Taking Their Breaths Away

I think one of the reasons I like damselflies and their larvae is because they look neat! The adults are so fragile, even more so than dragonflies, and the larvae have three beautiful “tails”, also known as caudal lamellae. In the summer of 2011, I was working in a fisheries lab and we were told to design individual projects. I got the crazy idea that I could raise damselflies, take pictures of them every week, and when they emerged, identify them to species and use the pictures to make a species key for larval damselflies. Little did I know that when I would go to identify my first emerged adult I would stumble across a larval key, get very upset that my project was useless and resign to the fact that raising macroinvertebrates was just too difficult a task for me anyways. The point of this anecdote was that when I originally collected the live damselfly larvae some of them lost their “tails” and I figured they would die within the first week. I knew that their three caudal lamellae were a source for respiration and would probably be able to live with out one, but not two. After stumbling across the following paper, I have reevaluated the main cause of death for may of the larvae to be their lack of interest in the fish food/mosquito larvae diet they were on, not that their gills had fallen off.

The following paper is entitled “Hypoxia and lost gills: Respiratory ecology of a temperate larval damselfly” by Sesterhenn, Reardon, and Chapman. The authors examined the effect of hypoxia (low dissolved oxygen) on the behaviors of an Ischnura (Coenagrionidae) species with respect to the number of caudal lamellae. This is important to understand as many lakes and slow moving systems, the favored habitat of most damselfly species, become eutrophic. Damselfly and dragonfly species are the top predator of aquatic macroinvertebrates in fishless systems and this role in an ecosystem with low dissolved oxygen is one reason that makes them a model organism for a study on hypoxia.

Caudal lamellae have many uses other than their function for respiration. They are also used for ion regulation, locomotion, intraspecific signaling, and weaponry. However, they are most well adapted for the uptake of oxygen. In looking at a close-up picture you will find that they look like a leaf with prominent veins. It is estimated that up to 90% of individuals in a population may lose at least one lamellae in its life just through the harshness of an environment. However, damselflies have other ways of breathing or increasing oxygen intake. One method is through rectal pumping which allows water to move in and out of the organisms back end and absorbing oxygen through specialized cells inside of the organism. Another method is abdomen waves in which the damselfly will “swish” its abdomen side to side to move water across its lamellae. Also, some damselflies (actually, most gilled aquatic macroinvertebrates use these methods) will do “pull-downs” or what I think looks more like a push-up. This also helps move water over the gills. Below is a link for a stonefly doing this action. Lastly, aquatic macroinvertebrates may just move to the surface of the water to access the oxygen rich layer right below the surface. Despite these other methods, the researchers hypothesized that the “number of caudal lamellae influences damselfly respiratory ecology, and that the role of lamellae may change at different levels of dissolved oxygen.”

With all of this in mind, Ischnura larvae were collected from low dissolved oxygen ponds in Kentucky. Only specimens with all three lamellae were taken for this experiment. The researchers successfully kept them alive (their methods for that make sense in retrospect as compared to mine) and prepared the larvae for two types of experiments. The first experiment involved measuring metabolic rates of damselfly larvae with either three or one caudal lamellae (yes, that means they pulled two lamellae off of some specimens). Through a complex set-up, the oxygen decline in a closed system in response to damselfly respiration could be measured. The second experiment was to observe behaviors of three and one lamellae damselflies at various levels of dissolved oxygen. Behaviors were recorded with a video camera.

What they learned from all of this is that the metabolic rate of Ischnura larvae was not affected by the number of lamellae. Also, smaller individuals and those with one lamellae spent more time near the surface of the water when the dissolved oxygen was low in the behavioral experiments. In addition, the alternate behaviors I previously described were observed more as the dissolved oxygen level decreased. From this we can understand that Ischnura is well adapted to living in low dissolved oxygen (hypoxic) ecosystems. It can also be noted that one caudal lamellae is sufficient for survival. The loss of all three lamellae would change the larvae’s ability to swim and then it wouldn’t be able to move oxygenated water over its body if needed. Therefore, the loss of all three lamellae is much more lethal.

Knowledge about specific tolerances of macroinvertebrates of environmental conditions will prove to be critical as water systems are degraded. A loss of macroinvertebrates communities will hurt fish populations if the fish aren’t directly affected by the low dissolved oxygen in the first place. It also goes beyond dissolved oxygen. Specific chemical tolerance can be species specific which also makes it that more important to know what is going into our waterways (*cough cough* fracking fluid)!

Stonefly pushups:https://www.youtube.com/watch?v=c06Up7YGkY4

Link to original paper:http://www.sciencedirect.com/science/article/pii/S0022191012002752

More about respiration and caudal lamellae: http://link.springer.com/content/pdf/10.1007%2FBF00323773.pdf

Ischnura being parasitized by mites!:http://www.researchgate.net/publication/232664446_PARASITISM_OF_ISCHNURA_POSITA_%28ODONATA_COENAGRIONIDAE%29_IN_FLORIDA_BY_TWO_SPECIES_OF_WATER_MITES