Do Late-Stage Anguillid and Other Leptocephali Use Swimming to Reach Their Recruitment Habitats?

The larvae of eels have various unusual morphological features such as extreme lateral compression and transparency, and large size, but little is known about their behavior.  Leptocephali are generally considered to drift with ocean currents, but various types of evidence indicates that some species must swim at critical times to successfully reach their metamorphosis and recruitment areas.  Active directional swimming seems to be the only way American eel leptocephali can cross the Gulf Stream, while European eel larvae remain within the current, and the larvae of the American conger, and a subpopulation of Ariosoma balearicum must also cross the current after spawning in the western Sargasso Sea.  Leptocephali of both anguillid species disappear from the Sargasso Sea gyre at sizes of > 60 mm, which also suggests active swimming occurs at large sizes.  Similar factors are present for anguillids and congrid eels in the Pacific, and some taxa of marine eel leptocephali show greater levels of larval retention than others, which may be achieved through swimming, even at small sizes.  It is unclear if there are physiological constraints on long-term swimming in leptocephali, but evidence indicates that they may use swimming at key times during their larval stage.