Hidden BRITAIN
Aglance into a pond on a late-winter day may reveal the first stirrings of aquatic life. At this time of year, the water is clearer, with fewer plankton, algae and plants (such as duckweed), making the twitching masses of water fleas much more obvious.
Water fleas are crustaceans – it is their proportions (no more than 5mm long) and jerky movements that give them their common name. Of the UK’s 80 or so species, Daphnia pulex is the most common and widespread.
Have a close look at one under a microscope or under hand lens, and you’ll see a beautiful, delicate, glassy animal. A single compound eye stares back at you, long feathery antennae (used for swimming) flick out on either side of the head. The whole body is encased in a transparent hinged ‘clamshell’ arrangement, which enables you to see everything: guts, a beating heart, even eggs or a brood pouch full of developing babies.
Modified feeding legs protrude through the gap in the species’ shell. The legs draw microscopic algae and bacteria towards Daphnia’s mouth. So efficient is this feeding current, it can filter more than 4ml an hour. Multiplied by thousands, you can see why these superabundant organisms have a significant impact on water quality.
Daphnia are also a major food source for many other creatures, including the larvae of phantom midges, dragonfly and damselfly nymphs, bugs such as water boatmen, newt larvae and fish fry. However, they’re not merely nature’s cannon fodder. They fight back, albeit in a surprising way: when conditions are right, Daphnia multiply phenomenally fast.
Part of the trick is a strategy called parthenogenesis, meaning they reproduce without males. The females give birth to clones of themselves; males are formed only when temperatures drop, or the water they’re in is drying up. Entire populations may also undergo a process known as cyclomorphosis, which modifies the appearance and shape of the next generation to make it less palatable.
What triggers these changes in Daphnia? Chemicals released into the water by the very animals attacking them! The warning chemicals are by-products of digestion, as the
predators enjoy their meal. In response, the development of the next generation of Daphnia
embryos is altered. Specific genes are turned on, leading to the formation of a variety of extensions to their bodies, making them difficult to swallow.
Each type of predatory animal induces a slightly different shape-shifting response in the water fleas. The development of long tail spines, neck teeth and huge hooded helmets are just some of the options available to them. These adaptations may be more energetically expensive, but it’s worth it. By rendering the owner impossible to swallow, they can ensure a higher number survive to breed.
Size is a defence, too. In response to predation, Daphnia
are able to change how they invest in growth. For example, by speeding up development and maturing at a larger size, they become too big to be eaten by their micro-predators (though at a cost, as it reduces their reproductive potential). Conversely, if it is fish that are hunting them by eyesight, Daphnia do the opposite. They now mature earlier at a smaller size, becoming thinner and so less easily seen.