Turtleweed (Chlorodesmis fastigiata) is a common and widespread soft, upright filamentous green macroalga ("seaweed") in the tropical Indo-Pacific and waters off eastern Australia, including the Great Barrier Reef (Huisman 2000). Turtleweed occurs both inshore and on coral reefs farther offshore (Schaffelke 1999). It grows as thick tufts or patches, often surrounded by live coral tissue. Although common, these algae are rarely if ever abundant or extensive, with patches rarely growing larger than 20 to 30 cm in diameter. Despite being highly conspicuous, Turtleweed patches are able to persist even in areas with relatively high levels of herbivory, probably because the filaments contain high levels of a secondary metabolite with herbivore-deterrent properties. (Jamaluddin and McCook 2003)
Turtleweed has been a focus of several studies in recent years examining the potential impacts of macroalgae on coral reefs. In recent decades, some reefs that were formerly dominated by coral are now dominated by macroalgae. There has been significant concern that interactions with some species of macroalgae may suppress the recovery of some coral species following disturbance or damage. Rashera et al. (2011) found that numerous species of macroalgae directly damage corals by transferring competitor-suppressing hydrophobic allelochemicals present on algal surfaces. These hydrophobic compounds, which include two acetylated diterpenes from Turtleweed, cause coral bleaching, decreased photosynthesis, and even coral death. Some studies have found negative impacts on corals by Turtleweed and some other algae even without physical contact. Birrell et al. (2008) found that the presence of nearby Turtleweed delayed the settlement of larvae of the coral Acropora millepora, apparently via some sort of waterborne effects.
Bender et al. (2012) found that Turtleweed significantly reduced tissue recovery in one coral species studied, Acropora pulchra, but not in the related A. aspera. The presence of Turtleweed led to the infection of A. pulchra with ciliates, possibly due to an algal-driven impairment of the coral's immune systems.
Mantyka and Bellwood (2007) studied the impacts of grazing herbivorous fishes on a dozen species of macroalgae on the Great Barrier reef. Turtleweed was one of two algal species that were strikingly less impacted by grazing than the other ten algae studied. In studies examining the feeding-deterrent effects of algal extracts and isolated metabolites, the extracts of Turtleweed and the isolated metabolite chlorodesmin were significant feeding deterrents to mixed-species populations of herblvorous fishes (Paul et al. 1990 and references therein). Interestingly, chlorodesmin deterred adult but not juvenile rabbitfish(Siganus argenteus) (Paul et al. 1990).
Dixson and Hay (2012) documented a remarkable mutualistic relationship between a coral, Acropora nasuta, and two goby fishes, Gobidon histrio and Paragobidon enchinocephalus, recruited by the coral to keep it free of Turtleweed. Within minutes of Turtleweed (or even a chemical extract from the Turtleweed) contacting the coral, the coral releases an odor that recruits gobies to trim the Turtleweed and dramatically reduce coral damage that would otherwise occur. Interestingly, only one of the two goby species, G. histrio, actually consumes the Turtleweed it removes. This goby normally produces toxic skin secretions that repel predators and contact with the Turtleweed appears to increase their toxicity.
Franklin and Lasrkum (1997) investigated the adaptations of Turtleweed for the high-light conditions at the tops of coral reefs.
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