Chemical mediation of coral larval settlement by crustose coralline algae.

The majority of marine invertebrates produce dispersive larvae which, in order to complete their life cycles, must attach and metamorphose into benthic forms. This process, collectively referred to as settlement, is often guided by habitat-specific cues. While the sources of such cues are well known, the links between their biological activity, chemical identity, presence and quantification in situ are largely missing. Previous work on coral larval settlement in vitro has shown widespread induction by crustose coralline algae (CCA) and in particular their associated bacteria. However, we found that bacterial biofilms on CCA did not initiate ecologically realistic settlement responses in larvae of 11 hard coral species from Australia, Guam, Singapore and Japan. We instead found that algal chemical cues induce identical behavioral responses of larvae as per live CCA. We identified two classes of CCA cell wall-associated compounds--glycoglycerolipids and polysaccharides--as the main constituents of settlement inducing fractions. These algae-derived fractions induce settlement and metamorphosis at equivalent concentrations as present in CCA, both in small scale laboratory assays and under flow-through conditions, suggesting their ability to act in an ecologically relevant fashion to steer larval settlement of corals. Both compound classes were readily detected in natural samples.

Interspecific transmission and recovery of TCBS-induced disease between Acanthaster planci and Linckia guildingi.

The susceptibility of the coral-feeding crown-of-thorns starfish Acanthaster planci to disease may provide an avenue with which to effectively control population outbreaks that have caused severe and widespread coral loss in the Indo-Pacific. Injecting thiosulfate-citrate-bile-sucrose (TCBS) agar into A. planci tissues induced a disease characterized by dermal lesions, loss of skin turgor, collapsed spines, and accumulation of mucus on spine tips. Moreover, the symptoms (and presumably the agent) of this disease would spread rapidly intraspecifically, but interspecific transmission (to other species of echinoderms) is yet to be examined. Vibrio rotiferianus, which was previously reported as a pathogen isolated from lesions of experimentally infected A. planci, was also recovered from Linckia guildingi lesions after several days of direct contact with diseased A. planci, demonstrating disease transmission. However, all L. guildingi fully recovered after 31 ± 16 d. Further studies are in progress to understand the ecology of Vibrio infection in A. planci and the potential transmission risk to corals, fishes, and other echinoderms to evaluate whether injections of TCBS could be a viable tool for controlling A. planci outbreaks.