The mysterious Y-larva has a strange way of growing up. The speck-sized crustacean, found throughout the world's oceans, has only been observed in its larval form--until now. Researchers have discovered the next step in this animal's life, and the transformation is extreme--the many-legged, flealike larva turns into an eyeless, brainless blob. This unusual devolution from sophisticated to simple provides clues to how the final adult form of the animal, which still remains unobserved, makes a living.
Crustaceans, which include crabs, lobsters, and barnacles, metamorphose several times as they mature, often changing their form completely. But since the Y-larva's discovery a century ago, the animal's final form and ecological niche has remained a complete unknown to marine scientists--the captive larva always died before developing to the next level, let alone on to adulthood. In fact, it was the only crustacean with a formal taxonomy based completely on its larval stage.
Scientists wanted to force these larvae to grow up, but finding the biological trigger to do that was tricky. Some of the Y-larva's closest relatives are parasitic crustaceans, so investigators suspected it was parasitic as well, which would mean that hormones from its host were the most likely cue for the larvae's transformation. But the Y-larva's host is yet another mystery.
Attempting to bypass this problem, Henrik Glenner, a biologist at the University of Copenhagen in Denmark, and his colleagues exposed captive Y-larvae to a hormone known to induce molting in other crustaceans. After 12 to 24 hours, this chemical caused the Y-larvae to morph into something completely different: a balloonlike blob that lacked both a digestive and a nervous system, the researchers report online in the 20 May issue of Biomed Central. "From a shrimplike animal it became a sacklike structure," says lead author Glenner. His team dubbed the new form an ypsigon and believe it represents an intermediate stage in the Y-larva's maturation process. Because the ypsigon lacks all the organs necessary for surviving independently, the researchers conclude that the animal is a parasite, most likely living inside invertebrates like coral, where it would then develop into its adult stage.
"It's a significant step forward," says Alistair Lindley, a plankton biologist with the Sir Alistair Hardy Foundation for Ocean Science in Plymouth, U.K. "Anything that casts a bit of light on their biology is very interesting." Scientists can now focus on discovering the final, mature form, says Geoffrey Boxshall, a zoologist with the Natural History Museum in London. "It's tantalizing because there was this huge gap in the life cycle, and this half-filled it."