Quick answer: It's the larval form of a green moray eel!
Now, here's the real story. A friend showed me his picture of this amazing animal, but I wasn't able to get a copy of his image. (Long boring technical failure story elided here.) So I had to do this search just from my visual memory of the animal.
I knew it was some kind of eel from the shape of the head. (And I gave this bit of inside knowledge in my problem statement.) So I did a fairly obvious image search for
[ transparent eel ]
and switched to Google Images to find another example that matched what I'd seen. It was fairly short work to find that this really WAS an eel and that it was called a leptocephalus ("flat head"). A few quick searches after that confirmed that this was the larval form of a moray eel.
Disclosure: This is the paper from which I got those great eel images. If you have any interest in the way eels work, live, breed and develop (and it's still one of the great mysteries of icthyology).
To quickly summarize: The lepthcephali are the juvenile form of eels (this picture happens to be of a moray). All eels go through such a larval stage when they're largely transparent and drift with the plankton. Oddly enough, muraenid leptocephali (i.e., of the moray eel family) have pectoral fins, very different from that of the adults, which do NOT have them. When they transform from leptocephalus to juvenile that the pectoral fins are resorbed.
Now, what do they eat? (And why do so many leptocephalids have such large, fang-like teeth?)
I learned from another paper (Diet of anguilloid larvae: leptocephali feed selectively on larvacean houses and fecal pellets, N Mochioka, M Iwamizu, Marine Biology (1996) v: 125, i: 3, p: 447-452) that leptocephalids consumed "...larvacean houses and zooplankton... No trace of the many other phytoplankton or zooplankton, which were found with leptocephali... On the basis of the importance of larvecean houses in the diet of several species of leptocephalus larvae, it is proposed that the peculiar, large, fang-like teeth of leptocephali are used for feeding, and evolved to pierce and grasp the mucous houses of larvaceans."
So, to save you the trouble, "larvacean houses" are the discarded rigid skeletons of tunicates that live nearly everywhere in the oceans. Wikipedia describes the larvacea as "...solitary, free-swimming tunicates found throughout the world's oceans. Like most tunicates, appendicularians are filter feeders. Unlike other tunicates, appendicularians live in the pelagic zone, specifically in the upper sunlit portion of the ocean (photic zone) or sometimes deeper. They are transparent planktonic animals, generally less than 1 centimetre (0.39 in) in body length (excluding the tail)."
I didn't know that eels even HAD larval stages, let alone that they rely so much on the discarded houses of tunicates. The zooplankton makes sense, but even simple searches result in really interesting discoveries.
Search on!
Now, here's the real story. A friend showed me his picture of this amazing animal, but I wasn't able to get a copy of his image. (Long boring technical failure story elided here.) So I had to do this search just from my visual memory of the animal.
I knew it was some kind of eel from the shape of the head. (And I gave this bit of inside knowledge in my problem statement.) So I did a fairly obvious image search for
[ transparent eel ]
and switched to Google Images to find another example that matched what I'd seen. It was fairly short work to find that this really WAS an eel and that it was called a leptocephalus ("flat head"). A few quick searches after that confirmed that this was the larval form of a moray eel.
Answer path: [ transparent eel ] --> Images --> which led me to this remarkable paper: “Ecology of Anguilliform Leptocephali: Remarkable Transparent Fish Larvae of the Ocean Surface Layer” Aqua-BioSci. Monogr. (ABSM), Vol. 2, No. 4, pp. 1-94 (2009)
Disclosure: This is the paper from which I got those great eel images. If you have any interest in the way eels work, live, breed and develop (and it's still one of the great mysteries of icthyology).
To quickly summarize: The lepthcephali are the juvenile form of eels (this picture happens to be of a moray). All eels go through such a larval stage when they're largely transparent and drift with the plankton. Oddly enough, muraenid leptocephali (i.e., of the moray eel family) have pectoral fins, very different from that of the adults, which do NOT have them. When they transform from leptocephalus to juvenile that the pectoral fins are resorbed.
Now, what do they eat? (And why do so many leptocephalids have such large, fang-like teeth?)
I learned from another paper (Diet of anguilloid larvae: leptocephali feed selectively on larvacean houses and fecal pellets, N Mochioka, M Iwamizu, Marine Biology (1996) v: 125, i: 3, p: 447-452) that leptocephalids consumed "...larvacean houses and zooplankton... No trace of the many other phytoplankton or zooplankton, which were found with leptocephali... On the basis of the importance of larvecean houses in the diet of several species of leptocephalus larvae, it is proposed that the peculiar, large, fang-like teeth of leptocephali are used for feeding, and evolved to pierce and grasp the mucous houses of larvaceans."
So, to save you the trouble, "larvacean houses" are the discarded rigid skeletons of tunicates that live nearly everywhere in the oceans. Wikipedia describes the larvacea as "...solitary, free-swimming tunicates found throughout the world's oceans. Like most tunicates, appendicularians are filter feeders. Unlike other tunicates, appendicularians live in the pelagic zone, specifically in the upper sunlit portion of the ocean (photic zone) or sometimes deeper. They are transparent planktonic animals, generally less than 1 centimetre (0.39 in) in body length (excluding the tail)."
(Larvaean "house" image from: Arctic Science Journeys, 2002. )
I didn't know that eels even HAD larval stages, let alone that they rely so much on the discarded houses of tunicates. The zooplankton makes sense, but even simple searches result in really interesting discoveries.
Search on!
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