By Liz Kimbrough Scores of tiny shrimplike crustaceans make a nightly pilgrimage from underwater caves out to feed in the open sea. Bellies full, they find their way back, homing in on their specific cave over vast distances. (Vast for a shrimp.) How these millions of miniscule mysids find their way back home has been largely a mystery — until now. Mysid shrimp (Hemimysis margalefi), nicknamed possum shrimp because the females carry their larvae in pouches, are about the size of an aspirin tablet and live in caves along the Mediterranean coast. Researchers found they use specialized receptors …