Mighty Midge: A New Niche

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Two words. Two of my favorite words are exsanguination and bioturbation. These are great words to drop at dinner parties. The first word, exsanguination, is best left to discussions of vampires or black flies and cows. The second word, bioturbation, refers to how organisms mix up sediments in the soil or at the bottom of lakes and rivers. This may not seem too exciting to most people, but it is terribly important. Earthworms turn over soils when they move around. Nutrients such as nitrogen, carbon, and phosphorus tend to descend deep into the soil over time and the worm’s bioturbation serves to bring nutrients back up, close to plant roots that then absorb the nutrients for use in metabolism and photosynthesis.  In other words, with a bit of sun and water, these nutrients kick start the terrestrial food web, keeping us all alive.  In lakes, nutrients descend, slowly settling down to the soft sediments at the bottom on the lake, a zone that is static, still and cold. The nutrients then descend deeper into the sediments where they are no longer available for the aquatic food web. Aquatic oligochaetes, relatives of earthworms, and midge larvae are able to live in the cold, low oxygen environment at the bottom of lakes. They worm their way through the sediments, releasing nutrients back into the water column via bioturbation.

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Nutrients descend to the bottom of lakes. Organisms like aquatic oligochaetes and midges move through these bottom substrates, releasing the nutrients and sometimes oxygen back to the lake for fish and invertebrate to use.

Far and away. One tiny midge has been receiving a lot of attention lately in the scientific news for its bioturbation work in the Antarctic (https://www.britishecologicalsociety.org/aliens-antarctica-terrestrial-ecosystems/). The parthenogenic Eretmoptera murphya, an orthoclad, semi-terrestrial midge was accidently introduced from a sub-Antarctic island to Signy Island in the maritime Antarctic probably around 1967. As a parthenogenic species, it reproduces with only females, an adaptation that facilitates spread of individuals in the extreme cold since no mating is required. It is also flightless with reduced wings, so the adult saves energy that could be used on flying and mating, on producing eggs. Reports of these midges often show the adults, but the adults are short lived relative to the immature forms, the larvae. The small larvae live in the soil, serving as decomposers and turning over the soil through which they move, making nutrients available for the ecosystem. This role is more commonly held by earthworms, but there are no earthworms in Antarctica, thus the invasive midge has filled the open niche space. The increased availability of nutrients sounds good until you realize that the ecosystem on Signy Island has evolved to exist without this level of bioturbation. The relatively sudden appearance of insect bioturbators is beginning to change the delicate ecosystem.

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Photo by Ben Tullis from Cambridge, United Kingdom – Looking down to the base, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=5882809

Cold thermoregulation. So, why are there no earthworms present in Antactica? Well, the region is isolated, far from other continents. Also, it is cold. Very, very cold. Rather than ask why no earthworms live in Antarctica, I ask instead how these ectothermic or cold-blooded insects have come to colonize extreme cold environments. In fact, one orthoclad midge, Belgica antarctica, is the only free living, endemic insect in Antarctica and is considered the largest terrestrial vertebrate restricted to Antarctica (Read that to mean that penguins and marine mammals only live part of their time on land or in Antarctica). This midge lives along the shores of the Antarctic peninsula and the islands of the maritime Antarctic region. Another insect, a podomine midge, Parachlus steinenii, also lives on islands of the maritime Antarctic. These three midges have colonized and adapted to the coldest, most remote regions of Earth. We know the most about the cold tolerance of Belgica antarctica. Like Eretmoptera murphya, it has reduced wings and does not fly. It can survive in only a narrow range of temperatures from -15°C to 10°C. The ambient winter temperatures of its environment go well below -15°C in the Antarctic winter, but the temperatures within a centimeter below the surface of the ice stay near 0°C. Thus, the larvae stay relatively warm during the coldest parts of the year.

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Map of Antarctica including maritime Antarctic islands. Photo from NASA.

A Tale of Three Midges. Belgica antarctic is the only species of the three that lives on the mainland of Antarctic, thus it is given the title of the Antarctic Midge. The larvae of Belgica antarctica live for two years before pupation and emergence as adults. Unlike Eretmoptera murphya, it has both sexes. Adults live for about 10 days, long enough to mate and lay eggs. The larvae of B. antarctica are also semi-terrestrial, feeding on moss and algae. Conversely, Parochlus steinennii has aquatic larvae and is fully winged. This species has the greatest distribution of the three and is found in mainland South America. The larvae are able to adapt to a wide range of environments which may be how they were able to colonize Antarctic islands.

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Belgica antarctica larvae. Photo by Richard Lee found at https://www.nsf.gov/news/mmg/mmg_disp.jsp?med_id=77761

A changing world. All three species, Eretmoptera murphya, Belgica Antarctica, and Parochlus steinennii are unique insects that have colonized extreme environments. All three function in the ecosystem to process nutrients, to serve as part of the food web. However, the introduction of E. murphya from the nearby sub-Antarctic has set in motion massive changes to the ecosystem of Signy Island. Jesamine Bartlett and her colleagues have demonstrated that the larvae are creating more soil and disrupting moss, a major part of the Antarctic system. The researchers expressed concerns that E. murphya may be able to colonize the mainland of Antarctic in the future and disrupt the most pristine ecosystem on Earth. The larvae are very small, nearly microscopic, so are easy to miss during inspections of materials being brought into the mainland of Antarctica. A warming climate will increase the chance that these species and others may be able to colonize and cause irreversible changes to the food web and ecosystem functions of the southernmost continent. New niches will be filled, and some will be created phenomena observed with rapid changes in the Earth’s past.

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Insects are not well known for their ability to colonize cold environments, yet many do. More information on cold adaptation of aquatic insects in future posts.

For more information

Baranov, V., Lewandowski, J., Romeijn, P., Singer, G. and Krause, S., 2016. Effects of bioirrigation of non-biting midges (Diptera: Chironomidae) on lake sediment respiration. Scientific reports6, p.27329.

 

Convey, P., 1992. Aspects of the biology of the midge, Eretmoptera murphyi Schaeffer (Diptera: Chironomidae), introduced to Signy Island, maritime Antarctic. Polar Biology12(6-7), pp.653-657.

 

Convey, P. and Block, W., 1996. Antarctic Diptera: ecology, physiology and distribution. European Journal of Entomology93, pp.1-14.

 

Cranston, P.S., 1985. Eretmoptera murphyi Schaeffer (Diptera: Chironomidae), an apparently parthenogenetic Antarctic midge. British Antarctic Survey Bulletin66, pp.35-45.

 

Edwards, M. and Usher, M.B., 1985. The winged Antarctic midge Parochlus steinenii (Gerke)(Diptera: Chironomidae) in the South Shetland Islands. Biological Journal of the Linnean Society26(1), pp.83-93.

 

Lee, R.E., Elnitsky, M.A., Rinehart, J.P., Hayward, S.A., Sandro, L.H. and Denlinger, D.L., 2006. Rapid cold-hardening increases the freezing tolerance of the Antarctic midge Belgica antarctica. Journal of Experimental Biology209(3), pp.399-406.

 

Usher, M.B. and Edwards, M., 1984. A dipteran from south of the Antarctic Circle: Belgica antarctica (Chironomidae) with a description of its larva. Biological Journal of the Linnean Society23(1), pp.19-31.

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