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Flatworms compete with corals for zooplankton
Written by Tim Wijgerde   

Coral flatworms are well-known in the marine hobby and research community. Both in the wild and in captivity, they lurk between the tentacles of many corals. The exact nature of the symbiosis between corals and flatworms has long been unclear. New evidence is in agreement with the theory that symbiotic coral flatworms are in fact parasitic. Next to suffocating coral tissue and feeding on coral mucus, flatworms have now been found to compete with their coral host for zooplankton.

There have been many debates about symbiotic coral flatworms, especially about their possible nature in relation to corals. Aquarists have long regarded flatworms as a nuisance in marine aquaria, mainly due to their unaesthetic appearance when present in large numbers, and make use of a variety of methods to control captive populations.

Scientists have long wondered whether these so-called epizoic acoelomorph flatworms should be regarded as mutualistic, parasitic or commensal coral symbionts. Recent evidence published by Naumann et al. (2010) has shown that next to blocking light, epizoic flatworms feed on coral mucus. By labeling Fungia and Ctenactis coral mucus with a stable nitrogen isotope, they were able to retrieve its isotopic signature in the flatworms. By feeding on coral mucus, the flatworms may render the corals more sensitive to sedimentation, exposure to air, UV radiation and bacterial infections. A new study published in the scientific journal Coral Reefs has demonstrated that epizoic acoelomorph flatworms also compete with their host coral for zooplankton.

Competition between corals and flatworms

To demonstrate the ability of flatworms to compete with their coral host for zooplankton, marine biologists used single polyps of the scleractinian coral Galaxea fascicularis infested with flatworms (tentatively identified as Waminoa sp.). The coral polyps were incubated in a flow cell for 30 minutes together with Artemia nauplii (10,000 L-1), during which their feeding activities were recorded. The flatworms were found to capture Artemia by rising from the polyp surface and encapsulating their prey (see video below). 

Goniopora sp. with flatworms

Flatworms, here tentatively identified as Waminoa sp., are usually considered a nuisance in home aquaria (photograph: Tim Wijgerde).

This video shows several flatworms capturing Artemia nauplii by rising their anterior edges from the coral surface. Prey is rapidly immobilized after which ingestion and digestion are likely to occur. 

Although it seems that the flatworms only capture moderate amounts of zooplankton relative to their coral host, competition between corals and flatworms could prove to be highly disadvantageous to corals under low prey concentrations, as flatworms seem to be highly efficient zooplanktivores. This may be especially true in the wild, where ambient zooplankton concentrations can be as low as three zooplankters per liter of water (Palardy et al. 2006).

In conclusion, limiting captive flatworm populations may be appropriate after all to prevent detrimental long-term effects of symbiotic flatworms on corals. However, the negative view people have on flatworms is not entirely justified. These interesting animals are simply part of the reef ecosystem, and serve as a food source for predatory fish and nudibranchs. It is even possible that the flatworms secrete wastes that are absorbed by their host corals. It this were to be true, our view of the symbiosis between flatworms and corals would change yet again.

References

Naumann MS, Mayr C, Struck U, Wild C (2010). Coral mucus stable isotope composition and labeling: experimental evidence for mucus uptake by epizoic acoelomorph worms. Mar Biol 157:2521-2531.

Wijgerde T, Spijkers P, Verreth J, Osinga R (2011). Epizoic acoelomorph flatworms compete with their coral host for zooplankton. Coral Reefs DOI: 10.1007/s00338-011-0781-z.