Tadpole of the Australian sandpaper frog (Lechriodus fletcheri). Credit: John Gould

Tadpoles are generally considered to play a peaceful role in the lower food chain of freshwater ecosystems, spending their time sifting through the mud or mindlessly gulping at water filter-feeding on algae. This simplistic view is due to the difficulties in obtaining reliable information on their diet and their ability to process large quantities of low-quality food due to their elongated guts. However, for many tadpole species, this simple depiction of their diet may not be completely accurate.

Image by M W from Pixabay

A prime example are the tadpoles of the sandpaper frog, Lechriodus fletcheri, which are found in short-lived puddles of water on the forest floor along the east coast of Australia. While considered a generalized tadpole that feeds on algae and plant material, they are also highly cannibalistic towards tadpoles of their own species (conspecifics) that are sick or injured. However, two recent studies by John Gould of the University of Newcastle have found that not only do they also cannibalize on their unhatched eggs, they hunt down and eat mosquito larvae as well.

Frog spawn with eggs. Image by Bill Kasman from Pixabay

These predatory behaviors likely evolved in this species as a result of the low quality of food found in the small temporary pools of water it calls home, which can sometimes last just a few days. As such, it is highly advantageous for the tadpoles to grow rapidly before the water dries up. To do this, they need much more nutrients than the algae tadpoles typically consume. Luckily, animal-based foods are highly nutritious, packed with the high levels of lipids and proteins they require to grow quickly.

When it comes to eating mosquito larvae, tadpoles are found to swim near the surface of the water where the larvae often reside and lunge at any that come too close. Occupying higher levels of the food chain and actively hunting aquatic insects like mosquito larvae is likely to improve the chances of offspring surviving the short-lived ephemeral waters. Especially since they share similar diets and are direct competitors for limited food resources.

Tadpoles attacking and consuming mosquito larva. Credit: John Gould

Amphibians that exploit temporary habitats will often synchronize their reproduction, resulting in these small pools often inundated with large amounts of eggs that are ripe for the picking by tadpoles that have already hatched. In order to consume the eggs, L. fletcheri tadpoles will suck at the exposed edges of recently laid spawn, often consuming eggs whole while they gorge themselves for hours. While offering a hefty meal, consuming conspecific eggs will also have the added benefit of reducing future competition, overcrowding, and avoid the threat of being cannibalized themselves.

Tadpoles eating unhatched eggs. Credit: John Gould

Though it may seem counter-intuitive for a species to evolve cannibalism, it will increase the chance of survival of some individuals, albeit at the expense of others, improving the odds that the entire population doesn’t starve. It does, however, give rise to some interesting questions on how adults protect their offspring from being the unfortunate victims of this strategy, or whether offspring have any strategies of their own to avoid getting eaten. Nevertheless, far from being placid and mindless, these findings reveal the active role tadpoles play in freshwater systems.

Sometimes it truly is a tadpole eat tadpole world.

Written by John Gould & Jose Valdez

Research articles:

Gould, J, Clulow, J, Clulow, S. Food, Food not friend: Tadpoles of the sandpaper frog (Lechriodus fletcheri) cannibalise conspecific eggs as a food resource in ephemeral pools. Ethology. 2019; 00: 1– 6. https://doi.org/10.1111/eth.12995

Gould, J. Tadpoles of the sandpaper frog, Lechriodus fletcheri, hunt mosquito larvae in ephemeral pools. Australian Journal of Zoology. https://doi.org/10.1071/ZO19065.