Not so free range? Oviposition microhabitat and egg clustering affects Eretmoptera murphyi (Diptera: Chironomidae) reproductive success. Published in Polar Biology, October 2018
When I started seeing midge egg sacs laid in a seemingly random fashion on Signy Island, Antarctica I chuckled to myself: “haha, Chironomids are free range, hehe free range midge … wait, I should totally write a paper with that in the title”. And so it began. At the time I was expecting to see little variation in survivorship depending on where the invasive midge Eretmoptera murphyi had lain its eggs, as it’s such a tough species with its larvae surviving desiccation, heat, cold, ice, flooding, you name it. I figured that it was at least worth a look as they appeared to have an unusual oviposition strategy for a Chironomid. And I wanted to use that paper title!
Conservative with a little c
Typically Chironomids lay hundreds of eggs in one batch, sometimes multiple times in a season. They have gone beyond the ‘breed like rabbits’ strategy, and in places can be apocalyptically rampant. As anyone who has encountered swarms of midges will testify (see below! Although I will point out that my midge neither flies or bites. Best kind of midge...other than the invasive bit). This species though, E. murphyi, is a little more conservative. It lays on average just 48 eggs per egg sac, and oviposits just once. Quite the gamble for a stressful environment. In my previous paper on the life cycle and phenology of this species, I found that less than half of those eggs are successful. So just laying once is a risky life choice. I figured that the random placement of egg sacs in either the soil layer or on the surface may be a strategy by the adults to mitigate some of these egg losses.
Where you lay your eggs is not always a comfortable home
On the surface of it – literally – laying your offspring on top of the moss seems like a bad shout. It is exposed to the worst of everything. When the sun shines I found that ground surface temperatures could peak up to 30ºC, and some clear windless days saw temperatures in excess of 20ºC for as much as 8 hours of the day. This is Antarctica remember! But the dark moss and peat absorb the heat like a dark t’shirt. Heat aside, these exposed positions are also vulnerable to frosts, and desiccation from the dry Foehn winds that blow over from nearby Coronation Island. Physiological experiments were needed. And because of the scale of the insect I took environmental measurements too, as typically air temperature measurements would not suffice for this level of microhabitat. I divided the moss bank substrates, where E. murphyi lives in two: Ground surface, typically the moss layer; and below ground, typically the soil layer. I recorded water content of these substrates and then set up temperature and humidity sensors to monitor below ground, surface level and 10cm above the surface, conditions.
"... ground surface temperatures could peak up to 30ºC... this is Antarctica!"
Friends with benefits
We found that you get two very different environmental profiles when looking at the surface and below ground habitats (see image above). Soils are vulnerable to desiccation much more than I expected, with a lower humidity and available water content. Whilst the surface vegetation experienced the greater temperature fluctuations, as expected, but remained a more saturated environment overall. Combined with physiological experiments we found that the egg sacs - which are balls of a gelatinous matrix surrounding the eggs within - were not at all desiccation resistant, losing water rapidly. And as you would expect, those adults that chose to aggregate their own egg sacs with other egg sacs from other individuals had a better chance of reducing water loss. By doing this they reduce the exposed surface area of each egg sac. And so prolonged exposure to low humidity reduced survival in egg sacs that had not been aggregated, but didn’t really affect those that had been aggregated together.
"... 30ºC would be detrimental to the eggs ..."
Overall, the desiccation experiments showed us that egg sacs are not desiccation resistant, but the eggs within are to a degree, and especially when aggregated with other egg sacs. All eggs did well at temperatures that would are currently experience on Signy Island, but 30ºC would be detrimental to the eggs for any period exceeding a couple of hours – remarkably this could be a possibility for Signy Island ground surface layers by the end of the century. We concluded that considering the current conditions and tolerance of the eggs, the soil habitat would actually be the more unsuitable habitat, unless eggs are aggregated. We weren’t able to survey for the occurrence of this as the very act of getting at the soil below the moss would disturb the ground and therefore the aggregations. But eggs were found in both substrates, and anecdotally were found laid singly and aggregated - hence the original investigation.
For foehn sake
Why does this even matter I hear you mumble. Well, E. murphyi is an invasive species, and it is currently the largest terrestrial organism on Signy Island. No, it’s not a massive midge, its just THE ONLY INSECT THERE! Furthermore, like most midge species, it is so abundant even with low reproductive success that we cannot ignore it. Therefore, understanding its physiology, life history and how these things will be impacted by climate is key to evaluating its risk as a species. If there are environmental limits on its reproductive success, its good to know what they are. In this instance I would suggest that the variability in where eggs are laid may be a result of soil/ground conditions on any given day. However, given the gestation is over 30 days long, this is still a gamble as conditions change.
But clearly they are still doing OK, and the midge has expanded its range significantly just in the last 10 years (See poster about this here). Where it may hit a snag is if more extreme events occur like the day the Foehn winds pulled air temperatures up to nearly 20 degrees, where very warm, very drying ground conditions followed caused by these 'hairdryer winds'. And these events are likely to increase with climate change.
All in all, where they lay their eggs has an effect on survival at the microhabitat level and the midge are not as ‘free ranging’ as I first thought!