Sediment sucking acorn worms ride the currents between meals on the ocean floor
– By Nicholas Holland
The question of the origin of the vertebrates (including humans) from invertebrates is the most celebrated paternity suit in biology. Since they were first accused over a century ago, acorn worms (technically enteropneust hemichordates) have been one of the usual suspects. Acorn worms have fish-like gill slits and, according to some biologists, a vertebrate-like brain and perhaps a notochord (a structure shared by all embryonic vertebrates).
In recent years, the relatively new field of developmental evolution has used molecular biology to show that the genetic programs directing the development of acorn worms and vertebrates are also surprisingly similar. As a result, the question of the evolutionary origins is now a hot topic, and acorn worms are one of the star attractions, although there are also aficionados of other invertebrate groups as the nearest ancestors of the vertebrates. In sum, acorn worms attract our attention way out of proportion to the very modest number of species in the group (under 100).
Any recent invertebrate textbook will inform you that acorn worms (named from their acorn-shaped front end) live in burrows in relatively shallow water. The book will also divide the known species into three families. Now, however, our ideas about acorn worms have been broadened by work by three current and former scientists at Scripps Institution of Oceanography at UC San Diego: Ken Smith, who invented techniques for studying the worms at home on the deep-ocean floor; Karen Osborn, who used molecular sequencing to elucidate their family tree; and me. It was my job to describe their detailed anatomy. Together, the new findings—now available online in the Proceedings of the Royal Society of London—have revealed that the deep sea is the exclusive home of a fourth family of acorn worms that probably comprises at least a couple of dozen new species.
This is a surprising increase in the diversity of a small group of animals that was thought to be well-understood. However, the interesting part of the story is more than just a numbers game. First of all, the worms’ anatomy is eye-catching. Their front ends never look at all like acorns. Instead, long lips project from either side of the head, sometimes resembling the ears of Dumbo the elephant; sometimes the ears of Yoda from Star Wars; and sometimes the spread hood of an angry cobra. The body sizes (up to 3 feet long) and colors (dark blue, deep crimson, etc.) are often striking as well. The consistency of the body is very flabby and gooey, not much more substantial than a big piece of Jell-o, which complicates the task of recovering the beasts in good condition from their home on the seafloor, several miles down.
Their lifestyle is also remarkable and surprising. Their antics have been recorded in high definition by time-lapse photography and by video recordings made from unmanned submersibles called remotely operated vehicles. Instead of living like stick-in-the-mud acorn worms living underneath the ocean bottom in shallow water, the deep-sea worms spend part of their time crawling slowly on the sea floor surface, not burrowed beneath it. The worms use cilia on their underside to glide along at about 3 inches an hour while sucking in their meal of surface sludge. In browsing along the sea floor, a worm’s progress is marked by the shape of its trail of feces continuously laid down. Depending on the species, the worm may travel in a random meander, or in a watch spring-like spiral, or in a series of tight switchbacks. Then, when the animal decides to seek greener pastures for browsing, it empties its gut to get lighter and ascends above the bottom into in the water column. There it drifts laterally in the current (evidently with the aid of a surrounding balloon of mucus) until descending once again to pick up another meal from the bottom.
These remarkable worms have so far been recovered from the North Pacific and North Atlantic oceans, but our studies have yet to include the South Pacific, South Atlantic, and entire Indian Ocean. So we expect a further surge in the number of known species in the deep-sea enteropneust family, which we have named Torquaratoridae (Latin for “neck plow”) to emphasize their mode of feeding. The family name is a tongue-twister, but it is sure to be included in the next generation of invertebrate zoology textbooks.
– Nicholas Holland is a marine biologist in the Marine Biology Research Division