Suzanne C. Mills, Ricardo Beldade, Greg Moutardier, Mehdi Adjeroud, Pascal Dumas
Limes evoke different emotions for different people: as an important accompaniment with raw seafood for some and for others with a Gin and tonic; however, for none do coral-eating crown-of-thorns seastars come to mind.
The lime, Citrus aurantifolia, is native to Malesia but was introduced by early Europeans into the majority of Pacific islands. Since then Pacific islanders have embraced limes in their diet and common names include: taporo (Tahiti); ihitoro (Marquesas), moli tupolo (Samoa); moli laimi (Fiji); laim (Marshall islands); remong (Yap); moli laimi (Tonga) and malchainged (Palau). Such nomenclature is indicative of their widespread use and availability throughout Pacific islands.
Also widespread through the Pacific and indeed worldwide, from Panama to the Red Sea, are outbreaks of the coral-eating crown-of-thorns seastar (COTS), Acanthaster planci. COTS outbreaks may reduce live coral cover immensely to values below 1-5% and only end when most palatable corals have been consumed. However, in the absence of other disturbances reefs show remarkable resilience and can recover to pre-outbreak levels of coral cover in around 10-15 years, but I stress, only in the absence of other disturbances.
COTS outbreaks greatly impact ecosystem services including subsistence fisheries and the primary source of revenue in Pacific island nations: tourism. As such local communities and/or governments have developed a range of methods to halt such outbreaks. There is some debate as to whether such interventions are beneficial for the reef in the long-term or whether the cyclic outbreaks are part of the natural succession of coral reefs and hence they should be left to run their course. Currently there is no evidence concerning the impact of human interventions on the reef, however, it is predicted that the resilience of coral reefs to COTS outbreaks is impacted by anthropogenic disturbances such as overfishing and climate change and as such further human intervention is required.
Numerous approaches have been developed over the last few decades, but manual collection followed by disposal ashore is the most commonly used across the Pacific. However, hand-removal may not be appropriate for severe outbreaks and/or large affected areas. Injection approaches –where COTS are injected with a variety of noxious solutions– are increasingly used, as they are highly cost-effective and fairly safe when handled correctly. However, they also include drawbacks: most solutions injected over recent decades were not only noxious for COTS, but for the coral community as well, or have potential knock-on effects on the coral-associated community, and also not always cost-effective.
In Vanuatu, a new COTS control alternative was tested based upon acidic injections of a cheap, 100 % natural product. Small volumes of pure fresh lime juice injected into COTS caused fast high mortality. Ten to 20 litres of lime juice could kill up to a thousand COTS at a cost of less than 5p per specimen. In stark contrast to previous alternatives, contagion to either conspecifics or a variety of other reef species was not observed, even at high COTS densities. Injections of lime juice offer great advantages when compared to current best practises and constitute a cheap and natural option for all countries affected by COTS where lime trees are a ubiquitous part of the landscape.
Moutardier G, Gereva S, Mills SC, Adjeroud M, Beldade R, Ham J, Kaku R & Dumas P. 2015 Lime juice and vinegar injections as a cheap and natural alternative to control COTS outbreaks. PLoS One 10(9): e0137605. doi:10.1371/journal.pone.0137605