I’ve been collaborating with another scientist to finish up some of my graduate school work. However, unlike me, he doesn’t work with voles. He works with Daphnia.
What are Daphnia, you ask? Daphnia are a genus of small crustaceans that are surprisingly cute.
Also called water fleas because of their flea-like movement, there are more than 200 species of Daphnia that live in freshwater all over the world. If you’ve ever been swimming in a lake, you’ve been swimming with Daphnia!
Seriously. You can find Daphnia in almost any permanent body of water!
Floating in the water column
However, Daphnia can’t survive in just any old body of water. They’re pretty small and weak, so they can’t live anywhere with a strong current (aka, a river).
But being weak doesn’t mean Daphnia are at the whim of the water’s current. Because they’re so light, Daphnia can stay suspended in water columns; they move up and down this water column using their legs and antennae. These movements are called diel vertical migration and are dictated by seasons and predators. During the day, Daphnia will avoid predators by moving to the bottom of their water column; at night, they’ll come to the surface to eat.
Okay, maybe not entirely invisible. But most of a Daphnia‘s carapace, or shell-like outer structure, is clear. This makes it easy to see most of the internal organs working.
On the Daphnia‘s head is a darkly colored compound eye. This eye contains a tiny, light-sensing organ called an ocellus; this organ lets the Daphnia keep track of where the surface of the water is.
This transparency makes Daphnia great for scientists. With a relatively low-powered microscope, you can see things like:
1) their young moving in the brood pouch
2) their eyes moving
3) their blood pumped around by their beating heart.
So, if a scientist wanted to test the impact of a drug like nicotine or alcohol on Daphnia, they would be able to easily see the physiological effects of the drug. Daphnia are also great for toxicological experiments.
Plus, Daphnia don’t seem to mind being put on a coverslip under the microscope; they appear to be totally fine when returned to the water.
As another boon for scientists, Daphnia have a very short lifespan and reproduce quickly. They’ll hatch and go through a few molts until they reach reproductive age at around 5-10 days old.
This quick reproduction is called in biology terms ‘r-selection’. r-selected species like Daphnia are generally small, grow to adulthood quickly, and produce many offspring. Each of these many offspring has a relatively low chance of surviving to adulthood. Rodents and mosquitos are other examples of r-selected species.
On the other end of the spectrum are K-selected species. These species are generally larger and invest a lot in raising their offspring. K-selected species tend to invest in only a few offspring over their lifetimes, each likely to survive into adulthood. We’re considered a K-selected species, along with animals like elephants and whales.
Sexual and Asexual
What’s especially interesting about Daphnia, though, is the fact that how they produce their offspring varies.
During the summer, Daphnia are generally asexual and produce offspring through parthenogenesis. The female Daphnia can create viable eggs without being fertilized by a male. All of these eggs will hatch into females. Daphnia use asexual reproduction when there are lots of resources around, resulting in many young Daphnia being born quickly.
But when conditions are worse (i.e., not enough food, it’s cold), Daphnia switch to sexual reproduction. Some of the parthenogenesis eggs hatch into males instead of females; these males will then mate with the females to create fertilized eggs. Once the eggs are fertilized, the female will lay the eggs in a container called an ephippium. This ephippium protects the eggs through the winter (and up for 20 years!) until it’s a good time to hatch.
For such a tiny, simple organism, Daphnia are pretty sophisticated. In addition to changing their reproduction habits based on resources, they can change their size to avoid predation. When there are lots of adult fish around, Daphnia will decrease in size so they’re hard to see; when there are lots of juvenile fish around, they’ll increase in size so they’re hard to eat.
Pretty neat for such a cute little guy.