Cysts and eggs from marine zooplankton: dimension of the phenomenon, physiology of rest and ecological and biogeographic implications

Encystment is a common strategy adopted by coastal marine plankton species to avoid adverse conditions. Spiny cysts have been identified in marine sedi- ments of more than 600 millions years ago, thus suggesting that this has been an original trait of metazoan life cycles. Protista, Chromista, and Metazoa share this trait which is not the result of a convergent evolution, but a plesiomorphy, probably dictated by low oxygen concentrations. During the evolution of life on the Earth, the morphology has been modified, and today some taxa show typical and recognizable patterns. The affirmation of parental cares (brood protection) is suspected to have provoked the disappearing of spiny covering of eggs and/or cysts. Cysts produced in the pelagos sink to the bottom sediments where their dormancy may extend for decades. In polar seas, cysts are suspected to be entrapped within sea ice and released with ice melting to refuel a new plankton community in the complex frame of the so-called resurrection ecology. Confined marine coastal areas accumulate cysts in sediments due to many drivers as (i) reduced water movement/hydrodynam- ics, (ii) concentration of cyst-producing species with high population densities, and (iii) absence and/or scarcity of possible cyst consumers in the benthos. The pelagic- benthic nexus, which affects both the sediments and the water column (and possibly sea ice), is still poorly understood. The presence of cysts in the life cycle is likely to have considerable consequences for the ecology of coastal plankton as well as the evolution and biogeography of species.