Complex sentence: In catadromous fish, which migrates from freshwater to sea, the porphyropsin is replaced by rhodopsin and in an anadromous fish which migrates from the sea to freshwater, the rhodopsin is replaced by porphyropsin.
Split into simpler sentences: In catadromous fish, which migrate from fresh water to the sea, the porphyropsin is replaced by rhodopsin. In an anadromous fish, which migrates from the sea to freshwater, the rhodopsin is replaced by porphyropsin.