The transparent leaf-shaped larva of an eel, drifting flat in the open ocean before it becomes a fish.
A flat ribbon of clear jelly with a thin dark gut line running its length, a small pointed head with two black eyes, and no obvious body at all. It is the larval stage of an eel, and it looks so unlike an adult that leptocephali were described as their own genus before anyone connected the two.
The transparency is real and structural. The tissue is largely a watery matrix with very little muscle or bone, which keeps the animal near neutral buoyancy and close to invisible in open water; on a blackwater dive the eyes and the gut are often all a torch beam picks up, and the outline only appears as the beam crosses it at an angle. They swim by passing a wave along the whole flattened body, and they swim backwards as readily as forwards.
Which eel it will become is generally not answerable from a photograph. Leptocephali belong to the Elopomorpha, which includes the morays, the snake eels, the garden eels, the conger and the tarpon, and the larvae of very different adults are hard to tell apart without counting myomeres — the V-shaped muscle blocks visible along the body — under magnification. The honest log entry is leptocephalus.
At the end of the drifting stage the animal shrinks, loses much of its water content and condenses into the small transparent “glass eel” that will settle. It feeds during the larval period on marine snow and dissolved organic material rather than by hunting, which is part of why it can be so lightly built.
They are easily damaged. A leptocephalus caught in a strong focus light will often coil and thrash; back the light off and it flattens out again.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
