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No one ever accused SpongeBob SquarePants of being smart, but we might need to give him a little bit more credit. New research conducted by Jacob M. Musser and Detlev Arendt, from the Developmental Biology Unit at the European Molecular Biology Laboratory (EMBL), along with colleagues, are uncovering cell interactions which look a whole lot like the precursor to what we might recognize as a nervous system. Their results were published in the journal Science.
Our last common ancestor with sponges is pretty ancient; there’s been a lot of time to diverge, and sponges haven’t changed a whole lot in the intervening time. In fact, some scientists believe the very first animal was a sponge-like creature. As a result, understanding the way sponge’s bodies are built and the way they operate in the oceans, can give us clues to how more complex animals evolved over time.
“For a long time, sponges were assumed to be these weird alien-like creatures with a body plan very different from what we have, with very simple behaviors, and it was assumed they didn’t really have anything in common with us,” Musser told SYFY WIRE. “When it came out that they had these synaptic genes it gave us a window into what the transition from simple organisms to more complex organisms might have looked like.”
The sea sponge genome was sequenced years ago, but we didn’t have the tools to make heads or tails of what all those genes were doing. Scientists were able to dissociate sponges, breaking them up into their constituent cells and get a transcript of what each cell was doing, but they couldn’t see precisely how those cells were working together in concert.
Recent technological advances, however, allowed researchers to rewind the clock and reconstitute those cells within the sponge’s body to figure out how they are interacting with one another, using a process called single-molecule fluorescent in situ hybridization (sm-FISH).
“You can sort the cells into clusters based on what genes they express, then single molecule FISH allows you to see where the cells actually are. Then you’ve made a link from knowing how the genes express to describing the cell type morphologically,” Arendt said.
Researchers already knew there were cells which were sending signals and other cells capable of receiving signals, but it wasn’t until they were able to nail down their geography inside the body that the pieces fell into place.
“The eureka moment for us was when we realized that both cells were together in the digestive chambers,” Musser said. “What we saw suggested directed communication going from the neuroid cells to the digestive cells. The digestive cells are responsible for taking up food and driving water flow through the body. It’s very possible the neuroid cell is helping to direct both of those things.”
Unlike ordinary neuron communication, which is directed and permanently connected, the sea sponge neuroid cells are scattered throughout the whole canal system of the sponge and are capable of moving and interacting with different digestive cells. It lacks some of the speed and complexity we find in human nervous systems but allows for a little more flexibility in function. Given the sea sponges’ sedentary, filter-feeding lifestyle, speed isn’t necessarily a priority.
“The neuroid cells also have several genes involved in innate immunity that are used to detect bacterial invaders. In one of the cells we imaged, we found a bacteria that looked like it had been eaten by a neuroid cell. This suggests another function might be taking in information about the microbes in the environment and communicating that to the digestive cells,” Musser said.
There has been some debate within the scientific community about what behaviors early precursors to a nervous system might have evolved to control. One possibility was sensing the environment and moving around based on the sensed context. This study provides some evidence that digestion got first dibs on communication and laid the groundwork for more complex cell to cell communication later on in the evolutionary process.
“The most primitive form of communication would be diffuse release of signals that go everywhere, and other cells might happen to have the right receptor to receive the signal,” Arendt said. “Then you can imagine intermediate stages where cells want to communicate faster and evolve extensions that make the process more efficient by bringing them into closer proximity. This is the stage where sponges are and is the penultimate step before you make it even more direct with a synapse, bringing the pre and post synaptic process together.”
Modern sponges likely aren’t the same as they were when they first arrived on the scene hundreds of millions of years ago, but their current body plans and they way their cells interact can provide a model for what a pre-neural system might have looked like. It gives us a glimpse into our ancient history and the potential early architecture that eventually led to complex thought.
“We want to understand what the first animals looked like. How did they live? What types of cells did they have and how did that give rise to the incredible complexity we have?” Musser said.
You might be surprised to learn you have Bikini Bottom’s favorite resident to thank for everything modern animals, ourselves included, have been able to accomplish but… credit where credit is due.