SLURPING OYSTERS from their shells may be a rare indulgence for humans, but these bivalve molluscs and their relatives, such as clams and mussels, slurp for a living. Most are filter feeders, ingesting microscopic organisms as well as debris from their aquatic environments. A handful of mollusc species, however, have formed symbiotic partnerships with algae that live within their tissues. These individuals augment their diet with sugars that their microscopic partners produce through photosynthesis.