We humans have to view water fleas under a microscope to see them with any detail. But to their predators, they can certainly become visible, quickly.
Water fleas are small crustaceans, of which all but eight species are found in inland bodies of fresh water. They range in size from 0.20-6.0 mm, and feed on bacteria and organic detritus. They respirate through the surface of their bodies, and when viewed under a microscope, that surface is transparent. It is for this reason that scientists study water fleas so closely, as their internal organs are clearly visible (1).
Though incredibly small, a recent study conducted by German researches has concluded that these tiny creatures grow body armor, developed specifically to ward off the predators in their environment. This means that armor varies widely among water flea species, depending on the environment the specimen lives in (2).
This armor development is accomplished by detecting a predator’s scent through appendages called antennules. These small protrusions send the enemy scent to the brain, triggering the release of dopamine. In turn, dopamine signals juvenile growth hormones to begin development of the armor throughout the body.
Some of these predators include fish, phantom midge larvae, and backswimmers. The research team has determined that these tailor-made defenses make the water fleas incompatible with the feeding apparatus of their predators, though they are unsure exactly how (2).
Check out this link to see these little creatures in action, darting around their natural habitat:
Photo : Flickr/Creative Commons/NTNU Faculty of Natural Sciences and Technology