Sponges are the oldest animals on our planet, yet they thrive without brains. A sponge silently stuck to the reef blends perfectly into its environment, so that divers often pass it without a second glance. And yet, beneath that still surface, a sponge is astonishingly busy.
Barrel sponges (Albert Kok/public domain)
Alive in ways we are only beginning to understand, it is pumping, filtering, sensing, repairing, coordinating.
A sponge survives by moving water. Through thousands or even millions of tiny pores, it draws seawater into its body. Specialised cells beat microscopic hairs that create a continuous current, bringing oxygen and food particles inside. A sponge the size of a football can filter thousands of litres of seawater in a single day.
Entire reefs depend on this invisible labour. Sponges clarify water, recycle nutrients, and support microbial communities that feed other animals. Some scientists call them the “kidneys of the reef.”
Yet herein lies the mystery. How does a creature with no organs coordinate such a complex flow?
Communication without nerves
Strangely, sponges have no nervous system. But though they lack both nerves and a brain, they respond to their environment. If sediment clogs their pores, they can slow or stop pumping. If damaged, they reorganise their cells and rebuild their structures. Some species even produce coordinated contractions as if deciding to pause and reset. But how?
Researchers have discovered that sponge cells communicate through chemical signals and electrical-like waves passing slowly across their tissues. It is not a nervous system, but it is not random either.
It suggests something extraordinary--that complex coordination evolved before the development of the centralised nervous system, in which a brain receives messages and coordinates actions throughout the body. Thus, sponges may hold clues to the deep time origins of animal behaviour, including communication.
Ancient beyond imagination
Sponges are among the oldest animal lineages on Earth. Their ancestors date back more than 600 million years, to the time before fish existed, and before the first primitive, moss-like plants began to colonise the earth. Since then, they have quietly persisted, surviving each mass extinction that erased so many other creatures.
What was it that made them so resilient?
Perhaps it was their simplicity. With no centralised brain, no fragile organs, and a body that can reorganise itself, a sponge is almost impossible to kill. Even if broken apart, its cells can at times reassemble into a new individual.
Sponges blur the line between individual and colony, and between animal and ecosystem.
Cities within their bodies
For when one looks closer, a sponge becomes more than just one animal. It is a community.
Inside its tissues live dense populations of bacteria and microbes. In some species, microbial partners make up nearly half of the sponge’s mass. These microbes help process nutrients, produce chemical defences, and may even communicate chemically with their host.
Its community is layered with partnerships that science is still decoding.
The mystery in plain sight
To dive among sponges is to drift through some of the most ancient architecture on Earth. Barrel sponges are like giant urns, while delicate lace sponges filter the sunlight. Other, bright, encrusting forms paint the reef in red, orange and violet. They do not chase prey or perform dazzling displays. Their mystery is quieter.
Their silent presence in our world sparks many questions. How does life coordinate without a brain? How did early animals organise themselves before the evolution of nerves? What ancient secrets of our own evolutionary past are written in their soft, porous bodies?
Sea sponges remind us that intelligence, communication, and survival do not always look the way we expect. Sometimes, the greatest mysteries are the ones we swim past without noticing.
Ethologist Ila France Porcher, author of The Shark Sessions, The True Nature of Sharks, and five other books on wildlife behaviour, conducted a seven-year study of reef sharks in Tahiti, resulting in several scientific papers. Her decades of first-hand observations of wildlife— from sharks to bears to birds— focus on the individuality and intelligence of wild individuals, challenging traditional views of animal minds. Her work has been featured on Shark Week, in scientific discussions, conservation debates, and international media for its unique blend of field observation, art, and science.
