Diadramous, Catadromous, Anadromous, YouTube Lecture Handouts

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Diadromous: Catadromous (Eel) & Anadromous (Salmon, Lamprey) Fishes - Euryhaline & Semelparous


Diadromous fishes describe species that spend part of their lives in freshwater and part in saltwater. There are two categories of diadromous fishes, catadromous and anadromous.


  • Catadromous fishes hatch or are born in marine habitats, but migrate to freshwater areas where they spend the majority of their lives growing and maturing. As adults, they return to the sea to spawn. The word “catadromous” means “downward-running,” and refers to the seaward migration of adults. The best-known groups of catadromous fishes are the true eels. In these species, females spend their lives largely in freshwater, while males live primarily in the brackish water of estuarine areas. Individuals breed in the seas and die after spawning once.
  • Catadromous fish are born in saltwater, then migrate into freshwater as juveniles where they grow into adults before migrating back into the ocean to spawn.

Examples of Catadromous

Examples of Catadromous


  • Anadromous fishes are the opposite of catadromous fishes in that hatching and a juvenile period occur in freshwater. This is followed by migration to and maturation in the ocean. Adult fish then migrate back up rivers “anadromous” means “upward-running” in order to reproduce in freshwater habitats. The lengths of the initial freshwater period and of the oceanic period vary greatly by species.
  • Anadromous fish are born in freshwater, and then migrate to the ocean as juveniles where they grow into adults before migrating back into freshwater to spawn

Examples of Anadromous

Examples of Anadromous

Salt Water

  • In marine environments, the challenges are the opposite. Saltwater species must deal with an environment in which their salt and ionic concentrations are significantly lower than that of the surrounding aquatic environment.
  • Saltwater species tend to lose water to the ocean and to gain ions from it. To obtain and conserve water, saltwater species increase their drinking rate, and excrete smaller amounts of a highly concentrated urine. In addition, they eliminate excess ions through specialized salt-excretion cells in the gills and in the lining of the mouth.

Fresh Water

  • Freshwater fish are in an environment in which they are hyperosmotic. That is, the concentration of salts and ions in their bodies is greater than that in the external aquatic environment. As a result, they have a tendency to lose important ions through diffusion across the skin and gills, and simultaneously to gain water from the environment.
  • To maintain homeostasis, freshwater species have special adaptations for retaining ions and getting rid of excess water. First, they actively take in ions across their gills and skin, a process that requires energy. Second, to get rid of excess water they excrete nitrogenous waste products in great quantities, in the form of a highly diluted urine.

Salt Water & Fresh Water

  • Euryhaline species must adopt the tactics of freshwater species while in freshwater environments, and those of marine species in saltwater environments. Frequently, physiological adjustments are made while organisms are in the intermediate, brackish waters of estuaries. These include changing their drinking rate, the degree of concentration of their urine, and the direction of ion pumping in the gills and integument.
  • Diadromous species are semelparous, that is, they reproduce in one large reproductive bout and then die. This is also known as “big-bang” reproduction. Semelparity is contrasted with the reproductive strategy of iteroparous species, which reproduce multiple times. Iteroparity characterizes numerous species, including humans.

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