Beyond Salinity Tolerance: Ecology and Evolution of Estuarine Occupancy in Cephalopoda

Cephalopods are predominantly marine organisms and are generally considered physiologically constrained by reduced salinity. Consequently, estuarine occurrence has traditionally been interpreted primarily through the lens of salinity tolerance, with comparatively little attention given to its ecological and evolutionary significance. Here, we synthesize evidence of estuarine occupancy across Cephalopoda and evaluate how physiological tolerance, behavior and ecological opportunity interact to determine habitat use. Available records reveal substantial variation among lineages, ranging from recurrent ingress into moderately brackish environments to predictable seasonal occupation, reproduction, feeding and use by early life-history stages. Lolliguncula brevis represents the clearest example of extensive euryhalinity coupled with recurrent estuarine occupancy, whereas Sepia officinalis demonstrates that strong ecological integration can occur through seasonal and ontogenetically structured habitat use without equivalent extreme hypoosmotic tolerance. Octopods and sepiolids provide additional evidence that estuarine exploitation has emerged through different ecological and physiological strategies. We therefore propose that estuarine occupancy should be understood as a multidimensional continuum in which physiological accessibility, ecological profitability and ecological dependence represent related but distinct dimensions. Behavioral flexibility further mediates this relationship by allowing individuals to track favorable environmental conditions within dynamic salinity gradients. The phylogenetically scattered distribution of estuarine occupancy is consistent with repeated independent evolutionary transitions, although explicit comparative analyses are required to test convergence formally. Major knowledge gaps include strong taxonomic and geographical sampling biases, poor integration between laboratory tolerance and field occupancy, limited individual-level movement data and unresolved demographic nursery function. Moving beyond salinity tolerance toward an integrated ecological framework may reveal estuarine occupancy as a recurrent and underrecognized dimension of cephalopod ecological and evolutionary diversity.