Sponges, belonging to the phylum Porifera, represent one of the simplest forms of multicellular animal life on Earth. These aquatic organisms, which have thrived for over 600 million years, notably lack a centralized brain or any form of a true nervous system. They do not possess nerve cells (neurons) or organized tissues and organs. This fundamental absence raises questions about how they manage to survive and carry out their life processes.
Functioning Without a Brain
Without a brain or nervous system, sponges accomplish their life functions through the coordinated actions of individual, specialized cells. Their functions are regulated at a local, cellular level. Sponges are primarily filter-feeders, drawing water through countless tiny pores (ostia) into their bodies.
Water currents are created by collar cells, known as choanocytes, which possess whip-like flagella. These flagella beat continuously, pushing water into internal chambers and canals, allowing the sponge to filter large volumes of water. Food particles, such as bacteria, microorganisms, and organic debris, are trapped by the choanocytes and then engulfed and digested intracellularly.
Sponges also exhibit simple reactions to external stimuli, demonstrating a form of communication at the cellular level. For example, they can close their pores (ostia) or contract their bodies in response to physical touch, changes in water currents, or irritating chemical substances. These responses are localized cellular reactions, often involving cells like myocytes, which enable shape changes and water channel regulation.
The Evolutionary Blueprint for Nerves
Sponges occupy a unique position on the animal family tree, representing one of the earliest branches in animal evolution. While they do not possess neurons or synapses, their genomes contain many genetic components found in the nervous systems of more complex animals, including those associated with neural cells. These “proto-neural” genes suggest that the genetic toolkit for building nervous systems existed in ancient ancestors even before true neurons evolved. For example, the sponge Amphimedon queenslandica expresses genes like Notch-Delta signaling and a proneural basic helix-loop-helix (bHLH) gene, which are involved in neurogenesis in other animals. The study of sponges provides insights into how complex systems, such as brains and nervous systems, may have first emerged in other animal groups by recycling these ancient genetic building blocks.