Eels have a unique larval form, leptocephali, which can utilize marine currents to travel long distances.
A catadromous fish hatches and spawns in salt water but spends most of its life in fresh water. Catadromous life cycles are much less common than anadromous life cycles. American and European Eels (Anguilla rostrate and A. anguilla, respectively) are among the more famous fish that utilize this unusual migration pattern. They have a highly specialized larvae, leptocephali, which resemble a transparent leaf. Their specialized shape allows leptocephali to ride marine currents to the continental shelf using relatively low amounts of energy. In coastal waters, they metamorphose into glass eels. As they continue to grow and start developing pigmentation, they enter the yellow eel phase. Yellow eels migrate into estuaries and onto fresh water where they will remain until they reach sexual maturity as a silver eel and return out to sea to spawn.
For more information, check out the “CAN YOU SAY ANADROMOUS, CATADROMOUS, AMPHIDROMOUS, OCEANODROMOUS, OR POTAMODROMOUS?” post on The Fisheries Blog!