October 9 2013

12:00 LSB 2320

Nicholas Holland
Scripps Institution of Oceanography, UC San Diego

A new family of acorn worms gives insights into deep-sea ecology and evolution A new family of acorn worms gives insights into deep-sea ecology and evolution


Fifty years ago, deep-sea photographs revealed acorn worms (phylum Hemichordata) crawling on the deep ocean floor. At the time, it was claimed that these worms blended morphological features of the two main hemichordate body plans: namely the tentacle-less enteropneusts and the tentacle-bearing pterobranchs. Consequently, the worms were christened "lophenteropneusts" and invoked as missing links between the two groups; they even found their way into scenarios of how vertebrates evolved from invertebrates. The crude state of undersea technology 50 years ago did not permit collection of the actual animals. In the last few years, we have used modern submersibles to collect numerous specimens of so-called lophenteropneusts from the abyss all over the world ocean. Morphological study showed that such beasts simply had unusually wide collars and not tentacles ­- so lophenteropneusts turned out to be just enteropneusts misinterpreted by wishful thinking. By now, seven new species of these wide-collared enteropneusts have been formally described and more keep coming to hand. Molecular phylogenetic analysis surprisingly showed all these new found worms grouped into a single, deeply branching clade, now recognized as the fourth family in the class Enteropneusta -- quite a jump in biodiversity. Ecologically these deep-sea worms differ strikingly from their burrow-inhabiting shallow water relatives; the deep-living worms crawl on the surface of the sea floor and rarely if ever burrow; instead they ascend into the water column and float along near the bottom to travel to new foraging sites. The evolution of the group is considered in the light of fossil evidence plus current zoogeographical and bathymetric data.











































































































































































































































































































































































































































































































































































































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