by Piter Kehoma Boll
Sometimes a single water drop from a freshwater lake can contain several different organisms from the plankton that lives near the surface. And a common group in freshwater plankton is the crustacean order Cladocera, known as water fleas. The most common and widespread species is Daphnia pulex, or the common water flea.
Having a worldwide distribution and measuring about 3 mm in length, the common water flea has a typical water-flea body. It is usually transparent and the head is small and smooth, with two easily visible black eyes and a well-developed pair of second antennae that are used for swimming, being the largest pair of appendices. The thorax and abdomen are fused and are surrounded by a transparent, somewhat oval shell, making the common water flea look like a pot-bellied creature. The shell has a posterior tip that looks like a pointy tail. The thoracic legs, more difficult to distinguish because of the carapace, are smaller than the second pair of antennae and are used to create a water current that brings food to the common water flea’s mouth.
This food consists mainly of algae and other small organisms, such as bacteria, as well as of organic fragments. The common water flea is, therefore, a filter feeder. Its predators include both invertebrates, such as predatory arthropods, and small vertebrates, such as some fish.
The common water flea is considered a model organism and has been extensively studied regarding several biological aspects, including, for example, ecological stoichiometry, which investigates the response of organisms to changes in resource availability. The response of the water flea to predators has also been extensively studied and revealed, for example, that it can increase in size in the presence of invertebrate predators, in order to become too big to be eaten, and decrease in size in the presence of vertebrate predators, in order to become too small to be seen. The common water flea can also develop special structures in the presence of specific predators, such as head protrusions in the presence of glassworms.
The reproductive cycle of the common water flea is another aspect that is very well studied. As in most species of the genus Daphnia, the common water flea reproduces by cyclical parthenogenesis. Most of the population consists of females and, during their growth season, females produce a brood of diploid eggs (which are clones of the mother) every time they molt. The eggs hatch very quickly, usually after only a day, but the newly hatched water fleas remain inside the mother for about three days before being released. After passing through about 5 instars, they can start to produce their own eggs.
When environmental conditions become difficult, the second mode of reproduction is triggered. Some of the offspring produced by parthenogenesis turn into males and females start to produce haploid eggs, which are then fertilized by males and turn into resting eggs with a hardened coat, called ephippia. An ephippium can remain in the environment for many years, withstanding cold, drought or lack of food, and hatch into females when conditions improve.
The common water flea was the first crustacean species to have its genome sequenced. It was revealed that this species contain about 31 thousand genes due to an elevated rate of gene duplication. This is about 10 thousand more genes than humans have and is the reason why the common water flea has such an amazing capacity to adapt to environmental changes.
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More model organisms:
Friday Fellow: Touch-me-not (on 19 April 2013)
Friday Fellow: Red flour beetle (on 6 February 2015)
Friday Fellow: Pea aphid (on 12 June 2015)
Friday Fellow: Many-headed slime (on 1 April 2016)
Friday Fellow: Baker’s yeast (on 4 August 2017)
Friday Fellow: C. elegans (on 20 April 2018)
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Colbourne JK, Pfrender ME, Gilbert D, et al. (2011) The ecoresponsive genome of Daphnia pulex. Science 331: 555–561. doi: 10.1126/science.1197761
Krueger DA, Dodson SI (1981) Embryological induction and predation ecology in Daphnia pulex. Limnology and Oceanography 26(2): 219–223. doi: 10.4319/lo.1981.26.2.0219
Tollrian R (1995) Predator‐Induced Morphological Defenses: Costs, Life History Shifts, and Maternal Effects in Daphnia pulex. Ecology 76(6): 1691–1705. doi: 10.2307/1940703
Wikipedia. Daphnia pulex. Available at < https://en.wikipedia.org/wiki/Daphnia_pulex >. Access on 22 October 2019.
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