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Most animals have complex bodies, with electrical signals constantly shooting around and carrying messages to and from the brain. This allows our brains to act as the command center for each of our trillions of cells. But on the other end of the complexity spectrum, there’s the sponge - one of the simplest animals in existence. They lack a brain and a nervous system, and instead just sway in the current and eat whatever food flows through them.
However, these simple animals may have evolved their own unique way of transmitting information through their bodies - using light. Sponges were some of the first animals ever, with the oldest ones living around 800 million years ago. They have evolved into more than 8,000 different species, but their general body plan has stayed simple. Unlike most animals, they don’t have any organs or even tissues, and all this is organized without a brain.
The sponge’s genes simply give each cell its marching orders, and the cells carry out their specific roles. But even though sponges don’t have a nervous system, they act like animals that do. For instance, they respond to light in their environment. Some sponges will contract or their metabolism will change in response to different light levels.
In the early 2000s, researchers found a clue - a deep-sea sponge called the Venus’ flower basket had fibers called spicules that were strangely similar to fiber-optic cables. Then, about a decade later, scientists found a sponge species that creates its own light. The cells creating the light were clustered around spicules, suggesting the sponge was creating flashes of light and channeling it through its spicules. The team discovered that sponges have special cells with proteins called cryptochromes that get activated by light. Researchers think that light hitting a cryptochrome produces hormone-like molecules, and these act as chemical messengers. These molecules move through the body much more slowly than nerve signals, but like nerve signals, they can also trigger certain behaviors in cells. This suggests that sponges might be using a combination of bioluminescent cells, glass fibers, and cryptochromes as a unique kind of sensory system, not yet observed anywhere else in nature.
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