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| Anurans' (frogs' and toads') eyes are important for understanding how vertebrates developed their vision. Photographer: Dan Koleska/Shutterstock |
When tadpoles grow into frogs, their body shape isn't the only thing that changes.
The vast majority of frogs and toads start their lives as aquatic larvae before undergoing metamorphosis. As their tails give way to their legs and they adapt to a life away from water, their bodies and behaviors change.
According to a new study, the changes are even more extensive than previously thought. CT scans of more than 100 different species revealed that the lenses of their eyes changed depending on their adult environment.
Aquatic and burrowing species kept their lenses the same way they did as young aquatic tadpoles, with spherical lenses to focus underwater and for short distances underground. Meanwhile, terrestrial species such as climbers, gliders, and hoppers flatten out their lenses as adults to improve their vision in the air.
"The lens focuses light onto the retina to provide a focused image," says Amartya Mitra, a former Ph.D. student at the Museum who led the research. "How much refraction is required depends on the environment an animal lives in." Frogs and toads are important animals to study eye evolution in because they live in different environments at different stages of their lives."
"Because the refractive index in water is nearly identical to that of the inside of the eye, aquatic animals require powerful, spherical lenses to focus the light." On land, the cornea helps to focus light, so the lens is flatter and less powerful."
The research findings were published in the journal Proceedings of the Royal Society B.
The life cycle of frogs and toads
Frogs and toads are all anurans, one of the three major groups that comprise amphibians. They begin their lives as eggs laid by their mother and externally fertilized by their father, as do the vast majority of anurans.
However, from this point on, the lives of different anurans begin to diverge. Some species develop in the egg and emerge as a tadpole rather than smaller versions of the adult.
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| Tadpoles become adults through a process known as metamorphosis, which involves a change in their body plan. Photographer: Steve Byland/Shutterstock |
Tadpoles develop into adults through a process known as metamorphosis, which changes their body plan. Photographer: Steve Byland/Shutterstock
This method, known as direct development, is relatively uncommon. The majority of species will instead hatch as tadpoles, which have long tails, gills, and no limbs to adapt to life in fresh water.
Tadpoles stop growing larger after a certain point and begin to change shape. Some tissues undergo metamorphosis, which causes them to break down and reform, allowing them to mature into their adult form.
"The most obvious parts of metamorphosis see tadpoles lose their tails and gain limbs," says Dr. David Gower, a Merit Researcher at the Museum who co-authored the study.
"For example, the skeleton hardens and becomes ossified, undergoing significant shape change to allow them to breathe air and, in many cases, move around on land."
"Their diet, in the meantime, shifts from mostly herbivorous as a tadpole to mostly carnivorous as an adult in almost all cases." It's a significant transition, especially for those transitioning to a more terrestrial life as adults."
As the rest of the body changes, so do the eyes, with the rate of development determined by the adult's environment.
"The lenses are laid down in layers," David explains, "so a tadpole lens is surrounded by additional layers to shape it into an adult lens." "Changes in the shape of a species' lens occur as a result of laying down new layers in a different way than when they were a growing tadpole."
Understanding how these changes occur can provide new insights into the evolution of vision in vertebrates as a whole, not just frogs.
How do frog lenses change as they develop?
The researchers studied the shape of 179 frog and toad specimens from 126 species, including 45 species where both tadpoles and adults were studied.
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| Burrowing anurans, such as the Mexican burrowing toad, have lenses that are surprisingly rounded in comparison to other terrestrial species. Photographer: Juan Aceituno/Shutterstock |
Burrowing anurans, such as the Mexican burrowing toad, have surprisingly rounded lenses compared to other terrestrial species. Photographer: Juan Aceituno/Shutterstock
Each specimen was scanned with a microCT scanner, allowing the researchers to create a three-dimensional model of each individual's lenses. The use of scans to study the eyes is a significant advancement for this field of science, which previously required destructive sampling of specimens.
"One of the ways lens shape has been studied historically is making a series of really thin slices through the eye and looking at how that two-dimensional shape changes as you go," says Dr. Katie Thomas, a researcher at the Museum and another co-author on the paper.
"Because that takes time, it's not practical to do it for hundreds of animals." Using CT scanning allowed us to examine many more species in a short period of time."
Due to a lack of younger specimens, the study was unable to investigate direct developing species, but it did discover some surprising results among other species. Burrowing frogs, for example, had much more spherical and powerful lenses than expected, making them likely short-sighted on land.
"This was a surprising result, and it could simply be that these frogs don't need to see that far underground," Amartya explains. "Being nearsighted may not be such a disadvantage for them."
"Alternatively, they may not use their eyes much in the dark, so the energy-intensive changes to adjust their curved tadpole lenses to flatter adult lenses may simply be not worth it."
"Finally, it's possible that they only use their eyes when they emerge for the breeding season, which is brief and mostly takes place in water." In that case, the eyes are well-suited to the tasks that they must perform in an aquatic environment."
In the future, the researchers, who include museum co-authors Brett Clark and Dr. Jeff Streicher, hope to investigate more subtle changes in the development of lenses. Their findings are part of an international study of the diversity and evolution of vision in frogs and toads.
"We have a very broad snapshot of adults versus larvae, so it would be interesting to look at the differences within different life stages," Katie says. "Some previous studies have shown that changes in lens shape occur rapidly in some species and more gradually in others, but we simply don't know how they occur in the majority of species."
More information: Amartya T. Mitra et al., Ocular lens morphology in frogs and toads is influenced by ecology and metamorphosis, Proceedings of the Royal Society B: Biological Sciences (2022). DOI: 10.1098/rspb.2022.1770