Not all organisms have a symbiotic relationship, such as the one between zooxanthellate and coral. A relationship you need to worry about in your reef systems is parasitism. The relationship between coral and flatworms is a parasitic relationship that is quite common in reef aquariums, as this particular parasite can remain undetected for quite some time due to their camouflaging strategy, allowing them to reproduce extensively before detection. The specific parasitic relationship between flatworms and coral is known as kleptoparasitism because flatworms are suspected of stealing resources from coral by eating their symbiotic algae, zooxanthellae. Flatworms often block out light that is essential for coral tissue. Additionally, flatworms feed directly on the coral host and damage coral tissue.

Although flatworms use a camouflaging strategy, we can still identify them in our coral systems by looking out for several characteristics. There are over 8,500 species of coral, so flatworms color can vary from tan, to brown, black, white, or transparent in accordance with their preferred host. They have soft, elongated, and flattened bodies. They have a two-way digestive tract, having one orifice for eating and excreting waste. Their bodies are also bilaterally symmetrical, meaning the right and left sides of their body are identical. They lack respiratory, circulatory, and skeletal systems. Flatworms are hermaphroditic, meaning they have reproductive organisms of both sexes in one individual.

Once a flatworm is identified in a saltwater tank containing coral, it is crucial to make efforts to eliminate them. As mentioned, they are hermaphroditic, so they can multiply quickly and overrun coral. Another reason to quickly eradicate this pest is that they feed on microfauna of live rock, destroying the biological filtration of your system. Moreover, they have the potential to kill all the coral and fish in a system if they get crushed in pumps or other equipment because they release a deadly toxin into the water column when they die.

In order to avoid problems with flatworms, maintaining low nutrient levels can help minimize the outbreak of flatworms. Low nutrient levels can be obtained through protein skimmers and carbon reactors. Natural control methods can be used to eradicate flatworms as well by incorporating certain species of wrasse. Some species of wrasse that can be used are leopard wrasse, six-lined wrasse, and canary wrasse (yellow wrasse). Other methods of eradicating flatworms from coral include manual removal of them or chemical treatment, depending on the severity of the outbreak.

Maintaining a coral system free of parasites and pests is pivotal to raising healthy coral and increasing the lifespan of all species living in that system. Checking coral daily for these pests is ideal but routinely checking at least once a week is necessary.