Tadpoles have an eye on the future as their vision develops

The life cycle of frogs and toads

All frogs and toads are anurans, which is one of the three major groups that makes up the amphibians. Like the vast majority of anurans, they begin their lives as eggs laid by their mother which are externally fertilised by their father.

From this point on, however, the lives of different anurans begin to differ. Some species develop in the egg and emerge as a smaller version of the adult rather than a tadpole.

This process, known as direct development, is generally uncommon. The majority of species will instead hatch as tadpoles, which are typically adapted for life in fresh water with long tails, gills and no limbs. 

Past a certain point, tadpoles stop growing bigger, and instead begin to change their shape. Some tissues break down and reform in a process known as metamorphosis which puts them on track to grow into their adult form.

Dr David Gower, a Merit Researcher at the Museum who co-authored the study, says, ‘The most obvious parts of metamorphosis see tadpoles lose their tails and gain limbs, but many other changes occur as well.’

‘The skeleton, for instance, hardens and becomes ossified, undergoing a lot of change in shape to allow them to breathe air and in many cases to move around on land.’

‘Their diet, meanwhile, changes from mostly herbivorous as a tadpole to a carnivorous one as an adult in almost all cases. It’s quite a major transition, especially for those that are transitioning to a more terrestrial life as an adult.’

As the rest of the body changes, so do the eyes, with the course of development depending on the environment the adult will live in.

‘The lenses are laid down in layers, so a tadpole lens is, in essence, surrounded by additional layers to shape it into an adult lens,’ David explains. ‘In species that change the shape of their lens, these changes come about as a result of putting down new layers in a different way to when they were a growing tadpole.’

Discovering how these changes take place can provide new information on the evolution of vision not just in frogs, but in vertebrates as a whole.