Let 'Em Eat Worms
By Al Hornsby
On night dives in the Caribbean, you'll often see a swirling, dancing swarm of tiny, reddish-brown worms that will gather around your dive light. A part of the planktonic collection of small plants, animals and the embryonic stages of many familiar reef creatures that floats about the ocean, these worms-Arrow Worms, by name-are actually fierce predators that are swept about the reefs, voraciously feeding on virtually anything they can find.
They in turn are fed upon by a multitude of other creatures, including large animals such as mantas, and others as small as coral polyps. It's easy to witness the complexity of this tiny eat-and-be-eaten world; near the reef, hold your light still for a few moments until a cloud of worms has been attracted. Once the worms are present, move the light near a fire or other coral that has its polyps extended. As the worms come in contact with the coral, the polyps attach to them with fired nematocysts, tiny poison-injecting darts that kill the worm. Within moments, the coral polyps will draw the fluids from the worms, leaving pale husks.
To see this on a larger scale, find a Giant Basket Star that has spread its branched arms wide from atop a prominence. Bring the cloud of worms in close; as they touch an arm, the Basket Star will quickly roll the arm in, transferring the worms to its mouth at the base of its body.
In the photo accompanying this article, shot on a reef off Cayman Brac, an interesting collection of feedings is documented. At the top right, a Branching Fire Coral has captured two worms; at left middle, a Boulder Star Coral has just begun to feed; and in the center, if you look closely, you'll see that a Yellowline Arrow Crab (no relation to the worm) has just reached up and snatched a worm out of the swarm-it is held firmly in the crab's left claw. Moments later, it popped the worm into its mouth; a satisfying snack for this small, nocturnal predator.