Marine fungi are not a taxonomically or physiologically defined group of organisms; rather, they are an ecologically defined group. According to a commonly accepted definition, they are divided into obligate marine fungi, which grow and sporulate exclusively in the marine or estuarine environment, and facultative marine fungi, which may grow in marine as well as freshwater or terrestrial habitats. Consequently, they range from residents, that is, those that complete their life cycles exclusively in marine habitats and thus are never found outside of the marine environment, to transients, that is, those that occur in the sea accidentally by being washed or blown into it.