(The photos above I took diving in Belize in June 2005.)
Sponges are really simple animals. They don’t have nervous systems. AT ALL. Well, if you’re like me, you’re wondering how the hell that works. Granted, pretty much all sponges do is sit on the ocean floor and filter stuff out of the water. However, some species are capable of movement (at prodigious speeds of 1-4 mm per day), and most can close their ostia and oscula (or, the water intake pores in the body and the central opening of the body where water exits after being filtered by the sponge, respectively).
Hold the phone—animals with no nerves and no muscles can move? Apparently. It seems that movement in most sponges occurs via continuous movement of cells within the sponge, which rearrange its anatomy through space. Weird.
Sponges do have everything that would theoretically be needed for a nervous system: synapses, pathfinding mechanisms, excitation…. (Source: PZ’s neuro lecture on 9/19/11)
Exaptation: stuff that evolved for one purpose is co-opted for another purpose. A common example is feathers, which may have originally evolved for thermal regulation but were then used for display and for flight. Exaptation is an important idea used to help explain how complex biological structures and systems came to be. Sponges are pretty old, probably having evolved in the early to mid Cambrian period. The phylogenetic descendants of sponges are ctenophores (comb jellies) and cnidarians (corals, anemones, etc), which do have sensory organs and primitive nervous systems. They didn’t magically create their nervous systems whole-cloth, though; instead, the basic features of nervous systems that were already present in the sponges were adapted to new purposes.
Sorry, not a very exiting post this week. I’m not feeling 100%, and I was thinking about exaptation from a biology senior seminar I went to this week on infrared-sensing organs in snakes. Stay tuned though, next week should be better, although I haven’t the faintest what I’m going to write about.