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From my high school level knowledge I can recollect that there are two types of migration in fishes-

  • From marine to fresh water (like in Hilsa which grows in the marine water but migrates to the fresh water only for reproduction and then the young return back to the marine water)

  • From fresh water to marine (like in eels that live in the fresh water but migrate to the marine water to reproduce)

The only mechanism I had read about osmoregulation in aquatic organisms was via the ionocyte cells in the gills of the fish.

So my question is:

How do they change their osmoregulatory mechanism during this migration?

Just a follow-up:

Are there any other mechanisms by which they osmoregulate

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I found a publication related to the osmoregulation in salmon that might be interesting to you. I'm copying the abstract because it basically answer your question.

Smoltification, also called parr-smolt transformation, is a complex developmental process that consists of a number of independent, but coordinated changes, in the biochemistry, physiology, morphology and behavior of juvenile salmon in their transition from freshwater to seawater life. A key component of smoltification is represented by the physiological adaptations that enable smolts to thrive in hyperosmotic environments. Instrumental to this process is the ability of smolt gills to gradually become capable of actively secreting salt through specialized cells known as mitochondria-rich (MR) cells, ionocytes or chloride cells. NaCl secretion by teleost gills is therefore accomplished via the secondary active transport of Cl - and the passive transport of Na + . The driving force for active transport is provided by Na + /K + ATPase, which maintains low intracellular Na + and high intracellular K + concentrations. However, this NaCl secretion mechanism needs at least two different ion channels: A CFTR type chloride channel for the passive exit of Cl - and a potassium channel to recycle extracellular K + , which is a thermodynamic prerequisite to work under conditions imposed by high extracellular salinity in seawater. The characteristics of K + channels required for NaCl secretion from MR cells into seawater are still unknown for Salmo salar and only recently have begun to be studied in other teleosts.

You can find the complete paper here.

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  • $\begingroup$ thanks for the reply.But how does the reverse happen...I mean can you throw some light on how the adult Hilsa or Salmon transform it's osmoregulatory mechanism during its migration from the marine to fresh water? $\endgroup$
    – user 33690
    Oct 22, 2018 at 2:20
  • $\begingroup$ I don't know. Maybe is easier to move from salt water to fresh water, since you'll only need to shut down the regulatory mechanisms?. @user33690 $\endgroup$
    – BPinto
    Oct 22, 2018 at 3:06
  • $\begingroup$ Possible, infact I thought that too at first. But in fresh water fishes the ionocyte cells (chloride cells) act to drive the reverse reaction...that is, they take up $Cl^-$ from the surrounding water. So that kept me wondering...what was actually happening- some sort of transitions or simply stopping the entire process $\endgroup$
    – user 33690
    Oct 22, 2018 at 3:43

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