Identification of Common Worms in Saltwater Aquaria

Appearance: Feather dusters are filter-feeding worms that live in a paper-like or cement-like tube (often encased in rock or the sand.)  The “feathers” are called radioles, and cilia (finger-like projections) on the radioles move captured food particles and sediment to the center.  The sediment is used to build the tube. 

Behavior: Most that come in as hitchhikers will do just fine in the home aquarium (like the ones shown encased in the liverock), but larger ones sold at stores (like the “Coco Worms” shown below) often slowly perish in all but a few aquariums (likely from a lack of food).  Two problems are frequently encountered with feather dusters:  they leave their tube or drop the crown of feathers (worm remains in the tube).  Dropping the crown may suggest unsatisfactory water conditions or starvation.  Be patient, and the worm will likely regrow the crown when conditions improve (assuming the conditions do improve quickly).  If the worm leaves the tube, do not try to shove it back in.  Just leave it alone, and it will find a new spot in most cases. Basically, if you have a feather duster hitchhiker, place it in lower flow (cilia are more efficient) and enjoy it! (And be mindful of predators, as certain fish may find these as a tasty snack.)

Taxonomy: “Feather dusters” are in the Phylum Annelida, Class Polychaeta, Order Sabellida, and Family Sabellidae (usually soft-tubed) or Family Serpulidae (usually hard-tubed).

“Cocoa Worms” are in the Phylum Annelida, Class Polychaeta, Order Sabellida, Family Serpulidae, Genus Protula.

Appearance: Christmas Tree Worms are gorgeous and live up to their names; they’re shaped like their namesake trees and come in a rainbow of colors.

Behavior: Changes to the local environment (e.g., sudden shade/light, different water flow) will cause these worms to quickly retract, which makes them a quirky novelty in the home aquarium. Unfortunately, their desirability in the hobby does not match their resiliency; they do not typically live long in the hobbyist aquarium (although, there are cases of explosive growth. Your mileage may vary.)

Taxonomy: Christmas Tree Worms are in the Phylum Annelida, Class Polychaeta, Order Sabellida, Family Serpulidae, Genus Spirobranchus.

Appearance: Spirorbid worms are super tiny, up to only a couple of millimeters in diameter. Their white shells curve around into a circle and are stationary upon the substrate (glass, shown here).

Behavior: These are essentially miniature hard-tube feather dusters that typically cover substrate in low-flow areas.  They are harmless filter feeders, but they can clog pumps and other equipment if their population gets out of control. And, they can create quite hard and sharp projections that can easily scratch skin, so use caution around them while conducting maintenance!

Taxonomy: Spirorbid worms are in the Phylum Annelida, Class Polychaeta, Order Sabellida, Family Serpulidae, and Genus Spirorbis.

“Bristle Worms” from a common-name perspective also apply to “Fire Worms” and just about any other Polychaeta class worm, as most worms with the prominent bristles just naturally get the former name (think of it like the “Unidentified Flying Object” (UFO) nomenclature of the ocean. If it has bristles, it’s a “bristle worm”). However, I’m using them as two separate names here, as the reefkeeping hobby does to help distinguish risk between the two main polychaetes encountered. (Note: These orange “bristle worms” shown are not “Oenone fulgida,” which are predatory.)

I have to admit, these things terrify me (they always have and always will). My husband has had quite a few chuckles from watching me doing the “heebie-jeebie” dance every time I nearly touch one. The stings from touching one are quite painful but can typically be resolved with home care (of course, seek professional medical attention when needed).

Appearance: The bristle worms shown here are long (about 2-6 inches in length, although they can be much shorter or longer), slender, and are covered in bristles used for movement (and probably protection, as a dual-function). They’re typically orange (sometimes described as pink, but my eyes don’t see it), and some are more gray/blue.

Behavior: Regardless of their scary outer appearance, these worms are scavengers. They eat detritus and decaying matter (e.g., dying fish, fish excrement, leftover fish food). If you see a bristle worm on a dead fish, the fish was likely dead when the worm came along (these rarely kill fish). However, some of these can become insanely huge and predatory. Removal is best for extremely large worms (i.e., over six inches long), such as the one in the photo below. Typically these larger bristle worms have more of a gray and dull orange hue, rather than the bright orange of the smaller ones.

Taxonomy: Both “bristle worms” and “fire worms” belong to the Phylum Annelida, Class Polychaeta. This is a very broad description to fit the vast number of different “bristle worms” found in aquaria.

Compared to bristle worms, the fire worm is the dressy cousin that will eat everything in your home. Get it out. Thankfully, fire worms are not typically encountered in the aquarium hobby. In all of my years reefkeeping, I’ve never encountered one outside of scuba diving. In other words, if you are new to the hobby and find a scary looking worm, it’s probably just a harmless bristle worm.

Taxonomy: Both “bristle worms” and “fire worms” belong to the Phylum Annelida, Class Polychaeta.

Appearance: Dorvilleidae worms are small (about 1-2 inches in length), slender, segmented, and have hair-like structures used for movement.

Behavior: This cute little one falls under the “good” “bristle worm” category (actually, it’s technically a Eunicid worm), and similarly, is reef safe. It scavenges and feeds on detritus and often lives on the undersides of corals. In fact, I frequently notice some sneakily stealing food from my corals (but never in an aggressive manner).

Taxonomy: Dorvilleidae genera worms belong to the Phylum Annelida, Class Polychaeta, Order Eunicida, and Family Dorvilleidae.

Appearance: When looking for a true “bobbit worm,” look for a long, slender, and segmented worm that can range from an inch or so to well over a foot in length. They typically live in the sand or rockwork, hiding so as to ambush their prey. Look for a pinkish or reddish-brown hue on a darting worm that likely has a light-colored band below its head.

Behavior: Oh I’m sure I’ll get flamed for this one. As opposed to the nightmare-inducing “bobbit worm,” there are some Eunicid species that appear to stay small and non-problematic (e.g., the Dorvilleidae genera previously described). Like all opportunistic wild creatures, use extreme caution. The eunicid worm pictured lived in colonies of over a dozen under my corals, and they always stayed under two inches in length. While they didn’t directly bother my corals, they did frequently steal food from them. I monitored them for years with no negative indicators.

Taxonomy: Eunicid worms are in the Phylum Annelida, Class Polychaeta, Order Eunicida, and Family Eunicidae.

Appearance: With a quick glance, the “hair worms” are easily confused with “spaghetti worms” (the Terebellids), but hair worms have gills down the length of their bodies (the red squiggly lines are usually mistaken as tentacles or legs), whereas the spaghetti worms have gills extending from a single point.

Behavior: Hair worms free-live (no tube) within sediment and often extend their gills above the surface (along with their feeding palps). They’re harmless filter feeders and are considered a part of a healthy environment.

Taxonomy: Hair worms belong to the Phylum Annelida, Class Polychaeta, Order Terebellida, Family Cirratulidae, and Genus Cirratulus.

Appearance: Despite its small size, it can stretch quite a long way. I’ll admit, the first time I saw a striped one stretching its way through my rockwork (it looks like an elephant’s trunk when outstretched), I definitely overreacted (thinking it was a eunicid worm or something else predatory).

Behavior: This Sipunculid worm is a harmless filter feeder.

Taxonomy: Peanuts worms are in the Phylum Annelida, Order Sipuncula.

Appearance: Polyclad flatworms, such as this possible Pseudoceros sp. or Phrikoceros sp. one, are large, with some species reaching up to six inches. The ones most frequently encountered in the aquarium hobby are typically 3-4 inches in length and are translucent brown with a purplish hue.

Behavior: This is a very large, very predatory flatworm known to feed on snails and sometimes other mollusks (i.e., clams). They tend to camouflage well during the day and hunt at night. I had several of these come in on a coral, and it took about every chemical in my arsenal to kill them while in a dip, but I was unable to treat the eggs (photo of polyclad eggs). Once they make it in to the aquarium, they’re nearly impossible to remove chemically. Thankfully, several aquarists have reported success creating traps with frozen clams (although I have not personally confirmed this.)

Taxonomy: Polyclad Flatworms are in the Phylum Platyhelminthes, Class Rhabditophora, Order Polycladida. There are many different “polyclad” flatworms encountered in the hobby, such as the ones below, so this broad classification is intended to cover the most commonly encountered worms. If you have an identification for the worms below, please post in the comments! This will help in hobbyist identification of common worms.

Appearance: AEFW, as shown, are small (few millimeters), flat, and translucent-brown. These flatworms blend in very well with whatever color the Acropora is, so bite marks are usually the most easily distinguishable clue (and their eggs are visible but tiny – see photo).

Behavior: These Prosthiostomum acroporae [formerly known as Amakusaplana acroporae] flatworms eat Acropora corals and reproduce rapidly.  If your Acroporas look sickly, inspect them for small white bite marks.   

Controls/Treatments: Various treatments exist, to include some natural controls. For more information on their lifecycle, see the References section.

Taxonomy: Acropora-Eating Flatworms (AEFW) are in the Phylum Platyhelminthes, class Rhabditophora, order Polycladida, Family Prosthiostomidae, Genus Prosthiostomum, and Species acroporae. There may be other “Acorpora-eating flatworms” of other species; the described species is the one most commonly encountered in the hobby.

Appearance: Please forgive my atrocious photography here – and my Paint skills from back in whatever early-2000’s year it was. Acoel flatworms, like the one shown in green outline, are transparent, two-tailed reef-safe flatworms. Compared to “Red Planaria,” they’re two-to-three times larger. Note that these are *extremely* broadly named (reference Taxonomy), so they are often called “Ghost” flatworms due to their translucent appearance.

Behavior: They’re typically only found in small quantities and supposedly eat small pods. This is the only one I’ve ever knowingly encountered in my reefkeeping time.

Taxonomy: Acoel flatworms belong to the Phylum Xenacoelomorpha, Order Acoela.

Appearance: Contrasted to the aforementioned acoel flatworms, these flatworms are brown instead of clear and are much smaller. Similarly, they have a forked tail.

Behavior: Contrasted with the aforementioned acoel flatworms, these seem much more content higher in the aquarium toward the light (photosynthetic).

Controls/Treatment: If I had to guess, Heterochaerus sp. flatworms are probably endemic to a majority of reef tanks and are just controlled to the point of acceptability. Although they can certainly reach plague proportions (without causing harm), a good fish population seems to keep them practically unnoticeable. Typical flatworm treatments (e.g., Flatworm Exit) do not work on them, at least not within manufacturer’s protocol.

Taxonomy: Heterochaerus sp. flatworms are in the Phylum Xenacoelomorpha (note the controversy with Platyhelminthes, stated above), Order Acoela, Family Convolutidae, and Genus Heterochaerus. Note that these are often referenced as Amphiscolops sp. flatworms, as many species within Amphiscolops were recategorized to Heterochaerus. Please use official taxonomy references for the latest information.

Appearance: These are free-swimming larvae, only a couple millimeters long, that have a forked tail for jerkily-swimming through the water. In the aquarium, they appear like bright red pods and may be found along the surface of the water.

Behavior: If you need nightmare fuel, read about my encounter with these fluke cercariae – fish parasites that use snails as an intermediate host. Their jerky-swimming, bright coloration, and penchant for the water surface may all be attributes to help them become fish food (and subsequent parasites within the fish).

Taxonomy: Thanks to Dr. Pieter Johnson, these are likely a species of Bivesicula (possibly B. caribbensis). They belong to the Phylum Platyhelminthes, Class Trematoda, Order Plagiorchiida, Family Bivesiculidae, Genus Bivesicula.

Appearance: These worms were black and gray, only a few millimeters long, and were somewhat rounded.

Behavior: While I was cycling a fairly new tank, these extremely tiny black worms (shown on a piece of eggcrate) appeared in full force.  They appeared to only eat algae, and within a few weeks I never saw them again.  I assume they were herbivores, and as soon as the tank finished its cycle, the food supply ran out. I do not have an actual identification of them (yet). Other aquarists have noticed these in new aquariums as well, with no known long-term issues.

Taxonomy: They do appear to fit the description of “roundworms,” but their behavior (herbivorous) does not quite align. If you have an identification for the worms shown, please post in the comments! This will help in hobbyist identification of common worms.