Red Flame Scallop, Lima scabra, from Florida and the Caribbean.  These are common throughout their range and seem to prefer inshore habitats where there is lots of sediment and organic matter in the water.  In an aquarium they will seek out a dark place, usually in a corner or under a rock and attach themselves there with basal threads that they secrete.  They are filter feeders and will need to be fed fine particle foods like those used for corals and gorgonia.  The tentacles are not poisonous and the scallops will not hurt any other inhabitants unless they swim into the scallop and it closes on them.