Abstract
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.
Highlights
The majority of marine invertebrates produce dispersive larvae which, in order to complete their life cycles, must attach and metamorphose into benthic forms
In assays performed in the Great Barrier Reef (GBR), WA, Singapore and Guam, TBP significantly induced larval settlement and metamorphosis of Acropora millepora, A. tenuis, A. globiceps, A. surculosa, and Leptastrea purpurea (p < 0.01, Fig. 1) compared to the 0.2 μ m filtered seawater (FSW) control
We investigated the ecological plausibility of Pseudoalteromonads and TBP as a biofilm-derived larval settlement cue, which has been proposed as a compound of widespread ecological importance for coral larval settlement in the Caribbean[12]
Summary
The majority of marine invertebrates produce dispersive larvae which, in order to complete their life cycles, must attach and metamorphose into benthic forms. Our understanding of the chemical ecology of surface colonization in marine benthic systems is far less advanced than studies of chemically mediated plant-herbivore or predator-prey interactions, or of deterrents of larval settlement[7] This is because, unlike these other interactions, few inducers of invertebrate larval settlement have been chemically characterized, quantified in situ and shown to induce settlement at naturally occurring concentrations[8]. In the case of Caribbean coral species, TBP induced comparatively high levels of settlement and metamorphosis, leading to the proposition that TBP was a compound of widespread ecological importance for larval settlement[12] This and our previous studies observed that TBP and TBP-producing bacteria triggered variable degrees of larval settlement and metamorphosis and larval metamorphosis without attachment[4,12], i.e. larvae often did not accomplish the complete transition to an attached, metamorphosed benthic juvenile
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