When a water volume is available and larger than the organisms living inside it, the conditions allowing a pelagic lifestyle are present. This means that although lagoons, semi-enclosed seas, and underwater caves are environments that are extremely coastal (and sometimes transitional between sea and land) and commonly not considered part of the pelagos dominion, they can be “pelagos” for swimming organisms of the smallest size. Generally, the body size determines the pelagic lifestyle, and what is a shallow lagoon for humans can be an “ocean” for Bacteria or unicellular plankton. The pelagic lifestyle of such organisms and the water chemical-physical parameters are however, affected by the vicinity of the benthic world, and possibly the same organisms live the two benthic and pelagic lifestyles. A semi-enclosed coastal habitat not widely communicating with the sea was given the name Paralic System. Such a system is characterized by an evident (and sometimes intense) gradient of some environmental characteristics (e.g., salinity). Pelagic organisms are consequently adapted to single situations (specialists of one area or one season) or indifferent to such variations (generalists). In most cases, they show the possibility of overcoming adverse periods, which periodically occur in confined environments, as resting forms that sink to the bottom and wait for the return of favourable conditions.