Small Swimmers at Kelp Point
1At low tide in May, Mr. Benedikt Oyelaran's eighth-grade marine biology class explored the rocky tide pools at Kelp Point on the Pacific coast. Marisol Takahashi and her group counted ochre sea stars clinging to the rocks, purple sea urchins tucked into holes, periwinkle snails, a bright orange sea slug, shore crabs hiding under stones, a few small shrimp and a tidepool sculpin, a small fish that rests on the bottom. The students looked and photographed, then left every animal where they found it.
2In the afternoon the class visited the marine lab next to the beach. Dr. Ansel Brightwater showed them plankton, the tiny living things that drift in seawater, under a microscope. Many tide pool animals start life as larvae, small swimming young that often look nothing like their parents. The lab also raises embryos of a few local animals from eggs so that students can watch them develop.
3Marisol was surprised by the sea star larva. An adult sea star has five arms arranged around a center, but its larva has a left side and a right side that mirror each other, and it swims with bands of tiny beating hairs. The sea slug larva swam inside a tiny coiled shell, much like the larva of the periwinkle snail. The periwinkle keeps and enlarges its shell, but the sea slug sheds its shell when it settles on a rock. The young crab had a long, jointed tail held out behind its body. In an adult crab, the tail is short and folded tightly under the body.
4Dr. Brightwater then described an early embryo stage. A few days after an egg begins to divide, the ball of cells forms a dent, which becomes the first opening into the gut. In some animals this first opening becomes the mouth. In others it becomes the anus, and the mouth forms later at the other end. The lab has checked this for five of the animals in the table. Fish, humans and other animals with backbones form the anus first.
5"Adults are shaped by the way they live," Dr. Brightwater said. "A sea star creeps over rocks, and a crab scuttles sideways. Embryos and larvae change less in some ways, so they can keep clues to who is related to whom."