First identified in 1899, facetotectans (aka y-larvae) have been a puzzle to zoologists for over a century. No one has ever found an adult of this puzzling crustacean, despite the plethora of larvae, leading generations of marine zoologists to wonder just what y- larvae grow up to be. A new study reports the transformation of the larvae into a previously unseen, wholly un-crustacean-like, parasitic form.
Facetotectans are found amongst marine plankton in oceans from the poles to the tropics. Despite being ubiquitous and similar to the larvae of barnacles, not one adult y-organism has been identified in over 100 years of searching.
"Facetotectans are the only crustacean group with a taxonomy based solely on larval stages", say Henrik Glenner and Jens Hoeg of the University of Copenhagen "but the great species diversity indicates that the adults play an important ecological role." The study authors collected over 40 species of y larvae from a site at Sesoko Island near Okinawa, Japan, and exposed many of them to a crustacean moulting-hormone to encourage them to mature. The free-swimming y-larvae shed their articulated exoskeleton, and a simple, slug- like, pulsing mass of cells emerged.
The authors explain "The musculature and compound eyes that you might expect to see in adult crustaceans were in a state of degeneration, and from our observations of the live, and also preserved specimens, we conclude that the adults of these larvae must be parasites -- but of what we do not know."
The great diversity of Facetotecta and the finding that they are most likely parasitic as adults hints at a major ecological role that future studies, both in laboratories and in the field will try to uncover. These finding provide a tantalising glimpse of the solution to this 100-year-old riddle.