Solid evidence or fluid ideas on the importance of phase transitions to diapausing copepods

The recent Horizons article by Pond (Pond, 2012) hypothesizes that solid–liquid phase transitions in the lipids play a key role in the life history of zooplankton species, including the overwintering stages of calanoid copepods. Pond argues that positively buoyant surface-dwelling individuals about to enter diapause actively swim downwards where the increased pressure and lower temperature cause their lipid reserves to solidify. The resulting increase in the body density together with the change in the compressibility of the now solid lipid store would have an effect on the depth at which neutral buoyancy is achieved during the dormant overwintering period. Wax esters (WEs) are the main component of lipid stores of diapausing species of calanoids, and the percentage of polyunsaturated WEs affects the pressure and temperature at which it undergoes a liquid–solid phase transition.