Marine worms are one of the most abundant forms of life on the reef. Worms show a remarkable degree of adaptability and have evolved to occupy many reef niches. Despite their abundance, marine worms are not very easy to spot. The majority hide under rocks or in crevices, although a few species are free swimming. Worms get their food in a variety of ways, grazing on algae, preying on other animals, and scavenging dead and decaying material.

Around the reef, worms fall into one of two primary groups: flatworms and segmented worms. Of the two types, flatworms are the simplest and most primitive. Most marine flatworms are very thin, delicate animals. Flatworms have a simple digestive system with only one opening, so undigested matter is regurgitated out the mouth. To feed, a flatworm extends its pharynx, a muscular tube, onto its meal. The tube secretes digestive enzymes on the food, creating a soupy mix of partially digested tissue that can be sucked into the intestine.

Flatworms of the family Polycladida can be found in reef waters. These worms swim in undulating waves by throwing the sides of their bodies back and forth. Their name, which means “many branched,” describes the highly divided digestive systems, which can be seen through the worms’ transparent skins.

Polyclads have many would-be predators, especially among the fish and crustaceans. As a result, they have evolved several strategies to avoid predation. For example, most species feed at night when it is more difficult for predators to see them. In addition, many polyclads are camouflaged to blend into their environment. Others contain chemicals that either taste bad or are poisonous to their attacker. To advertise their distastefulness, flatworms with toxins, such as the specimen featured in the upper color insert on page C-4, sport bright colors and bold patterns. Using color to warn predators to stay away is a defensive strategy called aposematic coloration.

Polyclad flatworms are hermaphroditic, having both male and female reproductive organs. To cross-fertilize their gametes, two worms copulate, each donating sperm to the other. Sperm is exchanged through hypodermic-like stylets that each worm stabs into the body of its partner. The stylets are sharp, and stabbing sometimes causes damage to tissue, but injured worms usually recover within 24 hours. Once a stylet is injected, sperm are released to swim to the eggs. Fertilized eggs are laid on the reef floor under rocks or rubble and near a source of food. The eggs develop into larvae that swim for a few days before settling back to the reef floor where they become adult flatworms.

Most of the marine flatworms living around reefs have very poor eyesight. To get information about their environments, the worms depend on folds of tissue at the ends of their bodies. These folds create tentacle-like structures that can detect chemicals in the water, helping the worms navigate and find their food.

Although flatworms are common on the reef, they are greatly outnumbered by segmented worms. Reef segmented worms are colorful and flamboyant compared to their terrestrial counterparts. The most common types of segmented worms in marine environments are the polychaetes, or bristle worms. On some reefs, scientists have found that as many as 75 percent of the animal species are polychaetes. Their large bristles, structures called setae, are found on each body segment. Coral reef polychaetes are usually found living on the bottom in sediment, sand, or reef rubble.

Many reef polychaetes have a crown of true tentacles around their heads that are called radioles. These structures strain plankton from water and are the sites of gas exchange. Radioles are arranged as inverted funnels so that they can direct food toward the worm’s mouth. Cilia constantly move the water around the radioles, creating currents that sweep food along. One polychaete species, the sponge threadworm (Haplosyllis spongicola) bores into sponges, piercing them with tunnels. Sponge threadworms live in these tunnels where they are out of sight of predators and free to feed on the sponge tissue. Another group of polychaetes bore into corals, both dead and living. These worms eventually weaken the coral structure and cause sections of it to crumble.

Most species of segmented worms have separate sexes, although some are hermaphrodites. Those that spawn often do so when the corals spawn. Some segmented worms develop long structures called swimming lobes that help them swim up toward the surface during spawning. As the adults near the top of the water column, their bodies rupture, freeing eggs and sperm.

Two tube-dwelling polychaetes are the feather duster tube worm, as seen in the lower color insert on page C-4, and the Christmas tree worm. The Christmas tree worm (Spirobranchus giganteus), a Goliath in the world of polychaetes, lives in a tube that it builds on the surface of coral. As the worm grows, coral begins to surround the tube and eventually encloses it completely. To maintain contact with the outside world and avoid
being sealed inside, the worm must continually lengthen its tube. When the tube is encased in the coral, only the worm’s radioles are visible. The body, which is embedded in the coral, may be nearly 3 feet (1 m) in length. To feed, the worm sticks out its head and spreads it radioles, which resemble Christmas trees. Radioles can be yellow, pink, red, blue, white, gray, or brown, depending on the species. Their sticky mucus helps hold food particles, and their upside-down funnel shape directs food toward the mouth. When danger threatens, the worm can pull its head and radioles down into its tube.

Fireworms (Hermodice carunculata) are free-living polychaetes that grow to 12 inches (30 cm). Sensitive smelling organs on their heads resemble pleated cushions. Fireworms are hunters that move boldly around the reef, protected by venom-filled bristles. Because the bristles can break off and embed in an attacker, they cause a long-lasting, burning sensation, hence the worm’s name.

Hermodice carunculata (Image from wikimedia.org)
Hermodice carunculata (Image from wikimedia.org)

Fireworms are vicious predators of coral animals. They consume the polyps by spreading digestive enzymes over them, then sucking up the partially digested slush. A group of feeding worms leaves behind a trail of dead, white coral skeletons. Their favorite food is the tip of a staghorn coral, which one worm can devour in just a day. The feeding activity of fireworms can permanently change the reef because once the corals are killed, they rarely recover. In most cases, algae or sponges overgrow the area, preventing the return of coral animals.