Bivalves are molluscs with two shells, such as oysters, mussels and clams. As adults, these are all sessile, unable to move far from their anchor point on rocks and other surfaces. But movement is necessary to avoid overcrowding and food shortages – these animals have mobile larval and juvenile stages which are pelagic (move through the water column) in order to disperse through their aquatic habitats. Here, a juvenile Lima clam has visible shells developing.

Image courtesy of Greg Rouse and the Nikon Small World competition.