A tide pool is one of the few places where an animal must survive as both a sea creature and, twice a day, something closer to a land creature stranded in a puddle. Water temperature can swing by twenty degrees between high and low tide, oxygen grows scarce as trapped water warms, and a hot afternoon can raise salinity as water evaporates faster than it drains away. The animals that thrive there, mussels, barnacles, certain small crabs, are not necessarily the strongest competitors in the open ocean; they are simply the ones able to tolerate extremes that would kill a more sensitive species within hours.