The choice of keeping or eradicating Medusa worms in reef tanks has been a popular controversial topic on online reefkeeping forums over the years.
To put it in a nutshell, on the one hand, Medusa worms are harmless scavengers that help the reef aquarium keep nice and clean. On the other hand, their body contains some chemicals that can cause a lot of problems to the tank.
Medusa worms are a fascinating group of legless sea cucumbers that often find their way into saltwater tanks as hitchhikers— clinging to corals, sponges, or décor, whereas many aquarists tend to visit local aquarium stores in search of them probably because of their uniqueness, simplicity, and cleaning habits.
Are you undecided whether to keep Medusa worms in your reef tank? or lacking adequate knowledge of the pros/cons, whether they are reef-friendly and what they eat, all these and more will be covered in this article.
Origin of Medusa Worm
Medusa worms are legless sea cucumbers. These unusual creatures can be traced to the family Synaptidae in the Class Holothuroidea and the Phylum Echinodermata. Essentially, all sea cucumbers are echinoderms and members of the Class Holothuroidea.
Holothuroidea comprises three sub-classes:
- the Apodacea (group of slender, legless sea cucumbers which includes the renowned Medusa worm),
- the Apidochirotacea (deposit-feeding sea cucumbers e.g. the popular tiger-tail cucumber, Holthuria spp.) and
- the Dendrochirotacea (filter-feeding sea cucumbers such as the notable sea apple, Pseudocolochirus spp.).
Medusa worms (Synaptids) inhabit coral reefs throughout the Carribbean and Indo-Pacific, and they can be found in large numbers lying on the deep seafloor or sandy reef bottoms.
Description of Medusa Worm
This particular sea cucumber resembles a giant worm from which an array or mop of feeding tentacles are constantly being whipped across the substrate before being retracted into the mouth. The name ‘’Medusa worm” stems from the prominent tentacles which are similar to Medusa Gorgon of Greek mythology.
They have long, elongated worm-like brown/tan bodies (soft and cylindrical) immersed in sticky cells that help them adhere to any surface with ease. Medusa worms are small in their earliest stages, but over time they grow very big and long, some species can even grow up to 6 feet (2 meters) in the wild.
In addition, they have a highly reduced skeletal system; this skeletal system is composed of a series of tiny calcareous plates (ossicles) embedded in the skin of the sea cucumber.
Their shape is specially adapted for crawling and burrowing through the sediment, sometimes in a pattern similar to that of earthworms, and their mouth is surrounded with 10-25 pinnate or peltate tentacles.
Unlike many other Echinoderms, the Apodids are legless because they do not possess tube feet, and the absence of this feature gives the order its name Apodida which means “without feet”.
Moreover, they also lack the complex respiratory trees and most of the associated structures found in other sub-classes of sea cucumbers. Therefore, they respire and secrete nitrogenous waste through their body surfaces. How cool is that?!
The most common genera seen in the aquarium trade are Euapta, Synapta, Synaptula, and Opheodesoman; they are all quite similar— soft and flaccid with large rounded knobs along the length of the body.
Behavior of Medusa Worm
Notably, the muscular hydrostatic body and anchor ossicles of the Medusa worm enables it to propel forwards.
Locomotion of this sea cucumber is made possible by sticking at their base and squeezing their flabby body into an elongate shape. Through this series of activities, the Medusa worm can crawl about, burrow, and extend or retract its body easily.
Medusa worms are also capable of moving around the aquarium, or rapidly withdrawing into tight spaces such as a crevice when it senses any serious threat.
Feeding Medusa Worm
Medusa worms are detritivores and excellent grazers, and they always crawl on the sand bed with their feeding tentacles extended as they rummage for detritus and organic matter to consume.
In nature, they play the roles in ecosystem as decomposers and nutrient releasers in the food chain.
While they are a good sifter of bedding and filter-feeding, also, some aquarists do feed their Medusa worms crushed shrimp pellet fish food, and they do enjoy ingesting it.
These legless sea cucumbers may also spend time cleaning the underside of rocks, while certain species will nibble on diatoms in the reef tank (if any).
Is Medusa Worm Reef-Safe?
If we are talking about corals, then – Yes, they are. There are no reports of the Medusa worms causing problems with corals. They will not harm corals in any way.
Medusa Worm and Toxins
They do not possess the Cuvierian tubules commonly found in some species of sea cucumbers such as Actinopyga, Bohadaschia, Holothuria, and Stichopus; these tubules are capable of releasing a highly potent toxin known as holothurin.
However, they still possess a variety of lethal chemicals associated with the skin and body wall to protect them from being eaten or harmed by crabs, fish, and lobsters dwelling on the coral reef. These chemicals/toxins are released when the Synaptid cucumbers are under severe stress, for example, if it is injured after being sucked into a powerhead or overflow aperture, or threatened by predators.
Important: Protect your filter intake or powerhead! There are several known cases when Medusa worms got stuck in powerhead and got cut in half. The next day almost every fish and invert were dead.
Pros and Cons of Medusa Worm in Reef Tanks
This fascinating marine invertebrate will help in keeping the reef tank clean by feeding on tiny particles of organic detritus on the bottom of the tank.
Another merit of keeping this critter is because of the diversity it adds to your ecosystem. It’s great to have a variety of marine creatures in your display, with each one actively showcasing their unique personality, and the Medusa worm packs a whole bunch.
Medusa worms may be effective at cleaning the sand bed and all that. Yet it still poses a threat to the tank inhabitants. As mentioned earlier, when the Medusa worm is severely stressed as a result of a serious injury or attack by a predator, it will release toxins that are highly lethal to fish.
During its activities, the Medusa worm might cause harm to the tissue of corals or other tankmates using its ossicles as it retracts and attaches them to surfaces.
Also, some species of Synaptids are capable of reproducing massively in your reef tank if conditions are suitable, and that would be quite messy.
Medusa Worm Toxin and What to Do
In situations like this, your fish can be saved from the mishap by changing a large percentage of the tank water immediately, in addition to running a protein skimmer and using activated carbon to minimize the effect of the toxin.
If you need to remove the Medusa worms from your reef tanks, you have to be careful. Their body tissue is sticky everywhere, and very delicate so avoid the mistake of pulling it off whatever surface it’s on.
For small ones, use a turkey baster or a siphon to pull them out. For big ones, use tweezers, but do not squeeze too much! Otherwise, there is a chance to break them in half while plucking them off.
Medusa Worm and Emperor Shrimp Partnership
Additionally, certain cleaner shrimps e.g. Periclimenes Imperator (Emperor shrimp) have a commensal relationship with the Medusa worm.
Note: To be precise, the Emperor shrimp form what are termed commensal relationships with various species of nudibranch and sea cucumbers.
This symbiosis serves for feeding, defense, and transportation.
While Medusa worms protect the Emperor shrimp from its predators by providing shelter, the shrimp clean the host by eating the food scraps and mucus off the body surface.
In its entirety, medusa worms are not completely reef-safe. You could wake up someday and discover that your aquarium fish have died from asphyxiation due to the toxins expelled from an injured or extremely stressed medusa worm.
It’s best not to have this marine invertebrate in your tank at all. But if you insist, then you need to research properly about the species you want to obtain. Provide optimal water conditions, and protect the pump intakes and overflow drains to prevent the worm from getting sucked in.
Furthermore, add them to well-established tanks where they can easily access organic detritus at any given time; or else they will starve to death.
If you no longer fancy the existence of Medusa worms in your saltwater aquarium, you can manually remove them with your hands or introduce species like pufferfish, triggerfish, wrasses, and hermit crabs to devour them.
However, do remember to move the fish and other tankmates to a different tank before introducing the natural predators, the purpose of this is to prevent contact with the toxin.