In our Study of the Day feature series, we highlight a research publication related to a John Templeton Foundation-supported project, connecting the fascinating and unique research we fund to important conversations happening around the world.
When scientists study the mysteries and mechanisms of thought, they have tended to take a top-down approach, beginning with the human brain and proceeding to simpler, less-neurologically-endowed creatures. That method has its advantages but may lead to a neurocentric view that can struggle to account for things that look like cognition in things like plants or simple animals that lack nerves.
Philosophers William Bechtel of the University of California, San Diego and Leonardo Bich of the University of the Basque Country argued recently for the value of a bottom-up approach, centered around something that every organism has in common: the need to eat, that is, to extract useful energy and matter from its environment. In reviewing the latest scientific research about two types of marine life, sponges and Trichoplax, Bechtel and Bich suggest that we can learn a lot about cognition at its most universal level from the ways the simplest multicellular animals engage in complex chemical signaling to procure, process and evaluate information in order to choose when and what to eat.
The Sneezing Sponge
The diverse members of the phylum Porifera vary in size from the three-millimeter L. blanca found off the coast of Sweden to a minivan-sized specimen spotted in deep water near Hawaii in 2016. Sponges are filter feeders, relying on a series of structural aquifers to draw nutrient-containing water towards chambers lined with specialized cells which whip their flagella to push food particles to the base of the cell to be absorbed and shared with other cells. Sponges can engage in coordinated movements, ranging from twitches and ripples to all-out sneezes in order to regulate the flow of nutrient-rich water, or even reverse it to expel non-food particles or toxins (in the lab, scientists have found that nicotine can make sponges sneeze). These large-scale movements involve waves of complex chemical signaling between the cells that detect a mechanical or chemical stimulus, and the ones that tense or relax to create the necessary movement. “Coordinating this behavior,” Bechtel and Bich write, “requires that different cells in the sponge acquire information and use it effectively to determine their behaviors. That is, the sponge must engage in cognition.”
The Wandering Gut
While sponges are content to draw food unto themselves, Trichoplax are masters of external foraging. Small flat discs about a millimeter wide and just three cell layers thick, Trichoplax are found in the tidal zones of most of the world’s oceans. They adhere to surfaces and move around, pausing when they are on top of food sources like algae to release digestive enzymes from their undersides and then soak up the digested products, a behavior that has earned them the nickname “the wandering gut.”
There are few multicellular animals simpler than Trichoplax, but even a wandering gut has to decide when and where it’s worth releasing their enzymes. Despite having no central control system, sensor cells guide Trichoplax’s movement along the detected chemical gradients towards food. Researchers have discovered an ever-expanding list of chemicals that Trichoplax responds to by initiating or ceasing feeding behavior, or causing its simple body to crinkle, fold or rotate — all thanks to what one study describes as “the richest repertoire of amino acid receptors among all analyzed animals, from sponges to humans.”
The Chemical Spark
In creatures with nervous systems, it’s easy to think of electricity as the medium of stimulus, cognition and response. But electricity is only paramount within the neuron — with a few exceptions, communication between neurons happens solely via chemical neurotransmitters. Cognition, Bechtel and Bich write, “is first and foremost a chemical phenomenon, common to all living organisms regardless of the presence of a nervous system.” Gathering, evaluating and acting on inputs doesn’t just happen in centralized communication structures like brains, but in individual cells throughout the organism as they communicate chemically with other cells. This is manifestly true for creatures like Trichoplax and sponges, but is also an accurate description of what happens in more complex creatures. Cognition, in some form, is common to every living thing, because every living thing has to figure out what’s for dinner.
Still Curious?
Read “Eating and Cognition in Two Animals without Neurons: Sponges and Trichoplax”
