Abstract
For species with complex life histories such as scleractinian corals, processes occurring early in life can greatly influence the number of individuals entering the adult population. A plethora of studies have examined settlement patterns of coral larvae, mostly on artificial substrata, and the composition of adult corals across multiple spatial and temporal scales. However, relatively few studies have examined the spatial distribution of small (≤50 mm diameter) sexually immature corals on natural reef substrata. We, therefore, quantified the variation in the abundance, composition and size of juvenile corals (≤50 mm diameter) among 27 sites, nine reefs, and three latitudes spanning over 1000 km on Australia’s Great Barrier Reef. Overall, 2801 juveniles were recorded with a mean density of 6.9 (±0.3 SE) ind.m−2, with Acropora, Pocillopora, and Porites accounting for 84.1% of all juvenile corals surveyed. Size-class structure, orientation on the substrate and taxonomic composition of juvenile corals varied significantly among latitudinal sectors. The abundance of juvenile corals varied both within (6–13 ind.m−2) and among reefs (2.8–11.1 ind.m−2) but was fairly similar among latitudes (6.1–8.2 ind.m−2), despite marked latitudinal variation in larval supply and settlement rates previously found at this scale. Furthermore, the density of juvenile corals was negatively correlated with the biomass of scraping and excavating parrotfishes across all sites, revealing a potentially important role of parrotfishes in determining distribution patterns of juvenile corals on the Great Barrier Reef. While numerous studies have advocated the importance of parrotfishes for clearing space on the substrate to facilitate coral settlement, our results suggest that at high biomass they may have a detrimental effect on juvenile coral assemblages. There is, however, a clear need to directly quantify rates of mortality and growth of juvenile corals to understand the relative importance of these mechanisms in shaping juvenile, and consequently adult, coral assemblages.
Highlights
Most marine organisms have open populations, where rates of settlement are decoupled from local abundance and fecundity of adult individuals [1,2]
Hughes et al [13] found that settlement rates of scleractinian corals varied by an order of magnitude along 2,000 km’s of Australia’s Great Barrier Reef, yet adult coral cover was very consistent among the five latitudinal regions studied
Hughes et al [13] suggested that these findings were due to large-scale variations in early post-settlement dynamics, which compensate for marked differences in settlement rates
Summary
Most marine organisms have open populations, where rates of settlement are decoupled from local abundance and fecundity of adult individuals [1,2]. A plethora of studies have examined settlement patterns of scleractinian corals, mostly using artificial substrates, and showed that settlement rates are highly variable, both in space and time [11,12,13,14] These patterns established at settlement may be modified substantially by post-settlement processes such as differential growth and survivorship [15,16,17]. Hughes et al [13] suggested that these findings were due to large-scale variations in early post-settlement dynamics, which compensate for marked differences in settlement rates This apparent disconnect between larval settlement and adult coral populations is poorly understood, and only few studies have focused on early life-stages of corals on natural substrata, mainly due to difficulties associated with identifying small colonies on natural substrates [19]
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