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International Waters: “Flat” Aquarium Pest Found in the Great Barrier Reef

Posted: Oct. 19, 2012, 7:00 p.m. EDT


Mention Amakusaplana acroporae to most marine aquarists or marine industry specialists and they are likely to have no idea what you are talking about. However, mention AEFW or, to give it its full label, Acropora-eating flatworm, and the chances are that they will know perfectly well what you are talking about.

The species is a tiny, almost transparent flatworm that, as its specific name indicates, is found on Acropora corals. Normally, such coexistence does not lead to any problems, but as numerous marine aquarists have discovered to their detriment, heavy infestations can cause serious damage to these corals. Adding to these problems, the flatworm also lays its eggs on Acropora, which if left unchecked can result in the eventual death of the colony.

At least, this is the story as far as aquarium corals are concerned. Its effect on wild populations is, however, unknown. In fact, there are considerable gaps in our knowledge of A. acroporae in the wild. We don’t even know what its natural range is. According to Kate Rawlinson of the Biology Department at Dalhousie University in Halifax, Nova Scotia, Canada (one of the scientists responsible for describing the species in 2011), “Given the Amakusaplana acroporae’s preference for Indo-Pacific Acropora species, it is assumed that the worm is endemic to that region.”

adult Amakusaplana acroporae
An adult Amakusaplana acroporae (1.3cm long) on an Acropora coral with feeding scars (white circles) to the right and above the flatworm. The image shows how well-camouflaged the pest is against the coral. Courtesy of Kate A. Rawlinson
The small size of this flatworm (3 to 6mm) and its cryptic coloration, which allows it to blend in with its host, make it very difficult to spot on the reef, but it obviously exists there, though its occurrence has never been reported. Its presence in aquaria is a different matter altogether, so much so that the species has long been referred to as a serious aquarium pest.

Now, for the first time, A. acroporae has been reported from the wild, but not from what was assumed to be its home range in the Indo-Pacific. Rather, the report comes from Australia’s Great Barrier Reef (see below for full reference). Ten colonies of A. valida were collected from Lizard Island in the northern region of the GBR and were flushed out to dislodge any attached micro-fauna that might be sheltering or living among their branches. Although none of the colonies showed any signs of tissue damage, this procedure revealed an assortment of tiny creatures, among which, to the researcher’s surprise, were specimens of A. acroporae. Seven out of the 10 colonies contained the flatworms, with numbers ranging from one to five individuals.

Interestingly, the gut contents of the worms contained Symbiodinium, the most important genus of micro-algae (zooxanthellae) that live in symbiosis with corals, and spirocysts (coral ”stinging” cells), indicating that A. acroporae is indeed a corallivore (coral eater). Yet the wild coral colonies appeared undamaged, most probably because the other micro-fauna found among the coral branches included small coral crabs (Tetralia nigrolineata), coral gobies (Gobiodon brochus) and palaemonid shrimp (Coralliocaris graminea), which may control Amakusaplana by preying on the worms and/or their eggs.

The discovery of A. acroporae so far away from its presumed home in the Indo-Pacific, allied to its high incidence in aquaria, might lead some to assume that aquarists, or the marine aquarium industry, may be responsible for the presence of the flatworm in the GBR.

This assumption, however, would be totally unjustified. Far more plausible is the likelihood that A. acroporae occurs quite naturally in Australian waters. The fact that it hasn’t been reported earlier is probably the direct result of it having escaped detection. Now that we know it’s there, though, the chances are that there will be further reports from the GBR and elsewhere.

Having seen the photos and read the description of the creature’s distinguishing features, it seems to me that there’s also a possibility that the A. acroporae from Lizard Island could turn out to be a separate species. Certainly, the researchers report several detectable differences between the specimens they collected from the wild and those obtained from aquaria. Irrespective of this, they believe that finding the flatworm in the wild opens the door to further studies “of its biology and ecological interactions, and this, in turn, could lead to the discovery of effective biological controls for this corallivore in captivity.” 

Acknowledgement and Footnote: I would like to extend my sincere thanks to Kate Rawlinson for supplying me with the photograph of A. acroporae that accompanies this report, and for granting me permission to use it.

Kate also informed me that she and her colleagues are continuing to work on the species, which they believe to be “of commercial, ecological and evolutionary importance.” Interested readers requiring further information may contact Kate at k.rawlinson@dal.ca.

Reference: Kate A. Rawlinson, Jessica S. Stella (2012): “Discovery of the Corallivorous Polyclad Flatworm, Amakusaplana acroporae, on the Great Barrier Reef, Australia—the First Report from the Wild.” PloS ONE 7(8): e42240.doi10. 1371/journal.pone.0042240

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