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So, sponges are pretty chill, right? They’re just gentle filter-feeders that bask in the sunlight and absorb nutrients from the water around them.

Except, that’s not true of all sponges. And here’s something you might not have expected: Some of them are carnivores! Normally, a sponge absorbs nutrients with the help of its aquiferous system: an internal series of canals, chambers, and spaces that filter water and nutrients.

But sponges of the family Cladorhizidae have either reduced or entirely absent aquiferous systems. To get their energy, they’ve evolved a very different strategy: They catch and eat small animals. To do this, they’ve developed some pretty amazing adaptations.

For one, they’ve taken on strange, plant-like shapes. They often have stalks or branches reaching up off the seafloor, and from their bodies, long, thin filaments stretch out around them like fishing lines. Each of these filaments is then lined with tiny spines called spicules.

And that’s where stuff starts to get really cool. These spicules act like hook-and-loop fasteners to catch the little legs and hairs of tiny crustaceans or other animals — and they seem pretty effective. The prey captured by some of these sponges can be as long as eight millimeters.

Which isn’t bad for a sponge’s dinner, especially since many sponges aren’t huge themselves! Of course, catching the food is just the beginning. You still have to eat it somehow.

And when these sponges are ready for their meal, they make use of another amazing ability: migrating cells. Like all organisms, sponges’ bodies are made of cells. But sponges can do something pretty cool: Their cells can make trips across their bodies.

Some sponges may use this ability to help repair damaged body parts, and others use cell motion to slowly crawl across the seafloor. But carnivorous sponges use their migrating cells as mobile stomachs. Once prey is captured in their filaments, stem cells near the base of the sponge begin to multiply.

Then, they turn into digestive cells and swarm upward. Slowly but surely, they surround the prey and begin digesting it and absorbing its nutrients. Some of these cells even produce their own digestive enzymes, while others harbor enzyme-producing bacteria.

So basically, these animals make a stomach, and then that stomach travels upwards to claim its meal! For a large morsel, digestion can take eight to ten days. But afterward, the sponge is ready to feed again.

You might expect such an odd lifestyle to be extremely rare, but scientists have identified more than one hundred seventy-five species of carnivorous sponges! They think this might have happened because these animals live in nutrient-poor waters — like inside sea caves, or hundreds or thousands of meters below the surface. Which makes them crummy places for filter-feeders, so any sponges that live there have had to adapt and find new ways to get their food.

And apparently, along the way, they developed a taste for tiny critters. It just goes to show that even the calmest-looking things in nature can secretly be really hardcore. Thanks for watching this episode of SciShow, which is brought to you by our patrons on Patreon!

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