Low Tide at Gull Ledge
1Every April, Ms. Noelani Kealoha takes her seventh graders from Seabright Middle School on a field trip to the Gull Ledge Marine Lab. At low tide, the class explores the rocky shore with the lab's biologist, Dr. Tomás Arriaga, who has studied the same stretch of shore for years. He lets the students compare their counts with the lab's records.
2Unlike most crabs, a hermit crab has a soft, curled abdomen with no hard covering. It protects its abdomen by living in an empty snail shell, and it moves into a bigger shell as it grows. Hermit crabs cannot make shells, so new empty shells appear only when snails die. A crab in a shell that is too small cannot pull its whole body inside, which makes it easier for gulls and fish to grab. The class counted crabs and empty shells in three pools and watched each pool for 10 minutes, counting shell fights: times when one crab grabbed another crab's shell and tried to pull it out (Table 1).
3Back at the lab, Dr. Arriaga showed the class a tank test. Two tanks each held 12 hermit crabs collected from the South pool, with the same water, food and temperature. Tank A got no extra shells. Tank B got 12 extra empty snail shells of many sizes. After 3 days, the class watched each tank for 10 minutes (Table 2).
4Ochre sea stars live on the same shore. A sea star wraps its arms around a mussel, pulls the shell open a little and digests the soft body. Mussels filter tiny bits of food from the water. Mussels, barnacles and many seaweeds all need a spot of bare rock to attach to. In 2021 the lab marked off two areas of rock, each 10 m by 10 m, with about 6 sea stars in each. In Area A, nothing was changed. In Area B, workers moved every sea star they found to a distant part of the shore each month. Each year, they measured how much of the rock mussels covered and counted the kinds of seaweeds and attached animals (Table 3).
5In the sunny pools, green anemones get much of their green color from tiny single-celled algae that live inside the cells of their tentacles. Inside the anemone, the algae are sheltered from grazing snails and take up nitrogen from the anemone's wastes. In light, the algae make sugar by photosynthesis. To test whether the algae help the anemones, the lab kept anemones without feeding them for 6 weeks: one tank had light for 12 hours a day, and the other was kept dark at the same temperature (Table 4).