Synopsis. Marine life histories differ from terrestrial life histories because seawater is denser and more viscous than air, because desiccation is not a problem for organisms in water, and because food is abundant in suspension and solution. (1) Mating and competition for paternity in the sea often differs. Female gametes are often spawned freely. Passively dispersed spermatophores could in some cases provide single paternity to an entire clutch of offspring. Penises of sessile animals reach far for copulation. There are no pollinators. (2) In many clades of benthic marine animals, greater dispersal of offspring is associated with large adult size, and greater parental care of offspring and reduced planktonic larval periods are associated with small adult size. (3) Many benthic marine animals are colonies with modular construction, and these also commonly brood embryos and have short-lived larvae, in contrast to related solitary forms. (4) Unlike dispersal of terrestrial animals, larval dispersal of marine animals is often obligate with sexual reproduction and often includes a precompetent period during which larvae cannot settle at good sites. Unlike terrestrial seeds, marine larvae have no clear adaptations for dispersal, often grow during dispersal, and often leave bad sites. Feeding planktonic larvae are common among marine animals and rare among other aquatic animals, perhaps because of persistent aquatic routes between habitable sites for marine animals. Peculiarities in marine life histories may influence many aspects of evolution in the sea. Closely related sedentary marine animals can differ greatly in larval dispersal with consequences for recruitment to populations, genetic exchange between benthic populations, adaptation to local conditions, sex allo? cation, interaction with kin, speciation, and extinction.