colmer suggest that many insects that live in riparian areas or around ponds have body surfaces capable of trapping air films too. These may prevent drowning if a terrestrial riparian insect becomes submerged, either accidentally or by choice. Air films are clearly important to a variety of aquatic and riparian insects and spiders.
But they’re also important to plants! The authors discuss how many wetland plants have surfaces that repel water and create gas films around the surface of submerged leaves. These gas films work the same way they do in insects – absorbing oxygen from the water and improving the respiration of the organism in water. Plants don’t have the necessary structures to create permanent plastrons, but a plant that is submerged (during flooding, for example) can often survive two weeks or more completely submerged thanks to a little film of air that surrounds it.
The authors did a short study comparing the oxygen uptake by both an insect (a true bug in the genus Aphelocheirus, one of the plastron-bearing insects that only very rarely goes to the surface) and a plant (reed canary grass, Phalaris). They found that gas films strongly improved the ability of both the insect and the plant to take up oxygen from the water and that the gas films worked in both high and low dissolved oxygen concentrations. The authors also removed the gas films and discovered that the oxygen uptake strongly decreased. In the end, they concluded that gas films increase the area through which organisms can absorb oxygen from the water, greatly enhancing their ability to survive underwater and the time they could remain submerged.
The authors further suggest that gas films might aid in plant photosynthesis. Plants require carbon dioxide to photosynthesize and normally it enters the plants through pores in the leaves called stomata. In water, however, stomata are thought to close, so carbon dioxide must travel directly through the leaf’s surface, a long and slow process. Plants with gas films have an advantage: they can both absorb carbon dioxide more readily through the gas film than without it and they likely keep their stomata open, allowing carbon dioxide to easily flow into the leaves and allow photosynthesis to take place.
Pedersen and Colmer concluded with a few comments about water quality and gas film respiration. They posit that these sorts of systems only work in relatively clean water, that in polluted waters the oxygen levels are too low to support submerged plants and animals with simple gas films. In dirty water, insects with snorkel or scuba tank like respiratory systems stand a better chance of getting the oxygen they need because they don’t depend on oxygen in the water and go to the surface for oxygen instead.
What I really like about this paper is the connection it draws between the plants and arthropods, how two very different groups of organisms have hit upon the same solution to functioning underwater. Clearly this system wouldn’t work for all wetland organisms as animals with lungs don’t passively absorb oxygen the way plants and arthropods do, but gas films seem to work well for things that have more passive respiratory systems, regardless of the type of organism. I think that’s pretty darned cool! Plants and arthropods are wildly different organisms and it’s simply amazing to consider that they’ve developed similar solutions to deal with living in and around water. Yet one more example of how fantastic the natural world is!
Pedersen O, & Colmer TD (2012). Physical gills prevent drowning of many wetland insects, spiders and plants. The Journal of experimental biology, 215 (5), 705-9 PMID: 22323192