In 1641, French philosopher René Descartes, writing his famous “Meditations on First Philosophy,” observed that a mind is fundamentally different from the body which contains it. He reasoned that, while physical objects like flesh and bone can be divided or merged together, consciousness is intrinsically different because it cannot be quantified. Scientists have long known that it is impossible to separate or fuse a conscious mind — at least, that is what they thought.
Yet a pair of recent studies about comb jellies raise provocative questions about Descartes’ maxim. The first is a study from the journal Current Biology found that ctenophores, a phlyum of aquatic invertebrates better known as comb jellies, can successfully fuse together after being injured. (Though the animals have a striking weirdness and similarity to jellyfish, they are not technically related.) The scientists studied a population of warty comb jellies (Mnemiopsis leidyi) in a tank, gazing in amazement as two organisms bonded with one another with the flesh, seamlessly integrating as if to form a single new animal.
“While maintaining a population of M. leidyi in a seawater tank, we noticed an atypically large individual with two aboral ends [referring to the area farthest from the mouth] and two apical organs [areas closest to the animal’s apex],” the authors wrote in a statement. To test whether this comb jelly was actually two individuals who had fused together, the scientists tested individual ctenophores collected at different sites on separate dates. After cutting off a part of each animal’s lobe and placing different creatures together, they found on nine out of ten occasions (90%) “independent grafting experiments were successful, and all fused ctenophores survived for the full holding time of about three weeks in the holding tank.”
Fused Warty Comb jellyfish (Mnemiopsis leidyi) (Courtesy of Mariana Rodriguez-Santiago)
Previous research has demonstrated that M. Leidyi fuse within themselves whenever they need to defecate, squashing together their outer skin and digestive system fuse to form an opening— essentially creating a "disappearing anus" — yet this is the first evidence for two separate jellies becoming one. But If these two individuals are now one, does that mean their minds literally melded — or that perhaps they were never even sentient in the first place?
"Comb jellies may be the oldest animal group that exists and so they provide a unique opportunity to study basic but fundamental aspects of how animals move and interact in the world."
“We know that comb jellies (aka: ctenophores) are sentient in that they can sense their surroundings to find food and change the direction of their swimming if they bump into something,” study co-author and Colorado State University biologist Mariana Rodriguez told Salon. “We don't know much else beyond this basic type of sentience. It's hard to answer what this implies for consciousness of two animals fusing into one since it depends on how we define consciousness.”
Fellow co-author Kei Jokura, a biologist at the University of Exeter, initially discovered the fused comb jelly. Jokura told Salon that although no other animal has demonstrated “functional fusion” in the manner seen with the comb jellies, other organisms like corals, sponges and ascidians can physically behave as if they are a single entity despite containing numerous individuals.
“However, this differs from our finding, where previously independent organisms fused in just a few hours, merging nervous and digestive systems functionally,” Jokura said. “This ability is largely influenced by a lack of an allo-recognition system, which usually differentiates self from non-self and triggers immune rejection. In our experiments, nearly 90% of individuals exhibited fusion, suggesting that comb jellies might lack this allo-recognition mechanism.”
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Jokura added he was most intrigued by “the synchronized contraction of the muscles, which indicates that nerve cells fused quickly, allowing the sharing of electrical signals or action potentials. This could imply a form of shared consciousness, making this phenomenon a valuable experimental model for studying integration mechanisms.”
Jokura was not the only scientist to make a milestone discovery after wandering past a tank containing a gelatinous animal. While studying comb jellies in a different tank, University of Bergen natural historian Joto J. Soto-Angel noticed that an adult ctenophore, also the species M. leidyi, had vanished from his tank and seemingly replaced by a larva. Working with Pawel Burkhardt, group leader at the University of Bergen’s Michael SARS Centre, Soto-Angel designed experiments to see if the comb jelly reverted to its larval form much like the so-called "immortal jellyfish" Turritopsis dohrnii.
Like Jakura, Soto-Angel’s curiosity was rewarded. After being exposed to the stress of starvation and physical injury, the comb jellies in Soto-Angel’s care reverted back to a cydippid larval stage.
"Witnessing how they slowly transition to a typical cydippid larva as if they were going back in time, was simply fascinating," Soto-Angel said in a statement. "Over several weeks, they not only reshaped their morphological features, but also had a completely different feeding behavior, typical of a cydippid larva."
"This is a very exciting time for us," Burkhardt added in the statement. "This fascinating finding will open the door for many important discoveries. It will be interesting to reveal the molecular mechanism driving reverse development, and what happens to the animal's nerve net during this process."
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Even scholars who are not involved in the pair of papers agree that they reveal the immense potential in studying ctenophores. University of Florida neuroscientist Leonid L. Moroz, who specializes in ctenophores but was not involved in either recent study, told Salon that “we know more about rocks on the Moon than about our oceans. It is estimated that we only know ~10% of marine inhabitants.”
Pointing out that this is disastrous for humanity from the perspective of our basic survival, Moroz added that “support for basic science, and marine biology in particular, is the only reasonable and cost-efficient way to ensure a healthy life for our children and grandchildren.”
As Rodriguez succinctly put it, “Comb jellies may be the oldest animal group that exists and so they provide a unique opportunity to study basic but fundamental aspects of how animals move and interact in the world.” Because of their remarkable abilities like fusing together and reversing their own aging process, they “can teach us new ways to think about the evolution of regeneration and sentience.”
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