A new study published in the open source journal PLoS ONE (Bongaerts et al 2010) sheds light on the connectivity of corals within and between reef habitats, with some pretty surprising findings. Whilst previous research has identified distinct differences in morphology and genetic structure over small spatial scales, these new findings from the outer-shelf reefs on the Great Barrier Reefs demonstrates that coral populations from directly adjacent habitats can show strong genetic isolation. To test this, the authors used the ubiquitous ‘birds-nest’ coral (Seriatopora hysterix), sampled across a depth gradient of ~30m across two outer-shelf reefs:
Whilst strong genetic structuring of both coral host and the symbiotic algae was observed across the samples taken across a depth profile (2, 6, 27 m depth), high genetic similarity was observed between reefs. This suggests that high levels of gene flow can exist between populations from the same habitats at geographically more distant locations (~20 km).
Scleractinian coral Seriatopora hysterix (the spikey pink / cream looking coral by the post in the second photograph) at 30m depth on Yonge Reef, GBR.
The results from the host and symbiont genetic profiling are pretty convincing:
Such striking differences between relatively shallow environments (e.g. 2 and 6m) implies adaptation of the entire coral (host plus symbiont) to distinct environmental niches. These strong associations to particular reef environments present a compelling case for ecological speciation in reef corals, in that evolutionary processes are occuring in the absence of physical barriers. In this case, corals become so adapted to a specific environmental niche, that selection drives them to become genetically distinct from neighbouring populations. While speciation is understood to be a critical mechanism for diversification on coral reefs, previously it was assumed that physical geographical barriers that isolated populations (i.e. allopatric speciation) were the primary driving force of diversification in reef corals.
The study not only highlights the complexity of connectivity in reef building corals, but also points to the importance of conserving different reef habitat types in the design of marine parks. In this instance, corals in neighbouring reef systems 20,000m apart were highly similar, whereas corals in adjacent habitats separated by ~25m in depth showed strong differentiation. Given the projected increases in coral bleaching under future climate change scenarios, conservation of deeper reefs (e.g. 27m depth) is of key importance, as these reefs may act as a vital reproductive source for shallower reef areas to recover following disturbance events.
Bongaerts et al (2010) Genetic Divergence across Habitats in the Widespread Coral Seriatopora hystrix and Its Associated Symbiodinium. PLoS ONE 5(5): e10871. doi:10.1371/journal.pone.0010871