I have a soft spot for platannas. They can’t be described as cute – like a kitten – and I’m sure not many maidens are tempted to kiss one.  But nonetheless they have their uses – more about that later.

Lone African clawed frog. PHOTO: Wikipedia Commons

The African clawed frog (Xenopus laevis) is best known to us as the common ‘platanna’.  Its Latin name is derived from the three short claws on each hind foot, quite unusual for frogs. The word Xenopus means ‘strange foot’ and laevis means ‘smooth’. Hence a smooth frog with a strange foot. They also have webbing on their hind feet to aid their swimming. The colloquial name ‘platanna’ comes from the Dutch plathande, meaning ‘flat hands’.

The species is found across most of Sub-Saharan Africa. So successful are they that they have invaded the territory of the Cape platanna (Xenopus gilli). The Cape platanna’s habitat is restricted to the brown acidic waters of the coastal fynbos. They are now highly endangered through habitat loss and competition from the common platanna, which is particularly partial to Cape platanna tadpoles.

Anyone who has studied biology (or life sciences as it’s now called) will be familiar with the common platanna. It was the animal of choice for anatomy lessons and dissection. Xenopus laevis is still widely used as a laboratory animal, and is particularly suitable for genetic studies to elucidate human genetic disorders. These frogs are also very easy to breed in captivity. They are aquatic and can therefore be kept in an aquarium – which is much easier to manage than a terrarium. Moreover, they will eat anything – be it dead or alive – so feeding them is really simple. 

Platannas have no teeth and no tongues. Where other frogs use their tongues and even their eyes to push food down their throats, platannas use their hands to shove it down. If the food is a bit too big, they use the claws on the hind feet to tear it apart. In addition, they have a suction mechanism called a ‘hyobranchial pump’ to pull food into their mouths.

An African clawed frog (Xenopus laevis) pair. PHOTO: Wikipedia Commons

Platannas lack true ears, but have lateral lines running down the length of the body. Through these they can sense movements and vibrations in the water. To find food they use their sensitive fingers, sense of smell, and the lateral line system on their bodies.

Platannas, like most other frogs, lay their eggs in water. Males have mating calls to attract females.  These calls sound very much like a cricket calling underwater. Females answer vocally, signalling either acceptance (a rapping sound) or rejection (slow ticking) of the male. (As a matter of interest, the endangered Cape platanna sounds like it’s snoring under water.)

Females are larger than males, appearing to be far plumper, with bulges above their rear legs where their eggs are internally located.

The male frog mounts the female frog and clasps her around the waist. This is called ‘amplexus’. Of the seven amplexus positions in which frogs mate, platannas breed in ‘inguinal amplexus’. The male clasps the female in front of her back legs and he squeezes until eggs come out, which are then fertilized by the male. The fertilized eggs develop into larvae (which we call tadpoles), and the tadpoles complete the metamorphosis into the next generation of platannas.  

During times of drought, Xenopus laevis frogs burrow into the mud, and stay dormant there for up to a year. This dormancy is called ‘aestivation’. Aestivation is different from hibernation in that it is a response to summer heat and drought, rather than to winter cold. 

During aestivation the frog’s metabolism changes to adapt to the fact that they are not eating and cannot excrete their nitrogenous waste in the form of ammonia, which would be highly toxic if it could not be rapidly evacuated into their watery surrounds. So their metabolism changes to make urea instead, which is less toxic and can be stored in the body until the next rains. My own doctoral research was aimed at trying to find out how they do this.

Xenopus laevis have been known to survive 15 or more years in the wild and 25–30 years in captivity. They shed their skin every season, and eat their own shed skin. No food goes to waste.

In the early 1930s two South African researchers at the University of Cape Town developed a test for human pregnancy. This involved injecting a woman’s urine under the skin of a female Xenopus laevis frog. If the frog ovulated, the woman was pregnant. This simple and reliable test was used worldwide throughout the 1930s to 1960s. Enormous numbers of common platannas were exported from South Africa to countries around the world for this purpose. When more reliable chemical pregnancy tests were developed, many platannas were released or escaped into the wild in these countries, where they survive and now thrive. 

We all know what happens if an alien species is released where there are no natural predators to keep the numbers in balance. And so it is that Xenopus laevis has changed from valuable pregnancy indicator to a common pest in many countries. 

Meet the Xenobots

A team of scientists at Tufts University in the US has created xenobots, tiny robots made from frog skin and heart cells that can walk, work together and heal themselves.

The computer produces a design (left) which is used to create the living robot (right).

Algorithms define the configurations of frog cells, which are then constructed by humans to create a living robot that the scientists have called a xenobot, after the Xenopus laevis species of frog they are made from.

These aquatic organisms live for up to seven days, and the team hopes that in future they can be used to deliver drugs into people’s bloodstreams, clean up microplastics from the ocean, or manage radioactive waste spills.

A number of variations of the 0.7 millimetre-long robots are designed using a computer algorithm.

“Computers model the dynamics of the biological building blocks (skin and heart muscle) and use them like LEGO bricks to build different organism anatomies,” the team explained.

“An evolutionary algorithm starts with a population of randomly-assembled designs, then iteratively deletes the worst ones and replaces them by randomly-mutated copies of the better ones,” they added. “It is the survival of the fittest, inside the computer.”

The manufactured organisms are currently able to walk and push things, and work collaboratively with other xenobots in a swarm.

Their advantage over metal micro-robots is that they can heal themselves if they sustain damage. Also, because they are formed of organic material, they would be potentially less damaging inside a human body or any natural environment they were introduced to. Once they have served their intended function they can biodegrade naturally.

For more information Google ‘Xenobots’. Source: www.dezeen.com/2020/01/15/xenobots-living-robots-frog-cells-technology/

About the Author

Whale Coast Conservation passionately lives by its slogan “Caring for your environment”.

Its small staff and volunteers are dedicated to

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WCC facilitates educator development programmes to improve the capacity of educators to offer informed environmental content in their lessons across all learning streams.

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