FLATWORM WORRIES

 Acoel flatworms can reach plague proportions in the home aquarium. The photos shows Waminoa flatworms on a bubble coral.

The acoel flatworms are some of the most primitive animals on the planet. They actually lack a body cavity and a gut. Most of the flatworms we encounter in the aquarium trade are innocuous, causing no harm to corals. The acoels that are most often encountered living on mushroom anemones, soft corals and large-polyped stony corals are reported to graze on minute crustaceans, algae and detritus that adheres to the mucus of these cnidarians. When in small numbers, they do not appear to harm their host, but severe infestations can interfere with the photosynthetic activity of the zooxanthellae-hosting corals they infest.

Chelidonura varians is a flatworm assassin that will slurp acoels off of the substrate. Once the flatworms run out, it is going to starve to death. Do not mistake this species for Philinopsis gardineri pictured below, which does not eat flatworms.

Flatworms are easily transferred from an infected tank to a non-infected one on equipment and on live and dead substrates. If you buy a coral from a tank that contains them, you will want to try and remove them off before placing that coral colony in your display aquarium. There are a number of ways to do this, including siphoning the flatworms off the coral, dislodging them by swishing the coral colony back and forth in a bucket of tank water or by blowing them off with a powerhead, or by giving the coral colony a freshwater dip (usually no more than 10 seconds in freshwater that is the same temperature and pH of the aquarium water). The later can have deleterious affect on soft coral. There are also some liquid remedies available on the market that can be used to rid new cnidarian colonies of flatworms. However, if they are not used correctly (and even some times when they are) they can cause serious problems in your home aquarium.

There are a number of fishes that have been reported to feed on flatworms, including leopard wrasses (Macropharyngodon spp.), Halichoeres wrasses (Halichoeres spp.), tamarin wrasses (Anampses spp.), lined wrasses (Pseudocheilinus spp.), mandarin dragonets (Synchiropus spp.) and certain gobies (e.g., Amblygobius spp.). Do not expect any of these fishes to decimate a major flatworm infestation. It is best to place them in the tank before flatworms become a problem.

A Chelidonura varians imposter! Notice the difference in the "head"and "tail" shape of this non-flatworm eating species. This is Philinopsis gardineri.

SLUGS TO THE RESCUE! 

One of the most effective flatworm feeders is not a fish, but a beautiful sea slug named  Chelidonura varians - known as the blue velvet headshield slug. This slippery beauty is a flatworm assassin!  It will slide over the substrate, vacuuming up the acoels in its path. They have sensory bristles on eat side of the mouth that help them find their prey and they simply suck the flatworms into their mouth once they overtake them. Unfortunately,  once the food runs out, these specialized opisthobranch will starve to death.  If food is not in short-supply, or if they are not attacked by resident fishes (e.g., wrasses, sharp-nosed puffers) or sucked into filter intake tubes or into powerheads, the normal life span is short (around three or four months).  They have been known to spawn in captivity, but raising the larvae is difficult. When they reproduce, they produce a white, wavy egg mass and the eggs embedded in this structure will hatch in three or four days. 

There may be other members of the genus Chelidonura that feed on flatworms, like this beautiful C. hirundinina.

 There are other members of this genus, some of which are stunning in color (e.g., Chelidonura hirundinina), that may also eat these flatworms, but to the best of my knowledge, they have not been employed in this capacity before. One word of warning: make sure you don’t mistake C. varians with the very similar Philinopsis gardineri in coloration and general form. In the later species, the “tail” is rounded and not as long and filamentous and it lacks the hammer-like head found in C. varians. The diet of the two species are very different – P. gardineri feed only on an unusual opisthobranch known as the bubble shell s (family Aplustridae), not on flatworms!

One final word about C. varians, unfortunately these animals are not the easiest to ship and require meticulous acclimation. Drip acclimation is the best way to help this animal adjust to a new conditions. Also be aware that they don't have a long life-span naturally, so they may only last months even if plenty of food is present. 

©  Scott W. Michael- Reef Tectonics