How an Ancient Animal Coordinates Without Nerves
Despite having no neurons, muscles, or brain, the humble sponge can orchestrate complex whole-body contractions. New research reveals this ancient creature holds secrets about the origins of animal coordination.
You might not give the sea sponge a second thought—a simple, porous creature fixed to the ocean floor, more often seen in bathrooms than in biology labs. Yet, this ancient animal possesses a remarkable ability. Lacking any semblance of a nervous system, a brain, or even muscles, it can still perform coordinated, whole-body contractions that function much like a sneeze, expelling waste and debris from its internal canals.
For decades, scientists have been puzzled by how the sponge achieves this feat. New research into its physiology is now unraveling the mystery, revealing a complex communication system that may represent an evolutionary precursor to the nervous systems found in all more complex animals, including humans 5 .
Sponges, belonging to the phylum Porifera (Latin for "pore-bearer"), have a body plan that is deceptively simple. It consists of just two thin layers of cells sandwiching a jelly-like substance called the mesohyl. They have no digestive, circulatory, or conventional nervous systems 3 .
How do cells communicate across the body without the dedicated wiring of neurons?
For a sponge to expel an irritant, its countless independent cells must act in unison. This requires a way to send a "contract now!" signal across its entire body. Researchers have discovered that sponges use a chemical toolkit that is surprisingly familiar to neurobiologists.
One of the most abundant neurotransmitters in the human brain. In sponges, it serves as the primary trigger for contractions 7 .
The universal energy currency of cells. In animals with nervous systems, ATP is often co-released with neurotransmitters to modulate neural signals. In sponges, it plays a crucial role in propagating the contraction signal 7 .
This discovery is revolutionary. It suggests that the fundamental molecular machinery for cell-to-cell communication was present in the earliest animals, long before the evolution of true neurons.
Cutting-edge genetic research has added another layer to the story. A recent study creating an atlas of gene expression in the freshwater sponge Spongilla revealed specialized cells that had never been described before 8 .
These mobile "neuroid" cells patrol the sponge's digestive chambers.
They express a suite of genes that, in more complex animals, are active in the part of a neuron responsible for sending chemical messages (the presynaptic bulb) 8 .
The choanocyte cells that line the chambers express genes for the receiving end of that signal (the postsynaptic scaffolding) 8 .
High-resolution imaging shows these neuroid cells reaching out with arm-like projections to touch the choanocytes, likely releasing glutamate to regulate the sponge's feeding current 8 .
This reveals a deep evolutionary connection between sponge physiology and our own neurons.
To understand how glutamate and ATP work together, a team of scientists conducted an elegant series of pharmacological experiments on the transparent freshwater sponge Ephydatia muelleri 7 .
The experiment yielded clear results. Glutamate, as expected, triggered a full contraction. But the role of ATP was more nuanced. ATP itself caused an expansion of the excurrent canals, while its breakdown product, ADP, triggered the complete contraction. When ATP was broken down by apyrase, contractions also occurred. Most importantly, when the ATP receptors were blocked by PPADS, sponges could no longer contract in response to glutamate 7 .
| Chemical Applied | Observed Effect on Sponge | Scientific Interpretation |
|---|---|---|
| L-glutamate | Triggers whole-body contraction 7 | Acts as the primary alarm signal, initiating the coordination cascade. |
| ATP | Causes expansion of excurrent canals 7 | May prime the system or play a role in the recovery phase after a "sneeze." |
| ADP | Triggers whole-body contraction 7 | The active metabolite for propagating the contraction signal after ATP is released. |
| GABA | Prevents contractions 7 | Acts as a "brake" on the system, counteracting the effects of glutamate. |
| PPADS (blocks P2X receptors) | Prevents glutamate- and ATP-triggered contractions 7 | Proves that ATP receptor activation is essential downstream of the initial glutamate signal. |
The coordination of a sponge's contraction relies on a paracrine signaling system where glutamate acts as the initial trigger, prompting the release of ATP, which is then rapidly converted to ADP to propagate the "contract" signal across the body 7 . This two-molecule system allows for a wave of communication without a single neuron being involved.
The discovery of a sophisticated signaling system in sponges challenges our ingrained, hierarchical view of evolution. Sponges are not failed attempts at complexity; they are supremely successful modern animals whose biology is perfectly adapted to their filter-feeding lifestyle 5 .
| Feature | Sponges (Porifera) | Ctenophores (Comb Jellies) | Bilaterians (e.g., Humans, Tigers) |
|---|---|---|---|
| Nervous System | None. Uses a paracrine chemical signaling system 7 . | Present, but uses a different genetic blueprint than other animals 5 . | Complex, centralized nervous system with a brain. |
| Key Signaling Molecules | Glutamate, ATP, Nitric Oxide 7 8 | A unique set of neurotransmitters and peptides 5 . | Glutamate, ATP, and a wide array of other neurotransmitters. |
| Muscles | None. Contractions are conducted by pinacocyte cells 8 . | Present. | Present. |
| Evolutionary Insight | Represents a potential precursor state to neuronal signaling. | May have evolved nerves independently, suggesting multiple paths to complexity 5 . | Represents one lineage's path toward complex coordination. |
Studying sponge physiology requires specific tools to culture these delicate animals and probe their biological functions.
The belief in an orderly progression from simple to complex is "complete rubbish" 5 .
The sponge, once dismissed as a mere rock with slime, proves that the story of animal evolution is far richer and more surprising than we ever imagined. By studying these ancient creatures, we don't just learn about the origin of our own biology—we discover radically different ways of being an animal 8 .