Population structure of the hydrocoral Millepora platyphylla in habitats experiencing different flow regimes in Moorea, French Polynesia

Abstract

While the fire coral Millepora platyphylla is an important component of Indo-Pacific reefs, where it thrives in a wide range of environments, the ecological and biological processes driving its distribution and population structure are not well understood. Here, we quantified this species' population structure in five habitats with contrasting hydrodynamic regimes in Moorea, French Polynesia; two in the fore reef: mid and upper slopes, and three in the lagoon: back, fringing and patch reefs. A total of 3651 colonies of fire corals were mapped and measured over 45,000 m2 of surveyed reef. Due to the species' sensitivity to fragmentation in response to strong water movement, hydrodynamic conditions (e.g. waves, pass and lagoonal circulation) corresponded to marked differences in colony size distributions, morphology and recruitment dynamics among habitats. The size structure varied among reef habitats with higher proportions of larger colonies in calm nearshore reefs (fringing and patch reefs), while populations were dominated by smaller colonies in the exposed fore reefs. The highest densities of fire corals were recorded in fore reef habitats (0.12-0.20 n.m-2) where the proportion of recruits and juveniles was higher at mid slope populations (49.3%) than on the upper slope near where waves break (29.0%). In the latter habitat, most colonies grew as vertical sheets on encrusting bases making them more vulnerable to colony fragmentation, whereas fire corals were encrusting or massive in all other habitats. The lowest densities of M. platyphylla occurred in lagoonal habitats (0.02-0.04 n.m-2) characterized by a combination of low water movement and other physical and biological stressors. This study reports the first evidence of population structure of fire corals in two common reef environments and illustrates the importance of water flow in driving population dynamic processes of these reef-building species.

Fig 1. Aerial views of the locations of each transect in the five surveyed habitats in Moorea, French Polynesia.

The names of these surveyed locations are: (A) Tiahura, (B) Papetoai, (C) Cook’s Bay and (D) Temae. Map data WorldView–2, Digital Globe.

Fig 2. Morphologies of Millepora platyphylla colonies in habitats experiencing contrasting hydrodynamic regimes.

(A) Encrusting wave-tolerant morphology in the mid slope, a fore reef habitat at 13 m; (B) sheet tree morphology vulnerable to wave-induced breakage in the upper slope, a fore reef habitat at 6 m and (C) massive wave-tolerant morphology in the patch reef, a lagoonal habitat (photo is courtesy of Gilles Siu).

Fig 3. Index describing the spatial distribution of Millepora platyphylla colonies across the five surveyed habitats.

(A) Density (B) cover (C) distribution index and (D) mean neighborhood distance. Values were average per habitat and error bars show the standard error for transect replicates. Similar letters indicate no statistical difference in post-hoc comparisons among habitats (P > 0.5).

Fig 4. Size-frequency distributions of Millepora platyphylla across the five surveyed habitats.

Colony size (cm²) data were distributed among 10 size classes based on a logarithm scale (log₂). Frequencies (%) for each size class were averaged by habitats with total population size (N in S2 Table) and error bars show the standard error for transect replicates.

Fig 5. Recruitment dynamics across the five surveyed habitats.

Proportions of recruits (< 1 cm²), juveniles (1–20 cm²) and adults (> 20 cm²) were averaged by habitats with total population size (N in S2 Table) and error bars show the standard error for transect replicates. Similar letters over each set of bars indicate no statistical difference in post-hoc comparisons for a given life history stage among habitats (P > 0.05).

Fig 6. Stock-recruitment relationship between the abundance of adults and coral new recruits and juveniles.

(A) Significant positive relationship in the lagoon (i.e. back, fringing and patch reefs) and (B) no stock-recruitment relationship in the fore reef (i.e. mid and upper slopes). Each dot represents the mean abundance for each transect surveyed. Note the different scales on x and y axes.

Fig 7. Morphology of Millepora platyphylla adult colonies across the five surveyed habitats.

Proportions of colonies with encrusting, sheet tree and massive morphology were averaged by habitats and error bars show the standard error for transect replicates. Similar letters over each set of bars indicate no statistical difference in post-hoc comparisons for a given morphology among habitats (P > 0.05). See Fig 2 for photos of each of the morphologies.

Fig 8. Non-metric multidimensional scaling (MDS) plot of Millepora platyphylla population structure across the five surveyed habitats.

Different shapes indicate the three transects for each habitat and grey lines show clusters given by dendogram based on Eucledian distance of 4 at a stress level of 0.09. The surimposed red lines define the Eucledian distance coefficient on normalized data based on Spearman ranking, with each vector having lengths ≥ 0.4: density, cover, distribution index, mean neighborhood distance, mean height and size of adults, and proportion of recruits (< 1 cm²), juveniles (1–20 cm²) and adults (> 20 cm²). The second transect of the fringing reef is shown as a single group mostly related to a small population size (i.e. 27 colonies, S2 Table).