The basic idea behind the theory of evolution behind natural selection is this: if you get eaten or otherwise die before you’ve had the chance to reproduce then your genes, or traits, don’t get passed on. It’s a dead end. But if you survive… some of your traits get passed down to a new generation. Maybe you were just a bit faster than the other wildebeest. Maybe your coloration gave you better camouflage than some of the rest. Who knows? It could be a ton of things. The point is, over time, these little changes in the proportion of traits will add up to big changes, maybe even entirely new species!

There’s a misconception out there though that’s related to this idea called Survival of the Fittest. That evolution works through the survival of the strongest, fastest, toughest, meanest of the bunch and that things get stronger, faster, tougher, and meaner over time. The thing is, that’s just not true. Problematic associations aside, that idea doesn’t explain the persistence of seemingly useless traits, or the loss of seemingly useful traits, or traits that simply don’t change for millions of years at a time. Sometimes, things persist simply because there’s no pressure for them not to. Sometimes, things just work.

In the case of sponges, there’s the Demo Model.

Between 75% and 90% of all sponges, nearly 8000 species by some estimates, belong to the Demosponges–the most diverse class within the entire phylum.

Unsurprisingly, this diversity in appearance and color is matched by their diversity of range and habitat. Most are marine, but one order, the Spongillida, lives in freshwater. You might spot some walking along a beach or diving in the intertidal zone. Deep sea submersibles have recorded them in the dark abyss of the deep sea. From the tropics to the poles, Demosponges have laid root across the planet.

The diversity of the demos is hard to justly express. Their asymmetrical bodies range from just a few millimeters to an excess of 6 feet across. In fact, all of the largest sponges are demos. They can form thin lichen-like layers of crust over vast surfaces, finger-like protrusions, or vase shapes. Pigment granules within their amoebocytes are to thank for their often Seussian coloration. Sunflower yellows, Poppy reds, fluorescent greens, oranges rivaling those of the flame bowerbird, and purples that would make wines look pale in comparison.

Their soft bodies cover hard skeletons made of one to four rayed spicules made of silica, spongin, or both, shaped most commonly according to the leuconoid structure.

Many demosponges, like their glass cousins, are incredibly long-lived, living lives of 500 to 1000 years. Their stationary nature in combination with their lengthy lifespans and dense, layered skeletons like the rings of a tree makes them the ideal chronological climatological cartographers, mapping and storing data about our planet’s past. 

This is something we discussed more in-depth in the last episode in connection to glass sponges

Sponges may be simple organisms, but if there is anything they embody, it’s the spirit of persistence.

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