Spatial patterns of coral damage affect tissue regeneration, skeletal growth, and coral morphology

Background/Question/Methods

Physiological connections among individual units of colonial organisms can influence how both individual units and the entire colony recover from damage.  Corals are colonial organisms whose predators often remove coral tissue, leaving small areas of exposed coral skeleton. Coral polyps share nutrients with their neighbors, and thus, scar distribution likely affects tissue regeneration and recovery rates after coral damage.  Scar distribution also likely contributes to variation in morphology due to differing growth rates across the surface of the coral.  However, the effects of the spatial distribution of scars on healing, growth and morphology are still unknown.   

I conducted surveys of coral damage to document the spatial distribution of predation.  I also conducted two field experiments.  The first field experiment tested the effect of distance between scars (no scars vs. scars separated by 2mm, 25mm, or 50mm) on healing (tissue regeneration) and skeletal growth of the colony.  The second field experiment tested the effect of distance between scars (no scars and scars separated by either 2mm or 25mm) on spatially explicit linear extension of the coral skeleton.  

Results/Conclusions

Coral scars occurred in varying spatial distributions, but most scars were near to one another (<25mm between centroids).  I detected spatial clustering in approximately one third of the quadrats.  Skeletal growth was decreased by 26% when scars were separated by 1mm compared to those separated by at least 25 mm, while tissue regeneration rate was higher on corals with separated scars.  Linear extension at the top of the coral was lower on colonies with scars close together than on colonies without damage or damage spaced 25mm apart, while growth at the base of the colony was similar across treatments.  Linear extension was lower around scars that were near to one another. 

These results show that damage spacing affects the recovery of the coral afterwards.  Thus, predators that inflict clustered damage will have larger effects than predators that feed evenly across the colony.   Also, settlement of competitors, such as algae, is more likely when scars are clustered (and heal more slowly), which will further reduce coral cover.  Finally, localized reduction in growth around clustered scars could alter coral morphology by creating more topographically complex reefs.