A new perspective on the role of lipids in zooplankton is proposed, with solidliquid phase transitions of lipids being a factor regulating their buoyancy. These phase transitions are controlled by zooplankton in relation to their physical environment, through the selective accumulation of specific lipids with optimum levels of unsaturation. The necessity to control buoyancy and maintain an optimum depth is a fundamental evolutionary force, driving anatomical, biochemical and behavioural adaptations of all organisms within the aquatic realm. It is hypothesized that each species adjusts the amount, composition and anatomical location of lipids, to maximize fitness according to their preferred habitat and life history traits. Recent discoveries regarding the role of phase transitions of lipids in marine zooplankton and their role in regulating buoyancy will require re-interpretation of existing data and stimulate future scientific endeavours in zooplankton research.