There are many elaborate ways of defending yourself in nature. The skunk uses a horrific chemical weapon; one type of lizard can squirt blood from its own eyes; and some sea birds literally vomit on their foes. But for the tiny so-called water flea called Daphnia – actually a freshwater crustacean, not a flea – the best way to get around threats is to mutate.
For example, if big-mouthed predators such as fish or large water bugs move into the vicinity, the water flea’s head may elongate up to twice its size in just a few days, making it harder to handle and fit in an animal’s mouth. If smaller foes show up, it turns prickly, sprouting ‘neck teeth’ or a spiky crown for protection. It may also grow a spear-like tail. This tail is more than just a weapon; a water flea swims in short bursts, ‘hopping’ forward, sinking and then hopping again (hence its name), and its new tail slows its rate of sinking so it doesn’t need to beat its oar-like antennae as much when travelling. Since many underwater predators hunt by detecting the vibrations made by their prey, this makes the water flea harder to find.
Daphnia is best at morphing when it is young – the development of new structures typically decline with age. The reason why these minute crustaceans morph in the first place instead of permanently evolving such defences is debated. Many experts believe that the rapid mutating skills of the water flea make it possible to invest in defence only in emergencies, saving energy for eating, growing and reproducing.
Water flea populations are mostly self-cloning females. Sometimes, if its pond starts drying up, a water flea may produce ‘resting’ embryos in thick eggs that can resist years, even decades, of drought. The embryonic Daphnia eventually wake up to a predator-free world just as soon as the water returns. But if conditions start to change dramatically, such as through fluctuations in oxygen levels or temperature, the females mate with males, which lead to genetically diverse ‘morphs’ that have a much higher chance of surviving unpredictable conditions. For Daphnia, change really is a good thing – it means the difference between life and death.