Updated: Sep 13
In my opinion, one of the most amazing things that biologists do is peer into the past by imagining what animals once looked like and how the animals of today are related to each other. If the molecular biological and phylogenetic concepts that biologists use were artistic media, their canvas would be their organism of choice. Any discerning artist would choose their canvas carefully, and so would the biologist: they choose the organism to fit the question of interest. For example, organisms like Drosophila melanogaster, otherwise known as the fruit fly, have short generation times, meaning inheritance of traits can be observed on short time scales. This allows studies of genetic inheritance that occur much more quickly than the inheritance we see across human generations. Other common “model organisms”, like nematodes, mice, and baker’s yeast, similarly give biologists many different ways to investigate questions. Then, why do some spend their careers examining a small, unassuming sea anemone when there are so many other organisms to look at?
Nematostella vectensis, the starlet sea anemone, is studied in labs such as the Gibson Lab at Stowers Institute, where I am working this summer, and in Professor Katerina Ragkousi’s lab at Amherst! It is a great model for biologists to study because of its ancient origins and developmental plasticity.
As an adult, Nematostella has a long body and tentacles that it uses to grab onto prey. Starlet sea anemones prefer to nestle into soft sand and grab onto whatever prey passes by with its tentacles over actively swimming after prey. However, this anemone starts out life as a microscopic swimming larva called a planula. These lively larvae are aptly named, as “planula” derives from the Greek word meaning “wanderer.” They travel in search of a good place to settle down, feeling around with a sensory hair-like structure called an “apical tuft” not unlike a bug’s antenna. Eventually, they undergo metamorphosis, settling down onto a surface and elongating their bodies. It is the planula larva that has captured the imagination of a select group of researchers, as they believe it can shed light on the evolution of our heads and what our ancestors were.
Nematostella, being an evolutionarily ancient animal, occupies a unique position for comparative studies in biology. It turns out that certain genes that function near the apical organ in Nematostella are directly related (orthologous) to genes that function in establishing the anterior end, or the head end, of bilaterally symmetrical animals, or bilaterians, meaning that it may organize its body much like we do. In fact, Nematostella is radially symmetrical, along with jellyfish, meaning they are not part of the "bilaterians", the group of animals that is bilaterally symmetrical. However, if you think of a random animal, it’s very likely that it is a bilaterian. For instance, birds, worms, humans, starfish, and snails all fall into this camp. This means Nematostella is evolutionarily quite far removed from these animals we are familiar with (at least by 600 million years or so). It is truly remarkable that the way in which it makes a head resembles the methods our bodies use in our infancies. Additionally, the fact that jellyfish share similar genes as us that are used for the patterning of our aboral/anterior ends means that our common ancestor also had this complex “toolkit” of genes that they used in a similar fashion. Thus, we can create an informed picture of the genes that our ancestors hundreds of millions of years ago may have used without having any genetic material from that era–we did that just by cleverly choosing a model organism.
In addition, if we look at the life cycles of other distantly related organisms such as sea urchins and annelid worms, we can start to see evolutionary patterns that tell us about the life cycle of their common ancestor, or the predecessor that these organisms evolved from. Despite being ecologically and phenotypically distinct, sea urchins, annelid worms, and Nematostella all have larvae that swim around and use an apical organ to study their surroundings. That leads to the hypothesis that the common ancestor of these three organisms had a larva with an apical organ as well. This type of comparison is only possible because Nematostella is so evolutionarily ancient compared to the other animals that we know.
Furthermore, Nematostella is incredibly developmentally plastic. This means that it responds to perturbations, chemical or mechanical, by altering how it develops. One of the most impressive feats that it can perform is the restructuring of its body after having its cells separated. During a phase in its early life called “gastrulation,” it is possible to take a pipette, submerge Nematostella gastrulae in water, and vigorously pipette them. This puts a lot of mechanical stress on their bodies and ultimately turns them into a collective “soup” of cells. The soup is put into a device called a centrifuge, which spins the cells at breakneck speeds until they are collected into a pellet. Over a few days, this formless pellet grows smaller and the pellet gains definition as its composite cells try to form structures normally found in sea anemones, such as its main body or its tentacles, and becomes a Frankenstein’s monster-like creature. This speaks to the capacity for the starlet sea anemone to re-establish structure after extreme disturbances, and makes it a good model for studying how cells can reorganize after trauma or even how processes like regeneration occur. Despite it being relatively simple in terms of its structure compared to other animals, we still have a lot we can learn from the robustness of Nematostella.
The Frankenstein's monster in all its glory.
The once little-known sea anemone is now a rising star in the field of evolutionary and developmental biology. It is a model that has profoundly surprised me by its weirdness and complexity. I was initially not particularly interested in studying Nematostella because I had previously learned in a biology class that cnidarians have simple nervous systems and overall, are less complex than other “higher” metazoans. However, I feel my time working with the starlet sea anemone has humbled me. While not even having a brain to speak of, these creatures have presented me with the fact that us humans have much in common with even the most distant of animal phyla. Of course, as I’ve come to understand, Nematostella is also a surprisingly challenging animal to study because its unique physiology and phylogenetic position naturally beget intimidating questions, such as why we have nervous systems or what the rules behind regeneration are. In my opinion, that’s also why I think it’s so fun to work with, because it reminds us that we sometimes have to study the simplest models to answer life’s hardest questions.