The brightly colored sea stars look like jewels scattered over the dark rocks. Dad comes up with a mystery to solve. “See if you can figure out how the stars are related to sea urchins, and sea cucumbers. Keep in mind they’re close cousins, all members of the group called Echinoderms.” He gives me a clue: “Echinoderms have brittle plates covered with a soft layer of skin. The plates can be large--check the skeleton of a sea urchin--or hardly visible at all in an animal like the sea cucumber.”
Sounds like I’ll need a little time to solve this puzzle. Dad says most of the sea stars we see are called ochre stars. They have five thick arms and come in orange, red, and purple, and many shades in between. “Remember that shape,” Dad says, “especially the number of rays. I’ve just given you another clue!” Dad has me run my finger lightly across an ochre star’s back. The skin is rough and spiny. On the underside are tiny legs, which Dad calls tube feet. Each one has a little suction cup at the end. “That’s how the star fastens itself to the rocks,” he says, “and also how it creeps around. It may be slow, but compared to some of these other critters, the star is an Olympic runner!”	Sunflower star & Ochre star
“Look at this one!” I yell. There’s a huge star moving through a nearby tide pool. It has more arms than I can count. “That’s a sunflower star,” says Dad. “Count the rays. They come in multiples of five, unless the star has lost an arm or two to some enemy. In that case, it will soon grow new ones.” He shows me how sea stars eat (it’s a little gross). [Photo below] We find one that’s folded itself around a mussel, and is using its tube feet to pry the shells apart. Eventually the star will inject its stomach right between the shells and digest the mussel’s soft parts. Yuck.
“See if you can find a sea urchin,” Dad says. “It looks like a pin cushion, with red, purple or green pins, depending on the species.” He says urchins like open coasts with lots of wave action, and we’re in just the right place. But they tend to camouflage themselves by pulling seaweeds, pebbles and bits of shell over their spines, so we have to search carefully. The shore is made of sandstone. Dad shows me where urchins have carved little round depressions to nestle down in when things get rough. “I found one!” I shout. Gently, Dad picks up the purple urchin and turns it over.
“See how short the spines are on the bottom side, compared to the top?” he says. “The urchin chisels the rock away with these short spines, and also with its teeth.” (Photo right) “Teeth?” I ask. “Are you kidding?” He laughs and points to a little hole in the center of the bottom spines. There they are: five tiny white teeth arranged in a circle! Dad says the urchin’s teeth, sometimes called Aristotle’s lantern, are also handy for scraping away at algae, its favorite food. “How do urchins move around?” I ask. “Well, look between the spines, and what do you see?” “Hey, tube feet! And, I see a star shape on the skeleton.” “Well,” says Dad, “you’re getting closer. I’ll bet you’ve figured out what the echinoderms have in common.” “Hmmm…there’s that star pattern. The sea stars, well most of them, have it. And it’s on the sea urchins too.” Aha! At home I have a sand dollar, which is related to the urchin. The shell has a star design on top. “OK, you’re doing fine.” Dad says. “Now, here’s a tough one: what about sea cucumbers?”
Dad finds a bright orange sea cucumber [photo left] in a tide pool, and I kneel down to examine it closely. Ten tentacles sway back and forth as the water moves. Dad explains that the main body is hidden in the rocks. “Well, there are ten tentacles, that’s two times five.” I answer, “So five is important to the cucumber too.” [see chart on right] According to Dad, the other clues are invisible. The hidden body has five rows of little tube feet down the side, holding the cucumber firmly in place. Also, if I could slice the cucumber like I’d cut up a cucumber from our garden, I’d see a star-shape on every slice, made by the muscle strips on the inside. So, with a little help from Dad, I’ve solved the mystery. Maybe I’ll be a biologist too someday!