Tadpoles of one frog species let out an audible "scream" when they come under attack, scientists have discovered.
They only make the noise, described as a brief, clear metallic sound made up of a series of notes, when in distress.
It is the first time any vertebrate larva has been found to use sound to communicate underwater.
The discovery that frog tadpoles can make sounds also raises the possibility that a host of aquatic larvae communicate in a similar way.
The distress calls are made by tadpoles of the horned frog Ceratophrys ornata which lives in Argentina, Uruguay and Brazil, researchers report in the journal Acta Zoologica.
That tadpoles communicate somehow is simply amazing
Dr Guillermo Natale National University of La Plata, Buenos Aires
Scientist Dr Guillermo Natale of the National University of La Plata in Buenos Aires, Argentina, and his colleagues, were studying the mating calls of adult frogs.
Many adult amphibians use loud sounds such as croaks to advertise their presence, and often to attract sexual partners.
Until now though, researchers did not realise that amphibian larva might also produce sounds underwater.
That changed when Dr Natale caught a horned frog tadpole in a pond using a hand-held net.
A tadpole "screams" when poked underwater
"We heard a brief, clear and very audible metallic-like sound," he told the BBC.
C. ornata tadpoles are difficult to find in the wild, so the researchers caught a wild pair of breeding adults, and began a programme to rear the young amphibians in captivity. This enabled the scientists to better study the noise they had heard in the field.
The team discovered that C. ornata tadpoles are naturally aggressive and carnivorous, often eating the tadpoles of other frog species that they encounter.
However, "much to our astonishment, they do not eat each other," says Dr Natale, who is also an assistant researcher Argentinean Research Council (Conicet). That may be because of the "screams" emitted by the tadpoles.
The researchers found that when C. ornata tadpoles come into contact with, or are prodded by, an external object such as a metal spatula, they let out a brief, metallic sound consisting of a short series of higher frequency pulses.