Episode 471: Mystery Larvae

Further reading:

I Can Has Mutant Larvae? 200-Year-Old ‘Monster Larva’ Mystery Solved ‘Snakeworm’ mystery yields species new to science Hearkening back to the hazelworm

Show transcript: Welcome to Strange Animals Podcast. I’m your host, Kate Shaw. A few weeks ago when I was researching big eels, I remembered the mystery eel larva we talked about back in episode 49, and that led me down a fun rabbit hole about other mystery larvae. Let’s start with that eel larva. Eel larvae can be extremely hard to tell apart, so as a catchall term every eel larva is called a leptocephalus. They’re flattened side to side, which is properly referred to as laterally compressed, and transparent, shaped roughly like a slender leaf, with a tiny head at the front. Depending on the species, an eel may remain in its larval form for more than a year, much longer than most other fish, and when it does metamorphose into its next life stage, it usually grows much longer than its larval form. For instance, the larvae of conger eels are only about 4 inches long, or 10 cm, while an adult conger can grow up to 10 feet long, or 3 meters. On January 31, 1930, a Danish research ship caught an eel larva 900 feet deep, or about 275 meters, off the coast of South Africa. But the larva was over 6 feet long, or 1.85 meters! Scientists boggled at the thought that this larva might grow into an eel more than 50 feet long, or 15 meters, raising the possibility that this unknown eel might be the basis of many sea serpent sightings. The larva was preserved and has been studied extensively. In 1958, a similar eel larva was caught off of New Zealand. It and the 1930 specimen were determined to belong to the same species, which was named Leptocephalus giganteus. In 1966, two more of the larvae were discovered in the stomach of a western Atlantic lancet fish. They were much smaller than the others, though—only four inches and eleven inches long, or 10 cm and 28 cm respectively. Other than size, they were pretty much identical to Leptocephalus giganteus. The ichthyologist who examined them determined that the larvae were probably not true eels at all, but larvae of a fish called the spiny eel. Deep-sea spiny eels look superficially like eels but aren’t closely related, and while they do have a larval form that resembles that of a true eel, they’re much different in one important way. Spiny eel larvae grow larger than the adults, then shrink a little when they develop into their mature form. The six-foot eel larva was actually a spiny eel larva that was close to metamorphosing into its adult form. Not everyone agrees that Leptocephalus giganteus is a spiny eel. Some think it belongs to the genus Coloconger, also called worm eels, which are true eels but which have large larvae that only grow to the same size as adults. But worm eels don’t grow much bigger than about two feet long, or 61 cm. If the mystery larvae does belong to the genus Coloconger, it’s probably a new species. Until scientists identify an adult Leptocephalus giganteus, we can’t know for sure. Another mystery larva is Planctosphaera pelagica, which sits all alone in its own class because the only thing it resembles are acorn worms, but scientists are pretty sure it isn’t the larva of an acorn worm. It’s not much to look at, since the larva is just a little barrel-shaped blob that grows about 25 mm across. This sounds small compared to the eel larva we just discussed, but it’s actually quite large compared to similar larvae. Acorn worm larvae are usually only about a millimeter long. Planctosphaera has been classified as a hemichordate, which are related to echinoderms but which show bilateral symmetry instead of radial symmetry. Hemichordates are also closely related to chordates, which include all vertebrates. They’re marine animals that resemble worms but aren’t worms, so it’s likely that Planctosphaera is also wormlike as an adult. Planctosphaera isn’t encountered very often by scientists. It has limited swi