y-Larvae - mystery of the "young"/ missing adults

We have puzzled over the identity of a tiny marine organism which is clearly a larval or immature form of an "adult" form which has never been found. The curious thing is that although smaller than a period, they appear in every ocean, and may play an important role, just from their sheer numbers.

As for the "adult" form, none has been found. Nobody knows what these babies become.

For more than one hundred years, we have resorted to classifying these creatures as "y-larvae". All attempts to raise them to adult forms have failed. Known as shrimp-like "facetotectans", they are the only crustacean with a taxonomy based solely on its larval stage, of which more than 40 varieties have been collected. What sort of abundant creature "disappears" completely when it grows up?

Biologist Henrik Glenner, University of Copenhagen, acting on a hunch, exposed the larvae to a hormone that causes other crustacean parasites to metamorph into juvenile adult forms. The result was that eyeless, limbless, juveniles lacking digestive tracts, wiggled out of their larval carapaces. It appears probable that the "adult" lives as a parasite inside the digestive tract of its host. That host has yet to be found.

We find yet another "niche" in which Life somehow manages to flourish. Life in the guts of other forms of life.