This is fascinating. Sand dollars are actually very flat sea urchins (weird, eh?). The rough “hairs” are actually miniature spines (sea urchin spines), and they have little tube feet they use to walk around and keep from getting buried. The hole in the center is a very powerful mouth, and the five little “doves” that fall out of broken sand dollars are teeth. The five gills make up the five-petaled “flower” on the top of a sand dollar.
Most sand dollars live about 30-40 feet under the ocean, in the subtidal region (so you have to dive to see them). They’re quite abundant at that depth, though. It’s quite unusual for them to be so shallow (here they’re only 2-3 feet underwater), but there isn’t much tidal variation here, and the slopes are long and gradual, so they get water all the time. They either feed by moving around on their little tube feet and eating whatever’s underneath them, or by sticking themselves vertically in the sand so water washes through them. The little “grooves” in live sand dollars are actually feeding areas, where food gets trapped in mucus and moved along to the mouth by little cilia.
Now I have to go find another sand dollar and take a closer look at it. I’d love to dissect one, but that would be kinda rude. (I might do it anyway, but curiosity isn’t a very good excuse for taking a critter apart, IMO. If it isn’t hurting me and I don’t want to eat it, I’d rather live and let live.)
However, I think someone brought a live (now dead) sand dollar to the inn yesterday–I may ask them if I can dissect it, since it’s there anyway.
At any rate, I’m sure that’s way more than you ever wanted to know about sand dollars, but dang, they’re cool.
Today the waters are a bit rough, so I’m going out on a river tour in an hour or two. I’m torn about what to do over the next few days–there’s snorkeling/fishing, but there’s also an Inca ruin and a very beautiful lagoon (the “Blue Hole”) that would be another good day trip. Either would be $150 for the day, which is a big chunk of change, not sure I can really afford that and the river trip. I may ask the guide if he can bring it down a little for the two-day package. Or I may just soak up the cost; I’m only going to be in Belize once, and I fly home (actually, to Guatemala) on Friday.
If I can find someone else to go along, of course, it’ll be cheaper. Even one other person would bring the cost down a lot.
Anyway, I’m off to prep for the river trip…