Italian agile frog tadpoles can learn to evade crayfish terror Free

An invader prowls the ponds and rivers of Northern Italy, taking out locals and threatening entire species; can they fight back? This is not a Hollywood movie; it's happening right now in the gentle streams of Lombardy and the chief intruders are alien red swamp crayfish. ‘This highly invasive and voracious predator … can have dramatic impacts on freshwater fauna and alter aquatic ecosystems’, says Daniele Pellitteri-Rosa from the University of Pavia, Italy. The crayfish have already devastated one local species: the Italian agile frog (Rana latastei). But hope for this threatened species could lie in the form of the next generation. If embryos in frogspawn can sense a lurking threat, they may be able to alter when they hatch to stand a better chance of survival. Alternatively, tadpoles might be able to improve their odds by taking evasive action when danger looms. Pellitteri-Rosa and colleagues decided to find out how Italian agile frog embryos and tadpoles respond to local predators – such as dragonfly larvae, which they always reside alongside – and the crayfish invaders that threaten their future.

Collecting both predators from nearby waterways, Andrea Gazzola fed them on Italian agile frog tadpoles, to make sure that they reeked of danger; predators release the alarming aroma of their dismembered victims in their vicinity. Then, Pellitteri-Rosa retrieved freshly laid Italian agile frog frogspawn from ponds in three nearby areas, taking them to the lab to find out whether the stench of feasting dragonfly larvae would affect when the tiny frog embryos emerged from their eggs. But it did not. The tiny embryos were unable to alter when they hatched to evade danger. So, how would the youngsters fare in later life when faced with peril?

Filming 15 day old tadpoles swimming in a cup, Gazzola then infused 2 ml of water carrying the odour either of unfed predators or of predators that had recently dined on tadpoles, while continuing to record the tadpoles’ reactions. Sure enough, when Giovanni Scribano and Alessandro Balestrieri (University of Pavia) and Andrea Fontana (Istituto Nazionale di Fisica Nucleare, Italy) scrutinised the movies, they found the tadpoles were afraid of the fed dragonfly larvae, freezing ∼80% of the time, while the fasted dragonfly larvae were almost as terrifying for the tadpoles. And when the youngsters did move after catching a whiff of their predator, they wove around more, in a bid to throw off the hunters, and switched course in all directions, to keep an eye out all around.

However, the tadpoles were less savvy when Gazzola added eau de crayfish to their water. Only the tadpoles retrieved from a pond where crayfish reside showed any sign of toning down their activity to evade detection. The tadpoles from ponds with few or no crayfish didn't alter their behaviour much, as if they didn't have a care in the world. However, when Gazzola added a dash of water from the container of fed crayfish to the youngsters’ home, the developing amphibians became more wary. Even the tadpoles that did not share their home ponds with crayfish were slightly more cautious, but less so than the youngsters from ponds where crayfish had already taken up residence.

It seems that Italian agile frog tadpoles can learn to evade alien predators, probably by learning to associate the scent of devoured tadpoles with the invader's aroma. But Pellitteri-Rosa and colleagues are anxious that time might be stacked against survival for species that are already teetering on the brink.