When Getting Slugged Is The Goal

Blue Velvet Flatworm Eating Nudibranchs are interesting in many ways, not the least of which being that they are not, in fact, Nudibranchs at all. (Also, not velvet, they do eat the heck out of the flatworms though, this part is the truth.) They are most accurately described with the name Chelidonura varians, and are actually a Sea Slug (Opisthobranchs). They are part of a larger group of critters we commonly call Headshield Slugs, so named for their hammer headed shape. This is a species with a huge variety of common names, in English, they are most often called: Blue Doris, Blue Velvet Headshield Slug, Blue Velvet Sea Slug, Blue-lined Slug, Blue-lined Sandflat Slug, Hammerhead Sea Slug, Tailed Slug, as well as multiple other combinations of the above words, and probably many more variations in other languages.

Sea Slugs (and Nudibranchs) are not the most common things to see in a marine aquarium. Even when they are in a display, most of them are small and their diminutive size keeps them mostly out of eyesight. Many, if not all of the commonly seen ones are monotrophic, meaning they only eat one thing. The name of the Blue Velvet Flatworm Eating Nudibranch (Sea Slug) should be all the hints you need about what this one eats. This diet is both the reason this slug is popular, and why it is commonly difficult to keep long term. Chelidonura varians requires flatworms to eat.

For many years, certain varieties of flatworms were a death sentence for a reef tank as they can reproduce quickly, and can quickly eat, or cover (also killing) every last bit of coral in an aquarium. There is a wide variety of flatworms that can reach plague proportions in an aquarium; a deep dive into them is coming.

There are a few popular medical treatments for most flatworms, but they all will require you to remove the corals from your aquariums and keep them in treatment tanks. The process of moving and increased handling of them, changing their flow and lighting is enough to cause a high percentage of your more sensitive corals to die. The medicine will frequently add 20% mortality on top of the handling impacts.

The only way effective way to treat for flatworms without this disruption is by adding effective predators, and this is where the Blue Velvets come in. Chelidonura varians is a flatworm vacuum. They will track down flatworms and mow them down at an astonishing rate. They are most commonly, and most effectively used for removing Red Planaria Flatworms (Convolutriloba sp.). Red Planaria flatworms reproduce readily both sexually via egg laying and asexually via budding and both mechanisms are very successful in-home aquarium environments.

This diet of only flatworms is also the reason that Chelidonura varians is sometimes dismissed as a reasonable option for fixing flatworm invasions. Once the Flatworms are gone, the Blue Velvets run out of food and will die unless removed. Many local fish stores are happy to take them back once the job is done, just don't expect to get credit back for them. Think of their cost more as a rental, than a purchase. If your Nudibranch starts to lose its color, it has run low on food, and you should remove it from your display. While the slug isn't toxic in and of itself, it has digested many thousands of flatworms who are toxic once dead, and if the Sea Slug dies, these toxins can be released into your aquarium. Occasionally, Chelidonura varians will slow down before the job is done. These can be held in another tank, or just separated from the system for a day or two to digest and get hungry again.

Other than this, these beautiful little creatures are simple to keep. If you have been successfully keeping corals, you have hit all the targets a Sea Slug will need. They like consistent temperatures in the mid 70s Fahrenheit, a specific gravity of 1.025 or so and low nutrient levels. They mostly are indifferent to lighting. They are susceptible to shifts in salinity, so be sure to acclimate them slowly to new displays. Chelidonura varians is a very peaceful critter, but some fish will try to pick at it. Most will avoid it, as the coloration of the Slug is a warning to most fish, but some large wrasses are notorious for trying to eat anything (looking at you Thallasoma!)

Chelidonura varians will readily lay eggs in your aquarium, but they have proved pretty much impossible to rear thus far. If you want to see this happen in your aquarium, you will need two individuals to get fertile eggs.

For those of you wishing to keep Chelidonura varians beyond the termination of your flatworm problem, you will actually need to culture flatworms. Yes, this seems counterproductive, but these are a beautiful little slug, worth keeping for their aesthetics. Red flatworms can easily be cultured at home, and you'll just need another aquarium to do so in. Just set up another small tank with similar conditions and lighting, add some of your flatworms and their population will explode as long as there isn't a predator around.

So, if you've noticed some Red Flatworms in your tank, do not wait until they reproduce into huge numbers. Add your Flatworm Eating Blue Velvet “Nudibranchs” now and stop their reproduction before it becomes a plague. Ask your Local Fish Store if they'd take them back when the job is done, and other hobbyists will get to use their services as well!