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Sponges were one of the very first animals, having evolved between 760 million and 485 million years ago. They’re so simple they don’t seem like animals at all. They basically have an outer and an inner skin with a jelly-like substance in between. They have no brains, muscles, tissues, or any internal organs at all. They just sit there and absorb whatever microscopic food enters their pores. They seem more like plants than animals, except when they’re in their larval stage and are actively swimming around before they settle down to a more sedentary life. They have a simple, quiet existence...or so it would seem. Things aren’t that simple.
Sponges are in the gray area between colonial animals and individuals. They do have specialized cells with different functions, but like stem cells, they can change function as needed. Transmogrify is the word for it. Glass sponges are strange in that they start off with many cells, but soon most of them fuse into a single cell bag with many nuclei. They still have a number of other specialized cells, but most of their body is a single cell.
Like corals and anemones, if a fragment breaks off a sponge, it will grow into a new individual. Also like corals and anemones, their cells recognize other cells that belong to them. If you take two sponges and push them through a strainer turning them to mush, mix it up, and they will collect back together into the two original sponges, although the cells with be in different places taking on new duties. In other words, they have self-recognition.
Sponges create their own water currents that run through their bodies. Their outside is riddled with small intake pores, which are the entrances to networks of passages, like a sponge you would use for cleaning. These are lined with waving whip-like flagella that keep the water moving through and out the exit holes, so there’s a nearly constant stream of water flowing into and out of them. Some sponges have a number of exhaust holes that are much larger than the entrance pores, or they have a central chamber with a large opening at the top where the water exits. Some sponges can open and close these large holes.
Bits of organic material and plankton get stuck to mucus lining the tube walls and are passed from cell to cell, whereupon food is absorbed where it’s needed. The waste is carried out to the exiting current. They also convert oxygen into carbon dioxide, just like we do when we breathe. They also process nutrients, like nitrogen, phosphorous, and carbon, making them available to other nearby animals and algae. And they filter the water, improving its quality. It’s very beneficial to have sponges on a reef.
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| Here are some unusual deep-sea sponges. The one on the top right is the largest sponge found so far. Those on the lower row are carnivorous sponges that adopted this alternative diet because of the shortage of food in their environment. They use Velcro-like hooks to snare copepods and crustaceans. Other deep-sea sponges resemble parasols, lollypops, and feathers. All NOAA, except for bottom second from left: Jean Vacelet, CC BY 2.5. |
They take on such a wide variety of shapes and colors that it can be difficult to recognize that they’re sponges. Some look like something melted over a rock, others look like puffballs, some like moss, and others like delicate vases. There are barrels, baskets, and pillars with holes down their centers. Some look like corals and some like ferns.
Much of this has to do with their environment. A melted-looking coral in a wave zone might grow upright and tall in calm water, or branch out like bush. You can’t always identify them by how they look.
The largest one found so far is about the size of a minivan and it’s thought to be hundreds of years old. We can’t tell for sure since there’s no way to measure its age. They don’t have growth rings like coral, so scientists have to use other methods to estimate their ages. Some smaller sponges are thought to be more than 2,300 years old.
What I find really interesting about sponges is that they can sneeze. Here are very simple animals with no muscles or nervous system, yet they actually sneeze, although it takes them a while to do it. Most of their time is spent sucking in water through their pores and then pushing it out through a larger hole or holes, but not all of the unwanted material is carried out on the current, so the sponge sneezes out accumulating sediment, waste, and mucus—sponge snot—but a single sneeze can take up to forty-five minutes. Scientists also use certain chemicals to make the sneeze and it looks pretty impressive in fast-motion videos.
So how does a brainless sponge know that it needs to sneeze? So far no one knows. We do know they can sense things with their cilia—the same hair-like structures we use for smell and hearing—and they do use electrical signals to communicate between their cells, but they have no nerves. Because inedible debris can get trapped in the mucus linings of their intake tubes, they have to get rid of it to prevent their tubes from getting clogged. They do this by reversing their current.
They suck water into their main chamber and then with multiple contractions, they force the water through the small intake tubes and out of their pores. These sorts of puffing movements push the mucus out onto their slimy thin outer skin, where it falls off or is picked off by fish and creatures who like to eat sponge snot.
Sponges can attach themselves to hard surfaces or send out structures to hold them in sand or mud, but this doesn’t stop some from crawling away. When looking at a video of deep-sea Arctic sponges, a group of German, American, and Norwegian researchers noticed they left trails in the seafloor sediment, some many yards or meters long. They’re able to change directions and even crawl uphill. They appeared to be extending forward needle-like structures that make up their skeleton from their bottom front surface, attaching them to the seabed, and then retracting them, dragging themselves forward while leaving behind a trail of broken bits of their anchored skeleton. They don’t seem to move very fast—perhaps only a few inches or centimeters a year—but that’s still quite impressive for animals we thought were permanently anchored to a particular spot.
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