Worms – yuck! There are many people who are repulsed by long, skinny, slimy life forms. But these creatures that we collectively refer to as worms are important members of many ecosystems, including coral reefs and surrounding marine habitats. Although there are many types of worms found in coral reef environments, the species most often encountered by aquarists and divers belong to the class Polychaeta (phylum Annelida). There are more than 8,000 described species of polychaete worms, with many species yet to be described. Most of these reside in marine habitats, with the vast majority practicing a cryptic way of life. There are a few frequently encountered groups of polychaete worms. I hope that this article will dispel some long-held misconceptions about these fascinating creatures.
Form and Function
The typical polychaete has a long segmented body with paddlelike appendages on each segment (called parapodia) and a “head” at the anterior end that is complete with eyes, antennae and a ventrally placed mouth that can have sizeable jaws and sometimes teeth. When it comes to lifestyle, polychaete worms can be broken into two groups.
The first group consists of the free-living species (what are known as errant polychaetes). The errant forms move about on the substrate, dig burrows in soft substrates, and lurk among coral rubble or within coral interstices. Most of these species are secretive and are more active at night. They are reclusive because they are a valued food source to many reef fish species. There are a few errant polychaetes that live commensally in the burrows of other worms or crustaceans, or live on hermit crabs, corals, sea stars, urchins and sea cucumbers. Most errant polychaetes are scavengers, but some are also voracious predators. For example, the leviathan of the group, the Bobbit worm (Eunice aphroditois), has relatively large bony jaws that it uses to grab passing fish. After snatching its prey from the water column, it jerks it beneath the sand surface and consumes it. This worm can reach a length of at least 10 feet!
Members of the second group in the class are known as sedentary polychaetes. These are more diminutive in size and construct a permanent refuge chamber on or in the substrate. Most of these polychaetes are specialized suspension-feeders that unfurl a conspicuous feeding structure into the water to collect nutrients. Because they have a refuge in which to retreat when in danger, they tend to actively feed during the day.
Bristleworms or Fireworms
The most common polychaete worms in the home aquarium are not intentionally introduced but come in as hitchhikers on “live” substrate or rock. Most of these belong to a group referred to as bristleworms or fireworms. They belong to a number of different genera (e.g., Amphinome, Chloeia, Eurythoe and Hermodice) in the family Amphinomidae. These worms are superficially centipedelike, but rather than having a lot of legs, each body segment is adorned with parapodia (limbs used for locomotion) and setae (bristles). The setae serve a defensive function. When these worms are threatened by a potential predator or inquisitive human, they flare open their bristlelike setae. The setae’s hollow bristles will break off on contact in the predator’s mouth (or in the hand of an aquarist). In the case of human flesh, this results in localized irritation and an itching sensation.
There are a few species of polychaetes (e.g., the Caribbean fireworm, Hermodice carunculata) that have venom glands associated with the setae. This worm’s “sting” can cause irritation, inflammation, numbness or a burning sensation (on very rare occasions, fireworm setae have resulted in the amputation of a finger due to secondary infection). While most bristleworms are not venomous, all should be treated with extra care and respect just in case. Wear thick rubber or neoprene gloves when handling live rock in order to prevent contact with bristleworm setae.
Bristleworms were once the bane of reefkeepers, not because of the noxious bristles but because they were thought to be a threat to soft corals, stony corals and clams. This attitude has shifted as a result of continuing education and understanding. It is now known that most of these worms are useful members of the reef aquarium community. They are effective scavengers that will ingest meaty food items that hit the substrate, and some species dig in soft substrates, which helps keep sandbeds from compacting.
While bristleworms may consume necrotic tissue, most do not prey on healthy ornamental tankmates as was once believed. That said, there are a few exceptions. For example, our aforementioned friend, the Caribbean fireworm (H. carunculata), is a larger species that can reach a length of at least 14 inches. The Caribbean fireworm will feed on sea anemones, hydrocorals (e.g., fire corals), gorgonians and stony corals (e.g., Acropora spp.). Fortunately for us, this animal is not a common live rock vagabond. It is, however, often confused with the more benign Eurythoe complanata. Eurythoe complanata is considered to be cosmopolitan, and it is a more common accidental introduction to the reef aquarium than the dreaded H. carunculata. Like H. carunculata, E. complanata can reach impressive lengths (at least 10 inches).
If you surf reef aquarium forums, you may see posts about bristleworms preying on tridacnid clams. This is a rare occurrence that is more likely a case of a worm scavenging on the necrotic tissue of a dying clam rather than attacking a healthy tridacnid. I have never seen one of these worms bothering a clam. Even more compelling testimony comes from tridacnid expert James Fatherree, who states that he has “never seen any evidence whatsoever of a bristleworm attacking a healthy clam in any way.” (Fatherree, James W. 2006. Giant Clams in the Sea and Aquarium. Liquid Medium Publ., Tampa, Florida. 227 pp.)
While most species usually do not cause problems in the reef tank, polychaetes can get out of control in aquariums where the owners dump in too much food. I have seen overfed aquariums that lacked predators of polychaetes where the substrate was writhing with bristleworms. There were so many of these worms that it was kind of creepy!
If you end up with too many bristleworms in your aquarium, you can use a number of different techniques to remove them. You may want to start by sucking out the worms you can see with a siphon tube when you do your regular water changes. You can also use a fish trap (I use a cylindrical acrylic trap with a sliding door at one end) to reduce the bristleworm population. Simply place a chunk of fresh seafood (scallop works well) in the food compartment at the end of the fish trap, turn off the aquarium and room lights, and leave the trap in the tank for a few hours. If you have worms in your tank, there will be worms in the trap when you come back. Drop the trap door closed, remove the trap, put the worms into a bag and throw them away in the trash.
Some people use more “sophisticated” bottle traps or nylon stocking booby traps hoping that the worms’ setae will get caught up in the stocking material, but simple fish traps work well. If you have larger worms that lurk about your tank, you can also grab them with a pair of tweezers or a fish net. This can be done after dark when most of the fireworms come out to feed or after food is added to the tank. Food will often induce these normally secretive beasts to come out and forage.
The worms have natural predators that, if added to a reef aquarium, can control the bristleworm population. Some of the most effective are arrow crabs (Stenorhynchus seticornis), serpent stars, sand stars, a variety of wrasses (e.g., Pseudocheilinus spp., Halichoeres spp.) and larger dottybacks. Some of these predators also eat the more desirable tubeworms, as well.
Tubeworms (Families Sabellidae and Serpulidae
The tubeworms are the most popular of the polychaetes. Larger species are intentionally purchased and added to the reef tank because of their elegant forms and lovely colors. Unfortunately, they often do not fare as well as their errant brethren, the bristleworms.
Tubeworms are sedentary polychaetes. They form a tube in which the body is hidden, and they have a spiral crown that projects from the tube. The crown is comprised of featherlike structures called radioles, which are used for suspension feeding and respiration. When threatened, the worms rapidly roll up and pull the radioles into the tube. Even though tubeworms can withdraw the crown rapidly, certain fish species may nip off radioles before they can be retracted. The body tube, which the worm creates, is comprised of sand grains and mucous (soft tube) in many sabellids (e.g., feather duster worms, Sabellastarte) or is calcareous (hard tube) in the serpulid worms (e.g., Christmas tree worms, Spirobranchus; and coco worms, Protula). Some Christmas tree worms have an operculum that plugs the end of the tube when the worm withdraws into it. Not only do the calcareous tubes provide shelter for these worms, the empty serpulid tubes serve as a home for a number of different blennies, including the tube blennies (family Chaenopsidae).
If you purchase a larger sabellid feather duster worm, the first thing you need to consider is placement in the aquarium. I recommend drilling a hole in a piece of live rock (with a masonry drill bit) and placing the worm in its soft or calcareous tube into the hole. You could also place the worm in preexisting holes or cracks in the hard substrate (they can self-attach). While they need current to push food items past their radioles, a strong, direct water jet can damage them and prevent them from unfurling the feeding crown. When selecting a feather duster, never purchase an individual that has crawled completely or even partially out of its tube. These individuals are not likely to live long.
The key to keeping these animals healthy is properly feeding them. If you are not willing to frequently feed your tubeworms (of any species), avoid purchasing these animals, as they are going to slowly starve to death in your aquarium. The larger species of feather duster and coco worms feed on phytoplankton, small zooplankton (including ciliates, rotifers and invertebrate larvae) and suspended organic detritus. The smaller species like the Christmas tree worms ingest bacterial floc (i.e., a chemical precipitate) and minute particles of suspended organic detritus. The best way to provide nutrients in the aquarium is to feed phytoplankton, small zooplankton and liquid foods for suspension-feeders (larger worms need larger food items). Smaller species of feather duster worms (those that often come in on live rock) tend to do better in home aquariums than larger species, which have higher metabolic needs and thus need to be fed frequently. When target-feeding, direct the food to the side and slightly below the feeding crown – do not direct it into the center of the feeding crown.
Shrimp and crabs sometimes feed on these worms. Sea urchins have been implicated in chewing through the sedimentary tubes of sabellids. If stressed, the tubeworms will shed the feeding tentacles, which will typically regrow in several weeks. They may lose their feeding tentacles if underfed, in which case the regenerated tentacles will be smaller. These worms are good plankton-feeding indicator species because they are so effective at suspension feeding – if your feather dusters are starving, then any other animals in the tank that rely on this feeding strategy will be in even worse shape.
The beautiful Christmas tree worms are often sold with their coral associate (often Porites spp.). Porites may live a long life in the wild (20 years or more), but they are typically short-lived in captivity because they do not get enough to eat. One common misconception is that Christmas tree worms rely on their coral host for food, and if the coral dies, the worms are soon to follow. This is not the case. Instead of using the coral for food, they embed themselves into the coral skeleton and use it as a growing surface. This protects them from predators.
That ends my look at some of the more commonly encountered polychaete worms. While the word “worm” may immediately turn you off to these animals, many species are welcome additions to the home aquarium because of their attractive appearances or because they serve a utilitarian function in reef tanks. Some, however, may have special care requirements, so before acquiring one, consider the time and energy necessary for keeping some of the more demanding species. Until next time, happy worm-watching! AFI
Scott W. Michael has kept marine fish for more than 25 years. He is the author of Reef Sharks and Rays of the World; Reef Fishes: A Guide to Their Identification, Behavior and Captive Care and more.