Forget those bombastic and profoundly daft Transformers movies – some of the best transforming creatures can be found within the natural world, from the masquerading northern white-faced owl to the stealthy mimic octopus.

A new study has suggested that the greatest transformers are some of the most common lifeforms on the planet: water fleas. Not only are these tiny planktonic crustaceans – known by the scientific name Daphnia – able to suddenly grow helmets and reinforced spines, but they are able to customize their defenses to whatever predator may be nearby.

“These defenses are speculated to act like an anti-lock key system, which means that they somehow interfere with the predator's feeding apparatus,” lead author Linda Weiss, a researcher at the Department of Animal Ecology, Evolution and Biodiversity at Ruhr-University Bochum, said in a statement.

“For example, Daphnia lumholtzi grows head and tail spines,” she said, so that three-spined stickleback fish would find it incredibly painful to eat them. Members of the species Daphnia atkinsoni are even able to grow a “crown of thorns” specifically in order to spear the predatory tadpole shrimp.

These swimming critters are found all over the world in a range of ecosystems, from the largest lakes to the smallest ponds. At only 0.2 to 5 millimeters (0.01 to 0.2 inches) across, it’s difficult to see these customized defensive body parts with the naked eye.

^{Undefended Daphnia longicephala (left) next to its defended phenotype (right). The crest and tail spines are specifically grown to stop the backswimmer Notonecta glauca eating them. Credit: Dr Linda Weiss}

Their defense mechanisms have, in fact, been known about for some time, and researchers have long thought that Daphnia are able to “know” which defenses to grow thanks to their antennules – microscopic appendages that detect biochemical-based predator cues in the water. However, knowledge relating to these little pylons have been relatively sparse, simply because they are so small and inaccessible that testing their biological role has been somewhat difficult.

Presenting their findings this month at the annual meeting of the Society for Experimental Biology in Brighton, the researchers discovered that there are specific neurotransmitters that can convert predator cues to hormonal responses, via these antennules. These hormonal responses in turn induce sudden and rapid changes in the crustaceans’ body shapes.

In particular, the neurotransmitter dopamine appears to play a particularly important role in Daphnia defense mechanisms. In humans, this hormone is associated with addiction, the regulation of movement, and attention in the human brain, but many types of life, including both plants and most multicellular animals, can synthesize it.

In Daphnia, dopamine release forces the levels of so-called “juvenile hormones” in their bodies to also change. In many related organisms, juvenile hormones affect how various parts of the body grows. The implication with Daphnia is that the sudden growth of defensive body parts is also down to these juvenile hormones.