Today’s Halloween in the Deep offering is a smorgasboard of odd creatures seen on June 30 this past summer on a dive site called "Twin Peaks" by Okeanos Explorer’s ROV Deep Discoverer. The most striking is the first creature seen—a giant acorn worm—but the sea cucumber doing a Mary Poppins impression and the crab with an anemone stuck to its butt are fun too. Also spotted: a long-legged isopod (very unusual as most isopods resemble pill bugs.)
Just to look at that velvety purple pile of acorn worm, you may wonder: what is all that stuff and what is it doing?
Well, here’s a bio-gram to the rescue:
The proboscis, when shorter and retracted into the collar, is what gives the acorn worm its name. Sticky mucus and cilia on the proboscis may collect food and pass it to the mouth inside the collar. Some live in burrows, and some on the sea floor and feed on detritus like this example.
As it turns out, acorn worms are also important creatures for understanding our own evolution. Why? Because inside and outside that oddly shaped and garishly colored creature are several familiar features: gill slits, a nerve cord along their back (and belly), and left- and right-handed symmetry.
It is not for certain that the acorn worm nerve cord (sometimes hollow like ours) shares the same evolutionary origins as our spine-encased nerve cords. But they may well have; we certainly share a common ancestor. Like the vertebrates and echinoderms -- the radially symmetrical group that includes sea stars and sea urchins -- acorn worms are deuterostomes. That is, our mouths develop from the second hole to form in an embryonic ball of cells (the anus forms from the first.) And like vertebrates, acorn worms have gill slits they use for both feeding and breathing. Most vertebrates with functional gill slits (like fish) use them for breathing only, though even land-dwelling vertebrates like ourselves still form gill precursors during early embryonic development.
It used to be thought that dull-colored little acorn worms lived only in U-shaped burrows in the sediment. But as I wrote about back in 2010, deep sea exploration like this dive has only recently shown that giant, frilly, brightly colored acorn worms can live right on the sea floor, occasionally rising into the water column to search for greener pastures. Some may reach eight feet in length. When you're missing giant, neon-colored, virtually sessile sea creatures that crawl along the sea floor at snail-speed, you know you got a whole lot more splorin' to do.