Just when it looked like biologists had finally settled on the true placement of sea spiders on the evolutionary tree, a new study of these bizarre arthropods has come to a dramatically different conclusion. The findings may bring scientists a step closer to understanding how evolution produced today's immense diversity of invertebrates.
The sea spider is a puzzle. Biologists traditionally place it within the same group as spiders and scorpions because its frontal grabbing appendages are similar to arachnids' muscular mouth appendages, called chelicerae. But whereas chelicerae grow out of the middle part of arachnid heads, sea spider grabbers seem to sprout from the front. Because the only arthropods with appendages mounted on the front of their heads are extinct species from 500 million years ago, it seemed likely that sea spiders are living fossils that have retained an extra head segment adorned with appendages.
The living fossil theory got a boost last fall with a study of sea spider development led by Amy Maxmen, a biologist at Harvard University (ScienceNOW, 19 October 2005). When Maxmen's team examined sea spider larvae, they found that the nerves that control the frontal grabbers are wired up to the frontmost part of the brain rather than to the middle, as they are in arachnids.
But a new study throws cold water on the theory. Michaël Manuel, a biologist at the University of Pierre and Marie Curie in Paris, and colleagues studied a gene, called Deformed, that guides appendage development in sea spiders. In arachnids, the Deformed protein steers the development of chelicerae; so if it is also in charge of the development of sea spiders' grabbers, then that would support their close relationship. The patterns of Deformed expression indicate that sea spider frontal grabbers are indeed modified versions of the chelicerae of today's spiders rather than primitive frontal appendages from days of yore. The strange positioning of the grabber nerves may be due to a rearrangement of the brain during sea spider evolution rather than an extra head segment lost in all other arthropod species, the team reports 25 May in Nature.
The study provides "clear and consistent results," says Max Telford, a biologist at University College London, but the debate isn't settled yet. Among the questions that remain is just what happened to the primitive frontal appendages seen in arthropod fossils. And Maxmen hasn't given up the ghost either. The developmental genetics of sea spiders are poorly understood, she says, so it is "premature" to conclude from a single shared gene that the frontal grabbers evolved from chelicerae.