For more than 100 years, biologists have puzzled over the identity of a tiny, mysterious marine organism known only as y-larvae.
Smaller than the period at the end of this sentence, the shrimp-like larvae are found in massive numbers scattered throughout the world's oceans, suggesting that their adult form plays an important role in ocean ecology.
But there is one problem: Nobody knows what they grow up to be. As they mature, they seemingly disappear. Efforts to raise them in the lab have proved futile.
The creatures, known formally as facetotectans, "are the only crustacean group with a taxonomy based solely on larval stages," said biologist Henrik Glenner of the University of Copenhagen.
Now, Glenner and his colleagues in Denmark and Japan have found evidence that the crustaceans are a parasite that must live in the body of a host as adults. They have not yet found that host, however.
Glenner and his colleagues studied more than 40 varieties of y-larvae collected at a marine station at Sesoko Island near Okinawa. Acting on a hunch, they exposed the creatures to a hormone that is known to cause metamorphosis in some other crustacean parasites, according to their report published Monday in the journal BMC Biology.
To their astonishment, the larvae transformed into a juvenile form to become wiggling, eyeless, limbless blobs of cells that resemble some other crustacean parasites. The juveniles "literally crawled out of the old larval carapace," Glenner said. The sight, he said, was "mind-blowing."
Because the juveniles lack digestive tracts and nervous systems, he speculated that they must live in some other organism, absorbing nutrients directly from their surroundings. That could be why the adult form has never been detected.
The team will now look for larval DNA in other organisms in an effort to find that host.