On my last trip to the beach, I had on my "shopping" list: some eelgrass with roots, because the hermits love to climb it and to eat whatever grows on it, hydroids and bryozoans and the like; some fresh sea lettuce, if I could find it, because the crabs had eaten all of the last batch; a bit of kelp, to keep my plumose anemone healthy, and a length of kelp stipe, because both the crabs and the hermits eat it, all the more enthusiastically as it disintegrates; a small stone with barnacles for the snails to eat; some snails for the anemones to eat. The little underwater community demands variety!
In the latest offerings torn up by the tide from the sea floor, I found the kelp and a good handful of eelgrass, and at the last minute, a nice blade of fresh sea lettuce. It all went in my bag. There were no barnacles this time; the carnivorous snails will have to make do with mussels for now.
Washing off the sea lettuce at home, I discovered that it was hiding a tiny sea urchin; lucky for it that I collected that piece; otherwise, tossed up by the tide and abandoned, the urchin would have died.
It's cruising around the upper levels of the wall of the aquarium, eating algae. And showing off its five-pointed star mouth, chomping away.
|Mouth side of the urchin. About 1 cm. across the whole animal. (It's a baby.)|
The arrangement of five teeth is called "Aristotle's lantern", not because they look like a light, but because Aristotle first described them. They open and shut, scraping at the algae; sometimes they can be used to scrape a hole for the urchin, even in rock.
|Zooming in. The suckers on the tube feet are like little cog wheels.|