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

From the Literature: The Power of Citizen Science

Ladybugs mating

Ladybugs mating

Last year’s Entomological Society of America Meeting featured several talks about citizen science and social media in entomology that I was very sorry to miss.  (I missed out on meeting Bug Girl in person!  Sad, though you can see her talk here.)  I honestly don’t go to the ESA meetings very often because it tends to be agricultural and pest management heavy and when you only get to attend one to two meetings a year you really need to make them count and go to the ones most applicable to your field.  Still, I was thrilled that they were featuring these sorts of topics!  And, even though I missed the talks themselves, the latest issue of American Entomologist, ESA’s quarterly journal, features articles about several of the citizen science projects that were presented at the meeting.  As a citizen science fanatic, I have really enjoyed reading about them!  One article especially caught my attention and I wanted to discuss it today.  Let’s talk ladybugs!

I’ve mentioned the Lost Ladybug Project a few times in the past as I consider it one of the most successful online citizen science projects dealing with insects.  It pops up in the news from time to time as citizen scientists keep finding rare and unusual ladybugs and it gets a lot of publicity.  I’ve talked about what I think the benefits of online citizen science projects are before so I’m not going to rehash it all now, but I think the Lost Ladybug Project is one of those projects that is perfectly suited for online citizen science because the information they seek benefits from having a lot of participants.  And they get a lot of participants!  Over 12,000 so far in fact.  The project is harnessing the power of the public to answer questions about ladybugs and it is a successful project as a result.

According to the American Entomologist article by Lost Ladybug organizers John Losey, Leslie Allee, and Rebecca Smyth, ladybugs studies are important because the colorful, well-loved, and easily recognizable beetles are also voracious predators of native and invasive pests and sensitive to environmental conditions.  Indeed, some researchers have proposed that they be used as indicator species for environmental change.  By tracking the location of populations via the online Lost Ladybug Project, the team hoped to learn something about what shifts in ladybug populations might say about environmental change in North America.  But those conclusions aren’t the focus of their article.  Instead, they’ve focused on the level of success of the project as an online citizen science project relative to what scientists have been able to glean without help.  The results as quite interesting.

First the authors detailed how they selected a group of the 12,000+ Lost Ladybug submissions for the comparison.  They included only ladybug sightings/photos verified by the team, counted overwintering groups of ladybugs as a single sighting, and counted sightings in the same location at least 24 hours apart as separate sightings.  They also described the data used for the scientist side of things.  Data were taken from a scientific review paper for data from 1991-2006 and from published scientific papers from 2006-present.  Then they compared the number of beetles observed and the distribution of the ladybugs reported by both scientists and citizen scientists, and made some detailed observations about a few rare ladybug species of particular interest.

What they found was, I think, amazing!  Scientists typically gathered more ladybugs per sighting than citizen scientists.  Most Lost Ladybug participants report a single ladybug at a time whereas scientists often collect over 1000 beetles in one go.  Scientists clearly collect more data about specific populations of ladybugs (especially in agricultural settings) and have collected more ladybugs since 1991 than the Lost Ladybug participants have since the project went online in 2008.  This isn’t particularly surprising as scientists know where to look and are trained in sampling techniques that will allow them to collect thoroughly in an area while most Lost Ladybug sightings are serendipitous findings and come in one by one.

However, Lost Ladybug participants, and in only four years, have collected over 60 times the total number of samples relative to pro scientists!  They might not collect as many individuals per sample, but the total number of sampling events is far, far greater.  Also, the sightings are much more widespread.  While the pros tend to stick to agricultural settings, the Lost Ladybug participants are spread far and wide in a variety of habitats.  There are a lot of eyes on the ground in any given area of North America out looking for ladybugs and the citizen scientists do a better job of sampling this larger area than the scientists ever could.  Citizen scientists are also better at finding rare species than career scientists.  Additionally, citizen scientists have collected more total species and have a higher average number of species per 1000 ladybug individuals than the pros.  In essence, citizen scientists are collecting better data than the pros when it comes to widespread sampling, cataloging species distributions, and finding rare species, essential information if one wants to compare current distributions and ladybug abundance  with those in the past.

The team thinks the reason their project has been more efficient than traditional science has less to do with the total number of individual participants and more to do with how widespread their observers are and the variation in habitat types that the participants sample.  Lost Ladybug participants have sampled a much greater area of the US, Mexico, and Canada than scientists ever have, or really every could.  As a result, the researchers have learned a great deal about the current distribution of ladybugs in North America and are starting to make inferences about habitat shifts and the causes of ladybug declines in the past few decades.  Though they don’t think that citizen science projects such as the Lost Ladybug Project is appropriate in every situation, they’ve collected valuable data simply by educating the public about ladybugs and asking them to report sightings, data that would be nearly impossible to collect without the help of enthusiastic volunteers who want to participate in science.

For more information about the Lost Ladybug Project and to get involved, please see their website at http://www.lostladybug.org/index.php!

Literature Cited:

John Losey, Leslie Allee, & Rebecca Smyth (2012). The Lost Ladybug Project: Citizen Spotting Surpasses Scientist’s Surveys American Entomologist, 58 (1), 22-24

(Want to read this article?  It’s available online for free!  Hooray for open access journal articles.)

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

The Purpose of Caddisfly Case Extensions: A Case Study

Well, Science Sunday ended up getting pushed to Monday, but that’s okay.  It happens!  But today I’m going to share a fascinatingly simple study with you about caddisflies, so I hope it was worth the wait.

I haven’t talked about caddisflies all that much on my blog yet, but I really should.  They are incredibly interesting little insects and hugely diverse as far as aquatic insects go.  Caddisflies fly around on land as adults and look like little bland moths:

caddisflies

Caddisfly adults

Their order name, Trichoptera, means hairy wings, and if you look closely at the wings you’ll understand why: rather than scales, as you see in their close relatives the butterflies and moths, caddisfly wings are covered in hair.  Their common name, caddisfly, is based on a peculiar structure that the aquatic larvae and pupae use, the case.  Not all caddisflies build cases, but those that do build them using silk that they produce and materials they gather from the stream such as algae, pebbles/sand, leaf bits, pine needles, small pieces of wood, etc.  There are a huge variety of cases, many of which are species specific so that you can identify the species based solely on the case.  Others are less distinct, but regardless of the structure of the case, they tend to be big, bulky, heavy things that are much larger than the larvae carrying them.

Over the years, researchers have proposed a variety of purposes for caddisfly cases.  Some are likely helping the caddisfly breathe by stirring the water around the larva, allowing it to collect oxygen from the water via gills that run along the abdomen.  Cases may protect some species from predation as fish and other aquatic predators are less likely to eat something that looks like a pile of rocks or leaves than a soft, squishy insect.  Still other caddisflies may use their cases to weigh them down, causing them to sink to the bottom so that they can move about fast flowing streams with less risk of being swept downstream.  Each species may use its case for a slightly different purpose, or even more than one.

One species of caddisfly, Dicosmoecus  gilvipes, builds a case from silk and plant bits, adding small pebbles as they get older.  The first through fourth instars also attach needles from Douglas fir to their cases (the fifth and last instar does not), attaching them near the top of the case so that they stick far out to either side:

Dicosmoecus gilvipes

Dicosmoecus gilvipes larva. Redrawn from Limm and Power 2011.

This is a rather peculiar arrangement of materials, so researchers Michael Limm and Mary Power wanted to figure out why those fir needle wings were so important.  They considered two hypotheses.  First, the wings might protect the larvae from predation.  Even if the little rocks didn’t discourage fish from eating the larvae, perhaps the pointy spikes sticking off the sides would.  Alternatively, the extensions could help stabilize the larva so that the larva would be less likely to tip over in areas of high flow in a stream.

To test these hypotheses, they did two simple experiments.  In the first, they released caddisfly larvae at the site where a stream flowed into a deep pooled area containing steelhead trout.  One person hid behind a boulder and released the larvae, which were swept into the pool.  A second person observed the fish and counted how many times each larva was approached by the fish, were “mouthed” by the fish, or eaten.  They did three treatments: caddisflies with the case intact, caddisflies with the douglas fir needles clipped off, and naked caddisflies that had been removed from the case prior to release.  In the second experiment, the researchers constructed a large rocking water tank that would roll the larvae over.  They placed a larva on the bottom of the tank and turned the machine on, then counted how many times each larvae rolled before it recovered its footing and how long this took.  They then compared the number of rolls and the time to recovery between caddisflies with cases intact and with the fir needles clipped off.  The team also measured the cases to determine how the width, length, mass, and center of mass changed with the addition of the fir needles.

The reseachers learned that the fir needles increased the width of the case by 410%, the length by 36%, and the weight by 24% and shifted the center of mass upward off the streambed.  They also learned that, while the steelhead readily consumed naked caddisflies, there was no difference in the number of approaches or the number of times the caddisflies were mouthed between the larvae with the extensions and those without.  Clearly the case alone was sufficient to prevent predation regardless of whether the extensions were present or not.

The results of the rocking tank were interesting though.  Larvae with the fir needle extensions rolled three times less than larvae without the extensions.  They also regained their footing more than three times faster with the extensions than without.  The cases might be providing other benefits to the larvae, but Limm and Power concluded that the function of the fir needles is to stabilize the larvae in areas of high flow.  Apparently it’s worth the extra effort of finding the fir needles and carrying around a much more unwieldy case if it means that you are more stable in the stream.

So, why does this matter?  According to calculations the authors did, the drag force required to tip a larva over is more than four times greater when the larva has the extensions than when it does not.  This means that the larvae can safely walk out into areas of the stream with flow up to two times faster without getting tipped over and washed downstream.  The extensions also help the larvae orient themselves so that they’re positioned parallel to the flow.  This decreases the chance of being swept away.  All of these benefits combined likely allow the larvae to wander out into areas of the stream where they would not otherwise be able to go.  Food limits the number of Dicosmoecus  gilvipes that can live in any particular stream, so by increasing their stability by the simple addition of a few Douglas fir needles, the larvae increase the area where they can forage for food in the stream, allowing more individuals to survive in any given area.  Pretty darned cool!

This is yet another example of how an insect can make a very simple, small change that provides huge benefits – just another example of why insects are such amazing creatures!

Literature Cited:

Limm, M., & Power, M. (2011). The caddisfly Dicosmoecus gilvipes: making a case for a functional role Journal of the North American Benthological Society, 30 (2), 485-492 DOI: 10.1899/10-028.1

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Unless otherwise stated, all text, images, and video are copyright © TheDragonflyWoman.com

Science Sunday: Impediments to Invertebrate Conservation

Hadrurus arizonensis giant hairy scorpion

Giant hairy scorpion (Hadrurus arizonensis)

Back in October, the New York Times Green Blog featured a post about a paper that had been recently released covering seven major reasons why invertebrate conservation isn’t garnering as much attention as it should nor being acknowledged as an important use of conservation resources.  I liked the blog post, so I recently read the paper that it was based on.  I think the paper makes some great points, so I’d like to take you through it here.

Insects and other invertebrates make up the majority of the described species on the planet Earth.  About 80% of all described species (this includes everything – plants, mammals, bacteria, fungi, insects…) are invertebrates.  Beetles alone make up 25% of described species and outnumber vertebrate species ten to one.  Clearly, invertebrates are an important part of the world.  They also perform an enormous array of environmental functions, from decomposing organisms and fixing nitrogen in soils to controlling pest species and processing leaf materials in streams to begin the nutrient cycling that drives freshwater ecosystems.  The services invertebrates provide are important for a wide range of other organisms.  Thus, it is important that we consider invertebrates and their role in biological and chemical processes when making plans for the conservation of organisms.

Water scorpion, Ranatra quadridentata

Water scorpion, Ranatra quadridentata

However, that’s not what’s happening.  Invertebrates are widely ignored by conservationists in favor of the showier organisms, the warm and fuzzy creatures that make people say, “Awwww…” before reaching into their pockets to fund research.  Far fewer people who say, “Awwww…” and shell out a few bucks to protect a parasitic wasp, a spider, or an aquatic beetle.  In fact, many people would probably rather let an insect species go extinct than pay to protect it.

This sort of attitude is also reflected in the endangered species lists, such as the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species.  These lists rarely include insects and other invertebrates so that the representation of invertebrates does not reflect their abundance and diversity in the natural world.  This is a problem, especially when you start to really think about the necessary services that invertebrates provide, the medical and research advances we’ve made based on invertebrate models (think fruit flies and C. elegans), and their the utility of invertebrates as indicators of ecological health.

Katydid

Katydid (I think this is the common short winged katydid, Dichopetala brevihastata)

Scientists recognize the value of invertebrates in the environment and are aware of the fact that invertebrates are often neglected when it comes time to conserve species.  Why, then, are there still so underrepresented?  Pedro Cardoso, Terry Erwin, Paulo Borges, and Tim New discussed seven reasons why these problems exist and recommend actions to solve them in their important paper.  Let’s go through each of them!

Problem #1: Invertebrates and the services they provide are not widely known among the public.  It’s hard to convince people that they should allocate funds (or at least support allocation of those funds) for invertebrate conservation when they’re not aware of the diversity of invertebrates or the valuable things they do to keep the world running smoothly.  Even worse, most people come to believe that most invertebrates are either pests or dangerous (neither is true) and fail to understand why anyone would want to prevent their extinction.  Solution: Cardoso et al recommend increasing awareness of invertebrates through media and outreach, a sort of invertebrate PR campaign if you will.  Even simply using common names when communicating with the public might be a step in the right direction.

Banded argiope, Argiope trifasciata

Banded argiope (Argiope trifasciata)

Problem #2: Politicians are largely unaware of the issues surrounding invertebrate conservation.  When our policymakers, the people who will ultimately determine the fate of research finding, are mostly unaware of invertebrate conservation issues, it’s hard to them to justify why invertebrates are important enough to deserve funding.  Solution: Educate the policymakers!  Working toward  better representation of invertebrates on the IUCN Red List and similar lists will also allow people to lobby on behalf of invertebrates to ensure that their conservation becomes a priority.

Problem #3: Basic taxonomic, ecological, and behavioral research is becoming increasingly understudied and underfunded.  It’s hard to determine which species demand our attention for conservation when we don’t even know what their role in the environment actually is.  Basic research helps answer these questions, but is becoming increasingly unpopular and funding for such work continues to decline.  Solution: Citizen science to the rescue!  Amateurs come across new species more often than you’d think and are able to provide useful data on distribution and abundance.  There are more non-scientists than scientists, so why not make use of hundreds of extra eyes and ears to cheaply answer some of the basic questions that are becoming hard to procure funding for?

Blue eyed darner, Rhionaeschna multicolor, flying

Blue eyed darner (Rhionaeschna multicolor)

Problem #4: Most species remain undescribed.  Estimates of the total number of invertebrate species in the world vary widely, but one thing is certain: we have probably only scratched the surface of invertebrate diversity.  According to Cardoso et al, a new invertebrate species is described every 35 minutes, but at that rate it’s going to take another hundred years or more to describe every species.  Just think of how many species might go extinct in that time!  Solution: Careful use of indicator species or surrogate species might be useful in applying conservation efforts to undescribed species.  Increased support for both taxonomic research and the speed of publication of new species descriptions will also help.

Problem #5: We don’t know the distribution of most species.  Describing a species is a start, but to protect it you need to know the extent of its distribution – where it actually lives.  Many species descriptions are based on 3-4 insects from a single location, so we don’t know the range of most species.  Solution: It is important that survey projects such as the Planetary Biodiversity Inventory continue to catalog and document life on Earth so that we know where species are actually located.  Online databases of distribution data such as the Global Biodiversity Information Facility will also help decrease the amount of time a researcher or conservationist must search for distribution information.

Problem #6: Changes in abundance over time and space are unknown for most species.  To conserve a species, it’s essential to know where that species is located, when, and how abundant it is.  We don’t, however,  have abundance data for most species.  Solution: By developing standardized sampling protocols, an effective biological inventory of an area can be undertaken by nearly any researcher for whatever purpose, yet provide information that is valuable to conservation efforts and other researchers.  Long term ecological and monitoring projects will also provide valuable information for conservation efforts.

crane fly side view

Crane fly (Tipula sp.)

Problem #7: Life histories and sensitivity to changes in the environment remain unknown for most species.  If we don’t know which ecological services a species requires or provides, it’s hard to develop invertebrate conservation strategies that will actually work.  Solution: Indicator taxa in an area might alert researchers and conservationists to problem within an environment (protect the environment, protect the species within it).  Determining which species make good indicators within an environment is a good way to start conservation efforts in an area.

Cardoso and colleagues identified seven impediments to invertebrate conservation, but they admitted that, in the end, it all boils down to one overarching issue: public perception of invertebrates.  We aren’t going to be able to solve any of Cardoso et al’s list of problems without the support of the public – support for invertebrates, support for science and research, support for conservation.  It is thus vitally important to get the public on board if we’re going to save invertebrate species from extinction.  And why should we save invertebrates?  I think Cardoso and his colleagues sum it up best: “Only by preserving all species and guaranteeing interactions and ecosystem services may we reach the goal of overall biodiversity conservation.”  And, ultimately, what’s best for invertebrates is best for us too.

Literature Cited:

Cardoso, P., Erwin, T., Borges, P., & New, T. (2011). The seven impediments in invertebrate conservation and how to overcome them Biological Conservation, 144 (11), 2647-2655 DOI: 10.1016/j.biocon.2011.07.024

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Unless otherwise stated, all text, images, and video are copyright © TheDragonflyWoman.com

From the Literature: The Cost of Breathing

I have a subscription to the science journal Science, but I rarely do more than shuffle through the contents every 6-8 issues.  While it’s considered one of the premier science journals, there are a lot of articles that are not applicable to what I do, so I usually choose to focus my attention on journals more closely aligned with my research.  However, I happened to look at the October 14, 2011 issue of Science and I’m happy I did!  I came across a great paper by Nick Lane that proposes a possible link between respiration and fitness, fertility, and life span within eukaryotes and explains how this might work.  I’ll talk about Lane’s hypothesis today, but I want to go a step further too.  The examples in the article are all vertebrates and the hypothesis fits well to these organisms.  But how well does it fit with insects?  I’ll discuss that at the end.

Note: if you don’t remember what the respiratory chain of cellular respiration is, you’re likely to get a little lost as you read through this.  I created a handy-dandy respiration refresher yesterday though, so you might want to take a look at that before you go any further.

Lane begins his paper by describing how respiration depends on two different sets of genes within cells, the mitochondrial genome (the genes found in the mitochondria) and the nuclear genome (the genes found in the cell nucleus).  The mitochondrial genome is passed directly from a mother to her offspring while the nuclear genome results from the combination of genes contributed by the mother and the father.  These two genomes must work together to create a functional respiratory chain, the important last step in cellular respiration.  If the mitochondrial and nuclear genomes are perfectly compatible, the respiratory chain will work nearly flawlessly and produce ATP and water as it should.  If there is a mismatch between genomes such that the respiratory chain develops incorrectly, electrons can escape the respiratory chain and react with oxygen to form free radicals, specifically reactive oxygen species (ROS).  Leaks in the respiratory chain can result in excessive ROS production, which can in turn cause damage and even programmed cell death.  It is thus essential that the mitonuclear compatibility be sufficient to form a functional respiratory chain capable of supporting an organism’s energy requirements.  In fact, offspring with severe mitonuclear mismatches will likely die during development or shortly after birth, leaving only those individuals with functional cellular respiration alive to reproduce.

hummingbird

A hummingbird has a very high metabolic rate.

But, Lane wondered, how good does this mitonuclear match really need to be for an organism to survive?  Consider an animal with a high metabolic rate such as a hummingbird or a bat.  These animals must have high mitonuclear compatibility to produce a respiratory chain capable of supporting the high energy demands of flight.  In these animals, any individual with even a minor mitochondrial mismatch should die because it cannot produce the necessary energy to survive.  However, more sedentary animals, such as rats and other rodents, can likely tolerate a higher ROS leak because their energy demands are lower.  It’s not as crucial that every electron make it to the end of the respiratory chain in these species.  If so, then offspring with mitonuclear mismatch (up to a point, the ROS threshold) may survive long enough to pass their genes on.

Let’s consider what this hypothesis, if correct, means with regard to reproductive success (i.e. fitness), fertility, and life span.  Think about the hummingbird and the rat for a moment.  If the hummingbird requires low ROS leakage, many of its offspring are likely to die during development because the ROS leakage threshold is low.  In contrast, a rat has a higher ROS leakage threshold and more of its offspring will survive relative to the hummingbird.  This means that fertility should be low in hummingbirds and high in rats, which is indeed the case.  Fitness could also be affected by ROS leakage thresholds.  Hummingbirds have fewer offspring relative to rats, but their offspring are also less likely to survive to reproduce themselves if their ROS threshold is very low. But there’s a trade-off too (there’s almost always a trade-off in biology!): a higher ROS leakage threshold means that more ROS are being formed.  Excessive free radicals can lead to cellular damage and death.  Rats, with their higher ROS leakage threshold, will have a higher ROS production as well as higher cell damage.  High free radical production is known to decrease life span, so the rat, with its high ROS production relative to the hummingbird, will have a shorter life than the hummingbird.

gerbil

A typical rodent has a low metabolic rate.

So Lane’s hypothesis fits the data for terrestrial vertebrates very well and he has generated a testable idea that people will be able to experimentally support or refute.  That’s good!  I think this idea, that a cellular process helps determine fitness, fertility, and life span, is very interesting and I’m eager to see the research that results from this paper.  However, at the end of the paper Lane suggests that the ROS leak hypothesis should apply across species, presumably across eukaryotic species such as fungi, plants, and invertebrates.  He might be right, but let’s consider insects for a moment.  There’s nearly always an exception among insects, and I think this is a case where there might be a big exception.

Let’s think about flies for a moment.  Many flies have high metabolic rates, which, according to Lane’s hypothesis, should mean that their ROS leak threshold is low with a corresponding low fertility and long life.  This isn’t necessarily the case.  Some flies produce few offspring, but many of them produce a lot of offspring!  Many flies also have very short lifespans of only a few weeks from egg to dead adult.  Let’s then contrast this to the flightless giant water bug I work with (not an altogether fair comparison as they are larger than most flies and that needs to be taken into account, but this is just a thought experiment).  The water bug spends most of its life sitting in one place, a highly sedentary insect with a low metabolic rate.  If Lane’s hypothesis applies to all eukaryotes, this giant water bug should have a short life and high fitness.   Giant water bugs have huge eggs, so they don’t produce as many offspring as some species, but 1000 or more offspring in a mating season is nothing to sneeze at and they seem to fit Lane’s hypothesis.  However, these bugs tend to have comparatively long life spans, sometimes a year and a half as adults!  Last, think about social insect queens.  These are sedentary insects that sometimes have very long lifespans (sometimes decades long!).  According to Lane’s hypothesis, these insects should have short lifespans, which isn’t always the case.

I don’t want to sound like I think Lane’s hypothesis is worthless because I think it can potentially explain several patterns of metabolism, fitness, and life span that we see in nature and I honestly hope it pans out.  However, I have two predictions, one or both of which I suspect will hold true as researchers begin exploring the limits of the hypothesis:

Prediction 1.  The ROS hypothesis will not apply to all eukaryotes and instead apply mainly to warm-blooded, terrestrial vertebrates.

Prediction 2. If the hypothesis holds up to experimentation, I suspect researchers will find other forces at play in insects and other invertebrates that overshadow or alter the impact of ROS leaks on life span and fitness so that they do not follow the same patterns that we see in warm-blooded, terrestrial vertebrates.

Ultimately, I don’t think the hypothesis in its current state is going to be as widely applicable to eukaryotes as Lane suggests, but I hope it will lead to some interesting new research dealing with the link between cellular processes and the life histories of organisms.  If it does that, even if it proves to be incorrect, I think this hypothesis can contribute something very meaningful to science.  And even if it doesn’t pan out, at least it was an exciting idea that got people thinking.  I consider that a success!

Literature Cited:

Lane, N. (2011). The Costs of Breathing Science, 334 (6053), 184-185 DOI: 10.1126/science.1214012

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Science Sunday: Dragonfly Accompanying Behavior

Happy new year everyone!  This year New Year’s Day also happens to be Science Sunday, so I’m going to start 2012 off with a science filled post.  Woo, science!  :)

Every now and then, I’ll get an e mail from someone during the dragonfly swarming season telling me a story of how the writer walked outside and had a dozen or so dragonflies follow him/her closely as he/she walked.  Most of the people who have this experience think it’s rather magical, but it’s unexpected and they want to know what’s going on.  Happily, this behavior has actually been covered in the scientific literature!  So, I’m going to start the new year by discussing a paper that deals with this interesting behavior that was released in the odonate science journal Odonatologica in 2010, authored by O and J Holusa of the Czech Republic.

For those if you who have followed my Dragonfly Swarm Project, you know that dragonflies often swarm because there are a lot of small prey insects in the area that attract them, forming a sort of all-you-can-eat buffet for lots of hungry dragonflies.  In essence, the dragonflies are taking advantage of abundant food that is easy to capture.  Swarms just happen to form if there is enough food for several dragonflies to eat.

A similar behavior called accompanying has been documented by a few odonate researchers.  It works like this.  When a large, slow-moving animal such as a hippo, rhino, or human walks through an area, two things happen.  Insects that are attracted to the animal, such as blood sucking insects, follow them as they move.  Other insects, those found on the grass, brush, or ground, also move out of the way by flying, jumping, or running.  Small clouds of flying insects therefore form around the large animal as it moves.  If you’re a dragonfly, these little clouds of insects are a great source of food.  And, if you are a dragonfly that lives in an area where there are few bushes and flying insects are unlikely to move around much during the day, accompanying large, slow-moving animals means that you’re more likely to catch a tasty snack than if you wait for the little insects to fly on their own.

Brachythemis leucosticta

Brachythemis leucosticta. Photo licensed under Creative Commons by F. Lo Valvo and is available at http://en.wikipedia.org/wiki/File:Libellula.jpg.

One species of dragonfly, Brachythemis leucosticta, has been well documented performing this accompanying behavior, following large mammals within its African range.  It tends to live in open, sparsely vegetated areas where movements of large animals could improve prey capture rates.  The Holusas were curious whether the European populations of B. leucosticta would exhibit the same pattern of behavior.  They also wished to document how far the dragonflies would accompany a large mammal and whether males or females were more likely to take part in the behavior.

They performed their very simple experiment with a population of B. leucosticta along the River Barbate in southern Spain.  They started in the area where the dragonflies perched in the shade near the water and walked perpendicularly away from the shore. When dragonflies followed them, they counted the number of individuals, determined the sex of the followers, and recorded the distance at which each dragonfly returned to its perch.  The researchers walked to the end of the floodplain and then made a wide arc back to the starting point, repeating the walk many more times over a two-day period.

With this experiment, the authors learned that dragonflies would usually follow them away from the water, flying close to the ground in front of them.  Fewer than 5 dragonflies would usually follow the researchers, but they recorded one group of 11.  Females were more likely to accompany than males, making up approximately 67% of the total observations.  Females also accompanied further than males (up to 111m in famales and 89m in males), though both sexes typically returned to their perches before the researchers had traveled 40m from the water.  Finally, of the 53 walks where dragonflies accompanied, dragonflies were observed catching and eating prey in only 3.

The authors made a few conclusions.  First, they noted that though the prey capture rate of accompanying dragonflies was rather low, less than 2%, this was probably a higher prey capture rate than they would experience without accompanying.  They also pointed out that few insects flew out of the grass as the researchers walked and the dragonflies captured prey every time small flying insects were observed.  Accompanying is therefore likely a valuable means of capturing prey in a habitat that is rather inhospitable for small flying insects.

The authors also attempted to explain the differences in accompanying distances they observed between males and females.  They suggest that because the females are lighter colored than males, they are not as noticable to the prey and are more successful at capturing prey via accompanying and therefore persist longer.  They also suggest that because the males are darker, they heat up more quickly upon leaving the shade.  I personally think this is highly unlikely, so I’d like to add a third possibility: that males return to their perches sooner becuase they are territorial and might lose their territory to other males if they stray too far.  Because females are not territorial, they can afford to accompany large animals further from the water to take advantage of the flying insect bounty.

Now, this behavior has not been confirmed in any US dragonfly species, but it is suspected in three: the green darner, wandering glider, and black saddlebags.  All three are commonly reported in static and migratory dragonfly swarms in the US, so these are dragonflies that are flying during the dragonfly swarming season.  This is also the time of year that those e mails start to come in from people asking why dragonflies are stalking them.  I think there’s a connection: the dragonflies in a swarming area are very likely accompanying people, forming little groups around people kicking up little flying insects as they walk.

Tomorrow I’m going to talk about another curious behavior that I haven’t been able to find any information about, but has been reported to me three times now.  It’s a bizarre behavior, so I hope you’ll check back!

Literature Cited:

O. Holusa and J. Holusa (2010). “Accompanying” behaviour of Brachythemis leucosticta (Burmeister) in Europe (Anisoptera: Libellulidae) Odonatologica, 39 (1), 63-70

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From the Literature: Oxygen, Temperature, and Giant Insects

I hope everyone liked the giant insects post last week!  It was one of my favorites to write.  The topic is just so fun!  I continue with the subject this week by describing a scientific paper that was released in July.  It combines several things I love (giant insects, aquatic insects, and respiration)  into one manuscript of pure science fabulousness!  Let’s get to it, shall we?

You probably learned as a kid that insects are ectothermic (aka, cold-blooded).  Ectothermic organisms are largely unable to regulate their body temperatures, so their bodies remain close to the temperature of their environments.  As the temperature increases, processes like metabolic rates speed up.  The opposite happens at cold temperatures and everything slows way down.  Ectotherms survive best under a range of temperatures where their body processes work efficiently, but the animal is still able to get everything it needs (food, water, oxygen, etc) from the environment.  They’re like Goldilocks: they don’t like things too hot or too cold and prefer for things to be just right.

Oxygen plays a big role in the interaction of ectotherms with their environments, especially at extreme temperatures.  Let’s consider a hypothetical insect, say a grasshopper.  As the grasshopper gets warmer, its metabolic rate increases and its body processes become more efficient.  However, as the grasshopper’s metabolic rate increases, so does its oxygen consumption.  At some point, the oxygen demand of the grasshopper may become greater than its oxygen availability and all sorts of bad things start to happen as its body processes start to break down.  Oxygen plays a role at very cold temperatures as well, leading scientists to propose that oxygen can set thermal limits (the maximum and minimum temperatures our grasshopper can survive) in ectotherms.

One problem though: terrestrial insects don’t fit the pattern observed in many other ectothermic animals.  This may be because their respiratory systems do not depend on lungs and blood to deliver oxygen to their cells and instead deliver oxygen directly to their cells via a series of tubes that connect to the outer environment.  This creates a terribly efficient system that provides enough oxygen even at high temperatures for many terrestrial insects.  Quite simply, their respiratory system provides enough oxygen even under the worst conditions.  But what if the insects live in oxygen-limited environments, such as water?  Might oxygen play a role in setting those upper thermal limits then?

stonefly

Image from http://www.glommaguiden.com/foto_2003/ bilder/030416_dinocras_cephalotes.htm.

Researchers Wilco Verberk and David Bilton considered this question and determined that if the thermal limits of any insects were to be limited by oxygen levels, aquatic insects were the most likely suspects.  So, they chose an insect that requires a low temperature and flowing water as their subject, the stonefly Dinocras cephalotes.  If the maximum temperature the stonefly could tolerate was limited by oxygen consumption, the maximum tolerable temperature would decrease in low oxygen water while it would increase in high oxygen water.  They then developed a simple experiment to determine whether this was the case.

The team placed stoneflies in flow-through chambers in a water tank and ran 10°C water containing various mixtures of oxygen and nitrogen (20% O2/80% N2 = normal, 5% O2 /95% N2 = low oxygen treatment, and 60% O2/40% N2 = high oxygen treatment) through them.  After letting the stoneflies acclimate for an hour, they ramped the temperature of the water up 0.25 degrees per minute until the critical temperature was reached, i.e., the stoneflies started showing signs of thermal stress such as lack of movement and leg twitching.  Then they compared the critical temperatures for each treatment to determine if their hypothesis was correct.

And it was!  They discovered that the upper thermal limit increased almost 3°C in the high oxygen water compared to water containing normal levels of oxygen.  Conversely, the upper thermal limit decreased in low oxygen water by about 1.5°C compared to that at normal oxygen levels.  The conclusion: oxygen levels can set upper thermal limits in larval aquatic insects!

Now you might be wondering why this is exciting or what any of this has to do with giant insects.  The results are interesting for several reasons, but largely because they show that some insects do experience oxygen-induced changes in their upper thermal limits.  This means that, while terrestrial insects might be able to obtain enough oxygen at any temperature to meet their needs, aquatic insects and other insects that live in oxygen limited environments can reach a temperature at which their oxygen demand outstrips the oxygen available to them.  Consider how an insect such as a stonefly gets the oxygen it uses.  They don’t have any spiracles (the pores through which terrestrial insects “breathe”), so oxygen is simply absorbed through the exoskeleton.  Many stoneflies have gills to make this process more efficient (the bigger your body surface, the more oxygen you can absorb from the water), but it’s still a very slow process.  The size of these insects may be limited as a result.  Aquatic insects that rely on absorbing oxygen from the water rather than going to the surface to breathe are also unable to regulate their oxygen uptake very well.  They can do various behaviors to increase the flow of oxygen into their bodies when they become oxygen stressed, but oxygen becomes toxic at very high concentrations.  Aquatic insects can’t do much to prevent oxygen from flowing into their bodies, so this can be a problem.

And this brings us to the giant insect part of the paper.  Verberk and Bilton propose that oxygen limitation at temperature extremes may have contributed to the rise of insect gigantism in the late Carboniferous and early Permian.  This makes sense considering how many of the giant insects were insects that probably had aquatic nymphs (proto-dragonflies, mayflies, and stoneflies, among other aquatic organisms).  The high levels of oxygen at the end of the Palaeozoic meant that oxygen could be absorbed more efficiently by aquatic insects and allowed them to become larger.  I covered this hypothesis last week, so check that post for more details.

Alternatively, Verberk and Bilton suggest that oxygen toxicity may have played a significant role in promoting insect gigantism.  How can an aquatic insect cope with increasing levels of oxygen in water and prevent oxygen poisoning?  They can get bigger!  If insects increased in size as oxygen levels in water rose, then they could counteract the negative effects of high oxygen levels on their bodies.  Oxygen levels at the end of the Palaeozoic were so high that aquatic insects likely had to become very large to prevent oxygen poisoning.  Giant immatures then led to giant adults.  Hence, giant insects that resulted from the limits of their respiratory systems in very high oxygen environments!  It’s a very interesting, new idea.  I suspect many people will do further tests in the future to determine whether this might really have been possible, so we’ll see if it holds up to further study.

I love this hypothesis!  Still, I have to point out that there is one major assumption that the entire hypothesis is built upon, that the giant proto-dragonflies, mayflies, stoneflies, etc had aquatic nymphs.  Modern dragonflies, mayflies, and stoneflies have aquatic immatures, so it’s likely that their predecessors did too.  However, there is no fossil evidence of aquatic nymphs for these groups at the time of the giant insects.  For all we know, the griffenflies and giant mayflies may have had terrestrial nymphs, which would make Verberk and Bilton’s hypothesis fall apart completely.  While the authors did acknowledge this assumption, I think their position would be strengthened if a fossil of even one aquatic immature could be found from that time period.  Without that piece of evidence, I fear this hypothesis is built upon a shaky foundation, one that might not hold up to scrutiny.

But wow!  A new explanation for how giant insects may have evolved!  And focused on the aquatic stages of insects!  You can see why I’m excited by it.  I can’t wait to see the research generated by this paper in the future.  It’s going to make for some very interesting reading!

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Literature Cited:

Verberk WC, & Bilton DT (2011). Can oxygen set thermal limits in an insect and drive gigantism? PloS one, 6 (7) PMID: 21818347

This paper is open access!  Full text available online here:  http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0022610

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