The intertidal: the area of shoreline that is covered in water at high tide and uncovered at low tide. Think about that for a minute. What would you do if half the time, your watery home became a dry and often hot place? Let’s take a look at some specific adaptations that have allowed life to thrive even in these difficult conditions.
1. If you have legs, use them. In addition to taking refuge in the cool, damp shadows of rocks when the water recedes, crabs can also simply follow the water to a spot lower in the tidal zone. In addition, their thick outer covering, made up of calcium carbonate and chiton, helps prevent evaporation (as well as help protect them during disputes over whose crack in the rock is whose). The shell is not easy to maintain, however; in order to grow, crabs must take in enough water to expand and crack the old shell, and then carefully extract all of themselves, including legs, mouth, eyestalks, and digestive tract lining. Once molted, the crab should avoid disputes until the new shell has hardened. Once they’re protected again, they can go back to their life of eating all the things that don’t have the luxury of legs.
2. Shut the door. Bivalves such as mussels can close their shells tightly when things begin to dry up, suspending activity until the water returns. Mussels also tend to group together, attaching themselves to rocks and to each other in a cluster. In this way individual exposure to the sun is reduced, much as the cluster behavior of penguins in the Antarctic reduces individual exposure to cold. However, mussels can’t share the burden equally; once they’ve glued themselves in place, that’s where they’re staying (and staying put is extremely helpful for survival once the waves return).
3. Create your own sunscreen. Anemones cover themselves in little bits of shell and rocks to deflect the sun’s gaze and hold onto moisture. You may not see the similarities when they’re out of water, but catch a glimpse of an anemone underwater and you will see tentacles; anemones are actually closely related to jellyfish, and use the same mechanism to catch their prey. If you’ve ever poked an anemone, you may notice that the tentacles feel sticky –as if there’s a bit of glue holding your finger to the tentacle. In reality, the anemone has launched barb-like nematocysts into your flesh, and is hoping that you’ll become paralyzed and allow yourself to be swallowed whole. Luckily for us humans, our skin is too thick to be pierced by the barbs, unlike the fish and invertebrates who venture too close. Indeed, an enterprising anemone has apparently taken on a young cormorant – and won! Let’s hear it for the invertebrates!