 Most of the bivalve larvae that hatch from eggs in the water column feed on diatoms or other phytoplankton . In temperate regions , about 25 % of species are lecithotrophic , depending on nutrients stored in the yolk of the egg where the main energy source is lipids . The longer the period is before the larva first feeds , the larger the egg and yolk need to be . The reproductive cost of producing these energy @-@ rich eggs is high and they are usually smaller in number . For example , the Baltic tellin ( Macoma balthica ) produces few , high @-@ energy eggs . The larvae hatching out of these rely on the energy reserves and do not feed . After about four days , they become D @-@ stage larvae , when they first develop hinged , D @-@ shaped valves . These larvae have a relatively small dispersal potential before settling out . The common mussel ( Mytilus edulis ) produces 10 times as many eggs that hatch into larvae and soon need to feed to survive and grow . They can disperse more widely as they remain planktonic for a much longer time . 
