The curious case of the comb jelly

Welcome to Science Sessions, the podcast of the proceedings of the National Academy of Sciences, where we connect you with Academy members, researchers, and policymakers. Join us as we explore the stories behind the science. I'm Paul Gabrielson. For most life forms, the arrow of time only moves forward. We are born, grow irreversibly older, and eventually die. But maybe not for all life forms. In a recent PNAS study, Joanne Joseph Soto of the University Museum of Bergen Norway, and Pavel Berkart of the Michael Sars Center also at the University of Bergen, report a species of comb jelly, Nemiopsis-Lady E, that reverts to an earlier stage of life when fed, following starvation or injury. One of the purposes of this phenomenon of reverse development, the authors say, may be to help comb jelly survive adverse conditions. Joanne, what is reverse development, and what did we know about it before this study? Reverse development is the capacity of an individual to go back to a previous life cycle stage, or in other words, reversing their normal development as if they were going back in time. Reverse development was actually first reported by the Slovenian sologist, Jovan Hatsi, at the beginning of the 20th century. In this study, Hatsi found a jellyfish species that was able to reverse from a very young Medusa to a polyp stage. Following this discovery, several other cases were reported, but always within phylum night area, the group that contains corals and moneys and true jellyfish. It was not until the 1990s, when a group of researchers from Italy found the first case of an adult jellyfish that was able to first degenerate to assess to then produce a polyp. This species, to retops its dorni, became quite popular worldwide for its ability to repeatedly rejuvenate and receive the name of immortal jellyfish. Prior to our study, the capacity of reverse development was considered unique among night aliens. With only a single case of a tapeworm, the bud also fit into the definition of reverse development. So, our study shows that Tinofores or Comjelis also have this capacity to reverse the balloon. Please introduce us to Comjelis and to their life stages. So Comjelis are actually not jellyfish. They belong to a completely different group called Tinofores. There's roughly around 200 species described and they come with many different shapes and sizes. Then can be tiny, just a few millimeters, so they can be quite large, even up to one meter in some cases. There is one group of Tinofores that are called Lovage, in which they go through two distinct life cycle stages. The first one is the Cdipit. They all are born as Cdipits, which is basically a small bowl with two long tentacles, and they use these tentacles to trap prey. They are some sort of ambush predators if you want to see it that way. And then we have the Lovate stage in which they have two very big lobes, and they also have structures that are pointy to create a current, like a funnel, go into what's above, and they have this kind of chamber and the current creating structures, and they basically filter the water by this means. So they eat in a completely different way as Cdipits. And in some cases, they still keep the tentacles for some particular species, but most Lovates, they degenerate the tentacles, they reabsorb the tentacles, so adult Lovates do not have tentacles. They eat in a different way. Pavel, the size differences can be dramatic, like a Cdipit is a few millimeters, and a Lovate can be 12, 15 centimeters, so really big. What's your attention to nemyopsis? We have been using nemyopsis-ladie in our lab for the past six years. Our main focus in the laboratory is to reveal the evolutionary origin of neurons and nervous systems. But for all of these exciting findings, we had to establish a permanent and reliable culture system for nemyopsis-ladie here at the Michael Sars Center at the University of Bergen. And this was challenging and took years. And sometimes we were pushing some variables to the limits to see how the Cdino 4 responded, and this has led us to better understand several aspects of the biology of nemyopsis, including reverse development. How did you demonstrate reverse development in the species? So we knew that reverse development in geolipish is triggered by a period of stress, such as lack of food resources or physical damage. However, prior studies working with nemyopsis survival during starvation conditions suggested that these animals could not reverse metamorphosis to go back to a previous larval state. Therefore we decided to try a different approach and see what would happen if we feed the animals after they shrank significantly. We decided to simulate two stressing conditions, either a period of stress by a starvation alone or by a physical damage, similar to what they experienced in the wild. In this case, we surgically removed the logs. For our surprise, when we fed them at a very small size, instead of growing back the logs and our ecles typical of an abled form, they developed tentacles and had a normal larval appearance and behavior. In brief, in order for reverse development to work in nemyopsis, the animals need to be free from the stressor that caused them to shrink and have enough food resources to develop the tentacles typical of a larval state. Why does reverse development occur? We have to admit that we do not know this for sure right now. However, we do have some ideas. Over the course of normal development from sydipid to low-bate, this xenophospesis minopsis drastically changes their diet, from a predominantly microplankton diet in sydipids to a larger plankton-based diet. So the larval state, also called sydipid stage of a low-bate ketenophore, is usually rather small, just a few millimeter in length. The ketenophore larva has tentacles, and thus feeds completely differently than an adult. So this may be a strategy to survive harsh conditions. Reverse development could help minopsis-ledi to have access to food resources that would not be available in an efficient way for the adult, and therefore survive in an environment with low food amounts. And this could also contribute to explain its success as an invasive species. What can we learn about rejuvenation from nemyopsis? Because this is something we recently discovered in order to find what can we learn we have to compare it to what we know in other groups that are actually displaying reverse development, and to compare it in a way that we try to identify which is the uniqueness within nemyopsis. So true jellyfish have a life cycle in which they alternate generations, meaning there is a polyp and a medusa stage in its life cycle. The medusa is the stage in which it reproduces sexually. In this sense, they are different individuals. The polyp is one individual and the medusa is another individual, even when they are genetically identical. Then the generating medusa through reverse development is able to reverse back to a polyp stage going through a profound transformation. While it has been compared with a butterfly and a caterpillar, entouritopsis is not the exact same individual which transforms back into the preceding stage. The cellular process behind this backwards transformation is called transdifferentiation, in which cells and learn their function and become some sort of ball of tissue and then relearn a new function in order to build a new polyp from this pool of undifferentiated cells. While the cellular and molecular mechanisms of reverse development in nemyopsis are still unknown, it is clear for us that it is the exact same individual that gradually rejuvenates into an earlier life cycle stage. And this is very exciting because it cannot just be compared with this so-called immortal geolipist to retopsis. So in this regard, nemyopsis is the first known case for us species without alteration of generations which is capable to rejuvenate after reaching their reproductive adult stage. Therefore, the species has a lot of potential to become a model species for reverse development and rejuvenation. But in order to reach that point, we still have to take a closer look at the cellular and molecular processes behind reverse development in nemyopsis. What are the caveats or limitations of the study? At the moment, we can only speculate about the underlying molecular mechanisms of reverse development in nemyopsis. So we look forward to reveal these in the next few years to come. The other thing I would like to add, which we as evolutionary biologists, marine biologists are extremely interested is, right now it is not known if reverse development is Only is found in the katina phoenixopsis lady, or if it's more widespread, and can be found in other katina pho species, or maybe even in other marine organisms.