Insects and Plants Use the Same Strategy for Breathing Underwater

Exposing the air store

A giant water bug, Abedus heberti, breathing using a physical gill

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.

cattails and algae

Cattails and algae help clean the water

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.

Sweetwater

Sweetwater Wetlands

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.

shallow treatment

A giant water bug going to the surface to get more oxygen

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!

Literature Cited:

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

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Unless otherwise stated, all text, images, and video are copyright © C. L. Goforth

A Trip to Sweetwater Wetlands

Before I get back to the water bugs, I want to continue my detour for a few posts and talk about some dragonflies I saw yesterday.  I needed to take some dragonfly photos, so my fiancee and I went to a constructed wetland in Tucson called Sweetwater to shoot.  Sweetwater is part of one of the wastewater treatment plants in Tucson and is fed entirely with reclaimed water and secondarily treated wastewater.  It can smell pretty bad at times (it IS made up of treated wastewater after all), but if you can overlook the scent it’s gorgeous and wholly worth a visit.  All those nutrients in the water do wonders for the plant life that grows in the water:

Sweetwater Wetland

Sweetwater Wetland

As you can see, the nutrient-rich water of the wetland is able to support a wide variety of aquatic plants.  Yesterday, the wetlands were full of cattails (the dark green bushy looking things on the right side of the picture), rushes, sedges, and duckweed.  Most of the bright green stuff floating on the water in this picture is duckweed, NOT algae, though there was definitely algae there as well.  The wetland also suports a variety of trees and shrubs and many different species of wildlife.  All those dark spots in the picture are ducks.  I’ve also seen several snakes and lizards, many other birds, and even a bobcat once!  And then there are, of course, the dragonflies.  Thousands of them.  I think Sweetwater is the best place in Tucson to see dragonflies.

We saw several dragonfly species, including green darners (Anax junius), arroyo darners (Aeshna dugesi), blue-eyed darners (Rhionaeshna multicolor),  flame skimmers (Libellula saturata), roseate skimmers (Orthemis ferruginea), blue dashers (Pachydiplax longipennis), Mexican amberwings (Perithemis intensa), variegated meadowhawks (Sympetrum corruptum), and black saddlebags (Tramea lacerata).  We also saw one species of damselfly (though we weren’t really looking hard for the damselflies either – there may have been more), the desert firetail (Telebasis salva).  Most of the dragonflies were flying above the cattails or were resting on them in various positions.  Some were even in the obelisk position, which I have talked about before in another post on dragonflies.

I got some good shots of several different species, which I’ll go over in my next post.  However, I was most thrilled with all of the large flying dragonflies, the darners.  I like taking pictures of flying dragonflies the best because they are the hardest to shoot.  This is the same reason I got interested in dragonflies in the first place – catching dragonflies for my collections for 4-H was the biggest challenge.  I have yet to get a really, really good shot of a flying dragonfly, but I keep at it, and I got some pretty decent shots today.

This is Anax junius, the green darner:

Anax junius mating pair

Anax junius mating pair

This is a mating pair.  The male, the more brightly colored one with the blue abdomen, had just grabbed his mate in preparation for mating, and then they promptly fell out of the tree onto the sidewalk, right next to me!  They sat there in this position for almost a minute while I snapped away with my camera.  Then they flew off to mate and lay eggs.  Anax junius was very abundant at Sweetwater yesterday, so I ended up getting a lot of pictures of them.  They are fliers, so they don’t land very often.  That also makes them fairly hard to photograph – they don’t sit still for very long.  So, most of the photos I got were of these insects flying, and most of them looked like this:

Dragonfly photo

Typical dragonfly flight photo

The little smudge in the photo at the tip of the big blue arrow is the dragonfly!   I took about 90 photos of Anax in flight, and these were the best:

green darner in flight

green darner in flight

green darner in flight

green darner in flight

green darner in flight

green darner in flight

What I particularly liked about these images, aside from the fact that they are actually mostly in focus, is that you can see the legs of the dragonflies folded up underneath their thoraxes.  Having the legs folded up under the body likely helps the dragonflies fly more efficiently.  If they let their legs dangle down underneath them, they are likely to slow the dragonflies down, get snagged on the vegetation when they fly low, and otherwise cause problems.  So, they fly with them tightly folded under their bodies and only stretch them out when they grab food in midair.  It’s fun to be able to see that they do this, but it would be hard to see by observing them directly because they fly so fast and so erratically.  It’s easy to see in the photos.

Like in my post on the species from the dragonfly swarm, you can clearly see from these photos that dragonflies are able to move all four of their wings independently from one another.  For example, in the top photo, you can see that the forewings are both moving down while the hindwings are moving up.  This ability to move their wings independently contributes significantly to the amazing agility that dragonflies exhibit.  If you’ve ever seen dragonflies flying, you know how fast and agile they are.  They can stop in midair, make 180 degree turns, fly backwards, hover, and do all sorts of other things that are nigh impossible for most flying animals.  They dart all over the place, which is why it’s hard to get photos of them in flight.  If you’re dealing with a flier species, such as Anax junius, they are going to be moving constantly too.  Another reason it’s hard to get good photos of dragonflies in flight is the behavior you see in this photo:

green darners in flight

green darners in flight

Dragonflies are highly territorial and protect their territories from other dragonflies that might be trying to steal their access to prime egg laying habitat or other valuable resources.  In flier species, the males typically patrol, or fly within the boundaries of their territories, looking out for females to mate with, food, and males who might want to try to claim the territory for themselves.  In this photo, the dragonfly at the top tried to steal the territory that belonged to the dragonfly on the bottom.  The owner of the territory, the dragonfly on the bottom, successfully chased the would-be thief out away from his territory.  In the photo, the territory holder is returning to his territory to continue patrolling while the loser is flying away to find another spot, hopefully one guarded by a wimpy male.  Because there are usually fewer territories available than dragonflies at a pond, these battles are constantly occurring.  When a male sees another male that might enter his territory, he will immediately change directions and charge the tresspasser in an attempt to protect his territory.  This means that the dragonflies are darting back and forth constantly.  You might track one dragonfly and just be ready to snap a photo when he stops, turns around, and zips off in a completely diferent direction.  It’s very hard to predict where a dragonfly is going to be at any given time, so it is difficult to get good photos of them in flight.

Next time I’ll go over how to identify some of the species that we saw at Sweetwater, then it’s back to the giant water bugs for a while!

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