The two-headed worms tell us something fascinating about evolution

Ashley Pap: This is the 60-second science of American American, I’m Ashley Pap.

You can say that Guillermo Ponz is a scientific monster hunter – although he believes that the term “monster” has never properly captured his subjects.

Guillermo Ponz: So they are ordinary animals that have gone through various developmental processes that would eventually build a body, this is not what you expect.

Pap: What this Madrid-based researcher in Spain actually loves are the amazingly amazing animals. In the end, he learns two-headed worms.

Ponz: We have these worms, which are usually ordinary worms, with one head and one tail, this is normal, but sometimes they can have two heads or two tails. And on the other side there are worms that always have one head and many tails.

Pap: Officially, he looks at bifurcated anelides, which means things like earthworms that have emerged from the larval stage with two heads, or spontaneously sprouted two tails, or … some other combination of mixed appendages.

We know that some species, such as salamanders and insects, have the ability to regrow appendages when needed. But there is one type of worm, the anelides, that can grow back, unlike anything we’ve ever seen in the kingdom.

Their segmented bodies, like earthworms with rows of annular compartments, help them easily grow a new head or tail at the first sign of trouble.

Or even crazier, they can grow back a whole new right side of their body if cut in half.

Ponz: … worms that do these crazy things that are very strange, very, you know, very, very strange things that these worms shouldn’t do, quote-don’t quote, shouldn’t do.

Pap: Once Ponz began to study the anatomically deadly lengths these worms would reach to grow and survive, he was completely attracted.

And he realized that he and his team were not the first to be fascinated. Ponz discovers that there was a golden age of research on “monster creatures” in the 18th and 19th centuries.

Ponz: … 100+ years of literature would be about the feelings of monsters, creatures, or monsters, or oddities, or, you know, they are, they are all those variations that describe them. After all, these animals are not monsters.

Pap: Fixing with the “reanimated monster” makes sense, especially then. The novel by Mary Shelley, Frankenstein, was published in 1818 and only further increased the interest – and part of that interest became real research.

Ponz and a team of international researchers conducted an extensive review of existing knowledge about monster worms. They immersed themselves in research worth 275 years – browsing the journals to observe scientists, reading historical texts and even turning to the wider scientific community to see if anyone knew anything about records of abnormal worms.

They wanted to understand all the different types and patterns of bifurcation and see if there was any clue as to how the oddities had developed.

Their search led to a jackpot of both history and science.

They came across documents and drawings of forked worms from all over the world – in Latin, French and German, all the way to Russian, Japanese and even Indonesian. In total, they spent more than a year working through the archives, translating old texts and following the clues to monster worms.

What they have learned is that bifurcation of worms has been observed in more than 60 species of worms in the family tree of the anelides, and in some species up to 20% of the young have some form of bifurcation. This work was recently published in the journal Biological Reviews. [Guillermo Ponz-Segrelles et al., Monsters reveal patterns: bifurcated annelids and their implications for the study of development and evolution]

Ponz: And this means, for example, in the case of bifurcation, that when an animal is cut and regenerates, for example, the tail, there must be some mechanism that specifies where this tail will be, how it will be oriented, what is rear, what is front , what is left, what is right, what is dorsal, what is abdominal. And these mechanisms can be disrupted. And this can lead to different anatomies.

And this gives us clues as to what is important during this process. Of course, these are baby steps. So we just point to this process, this phenomenon, we say, Okay, hello, this is happening, there are these animals doing these weird things. We should not forget about them, let’s look at them.

Pap: They also realized that there is a strong link between the type of bifurcation and the development of internal organs. This means that the way in which the worms were separated is reliably indicated if there are additional groups of organs.

With this type of information, Ponz and his team were essentially able to devise a plan or guide for the reliable and repeated creation of bifurcated worms … which is potentially a very useful resource for scientists interested in studying developmental mechanisms.

This long-forgotten study of worm developmental abnormalities seems ready to return. According to Ponz, this information can go far beyond the anelides and even insect worlds to help us better understand how things like growth and development actually happen … in both normal and monstrous ways.

Ponz: In a sense, we are now following this trend that they started then, studying these animals to try to understand larger pictures in nature. Usually development leads to a certain way of their establishment until a certain moment. So you have a development that ends in anatomy that is more or less preserved. But sometimes it doesn’t. And that can teach us something about development processes. And that’s interesting.

Pap: For Scientific American 60-second science, I’m Ashley Pap.

[The above text is a transcript of this podcast.]

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