The sea spider, a long-legged muncher of seaweed and smaller invertebrates, is not a spider at all, according to a new study. After analyzing the development of the creature's brain, researchers have concluded that the sea spider bears little relationship to arachnids or even other arthropods.
Sea spiders have always made natural historians scratch their heads. Known as pycnogonids--Latin for "knobby-knees"--they are traditionally put in the same group as spiders and scorpions because of a pair of pincered appendages on the front of their heads called chelifores, thought to be a modification of arachnid fangs. But others suspect that sea spiders have changed little since the origin of arthropods--segmented invertebrates that include crustaceans and insects--around 500 million years ago. Comparisons based on DNA have been so far inconclusive.
To penetrate this heady mystery, a team led by Amy Maxmen, an evolutionary biologist at Harvard University in Cambridge, Massachusetts, tracked the development of the sea spider brain from embryo to adult by staining neurons with glowing antibodies. The researchers wanted to see whether the nerves that control sea spider chelifores originate from the mid-brain embryonic tissue as spider fangs do, which would plant the creatures more firmly among arachnids. The tantalizing alternative was that sea spiders' chelifores might originate from the front part of the brain, making them seem closer to the oldest arthropod ancestors which, according to fossils, sported big grabbing appendages at the very front of their heads.
Sea spiders do seem to be as primitive as they look. Unlike spider fangs, both chelifores and their nerves sprout from the cells that form the front most part of sea spiders' brains, the team reports 19 October in Nature. This supports the theory that sea spiders belong to their own ancient lineage that predates the origin of all other modern arthropods. It also suggests that all of today's other arthropods inherited their heads from a subsequent ancestor that gave up such up-front appendages.
The study convincingly shows that sea spiders are "extraordinary living fossils," says Max Telford, an evolutionary biologist at University College London, U.K. But to fill in the bigger evolutionary story, he says, the developmental genetics of arthropods brains must now be teased apart.