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The larvae of anguillid eels are termed leptocephali, among other reasons, because they differ greatly in morphology and physiology from other fish larvae. Leptocephali grow to large sizes but are transparent and fragile and have an unusual feeding strategy, so their behavior or ecology has rarely been studied. We used leptocephali, metamorphosing larvae and glass eels reared artificially from eggs to study the ontogenetic change in phototaxis in Japanese eel Anguilla japonica. By study- ing the early life history of this eel, our intent was to learn more about behavioral mechanisms asso- ciated with their vertical migration in the ocean and their long migration from the offshore spawning area to coastal waters. Horizontal distribution experiments found a clear negative phototaxis in the leptocephalus and metamorphosing larval stages, but no phototaxis was detected after metamor- phosis into glass eels. Vertical distribution experiments found that overhead lighting caused down- ward movement in leptocephali and metamorphosing larvae. When kept in darkness, leptocephali and metamorphosing larvae were distributed near the surface layer at night and at the bottom dur- ing daylight hours, suggesting an endogenous circadian rhythm that may control their diel vertical migration. Glass eels exhibited benthic distribution under any light conditions (0.0039 to 3.9 µmol quanta m -2 s -1 ), but showed random vertical distribution in darkness during both daytime and night- time, a behavior which appeared to be controlled only by light. Swimming speeds of leptocephali during horizontal movement and rising behavior were significantly greater than during diving behavior. These swimming behaviors and ontogenetic changes in phototactic behavior may help reg- ulate the vertical distribution and inshore migration of anguillid eel larvae in the ocean.

Ontogenetic changes in phototactic behavior during metamorphosis of artificially reared Japanese eel Anguilla japonica larvae