Giant Pacific Octopus by Krista Anandakuttan copyright 2008, ink sketch painted in digital media
Why 8 tentacles? Exactly.
Sometimes a question is just really good. Recently while giving a demonstration of science illustration techniques in a natural history museum, a visitor asked me this question. I quickly reassured the asker that no, this was not a case of spider evolution. In fact I quickly drummed up everything I knew about extant cephalopods and even recalled the body plan of the hypothetical ancestral mollusk… yet slug-like ancestry, shell evolution, the appearance of tentacles, and loss of shells; mental images of nautilus, cuttlefish, squid and octopus– nothing yielded the answer.
So it’s a good question, and I looked it up. Embryology… 18 tentacle buds…fusion in development… 10 tentacles vs 8…. And finally, the sentence: “…ten-armed squids, and eight-armed octopus. What happened? We don’t know… Obviously, what we need is more octopus embryology!”
I did learn one tangential thing, and that is the distinction between tentacles and arms: tentacles have no suckers or suckers only near the ends, while arms have suckers throughout.
So I could deflect the question and look smart (and annoying) next time:
“Why do octopuses have 8 tentacles?”
Me: “They don’t have tentacles they have arms.”
Here’s a link to that article:
http://scienceblogs.com/pharyngula/2007/07/30/cephalopod-development-and-evo/
and the Art of Nature science illustration exhibit, at the Santa Cruz Museum of Natural History