Habitat selection, facilitation, and biotic settlement cues affect distribution and performance of coral recruits in French Polynesia

Abstract

Habitat selection can determine the distribution and performance of individuals if the precision with which sites are chosen corresponds with exposure to risks or resources. Contrastingly, facilitation can allow persistence of individuals arriving by chance and potentially maladapted to local abiotic conditions. For marine organisms, selection of a permanent attachment site at the end of their larval stage or the presence of a facilitator can be a critical determinant of recruitment success. In coral reef ecosystems, it is well known that settling planula larvae of reef-building corals use coarse environmental cues (i.e., light) for habitat selection. Although laboratory studies suggest that larvae can also use precise biotic cues produced by crustose coralline algae (CCA) to select attachment sites, the ecological consequences of biotic cues for corals are poorly understood in situ. In a field experiment exploring the relative importance of biotic cues and variability in habitat quality to recruitment of hard corals, pocilloporid and acroporid corals recruited more frequently to one species of CCA, Titanoderma prototypum, and significantly less so to other species of CCA; these results are consistent with laboratory assays from other studies. The provision of the biotic cue accurately predicted coral recruitment rates across habitats of varying quality. At the scale of CCA, corals attached to the "preferred" CCA experienced increased survivorship while recruits attached elsewhere had lower colony growth and survivorship. For reef-building corals, the behavioral selection of habitat using chemical cues both reduces the risk of incidental mortality and indicates the presence of a facilitator.

Fig. 1

Schematic of experimental habitats made with sanded PVC plates. Recruitment plates were oriented to create four habitat types: cryptic (inner) and low sediment (vertical plates and downward-facing horizontal plates) (1), cryptic and high sediment (upward-facing horizontal plates) (2), exposed (outer) and low sediment (3), and exposed and high sediment (4)

Fig. 2

a Density (no. m⁻²) of pocilloporid coral recruits (mean + SE) in exposed or cryptic habitats with high (14.67 ± 1.33 g m⁻² year⁻¹) or low (7.56 ± 0.44 g m⁻² year⁻¹) sedimentation after 6 months. Sample sizes (n) reported above each bar. b Selectivity indices (Manley’s α) calculated for each of the possible substrate choices for coral recruits from the genus Pocillopora (black bars), Acropora (gray bars), and Porites (white bars) after 6 months in situ within cryptic habitats that had low sedimentation rates and for Acropora tenuis settlers (hatched bars) after 3 days in the Harrington et al. (2004) laboratory assay. Substrate options were either a particular species of crustose coralline algae (CCA) or unoccupied space on the PVC plate (“bare plate”). The solid line represents random or passive settlement (α = 0.1667). NA A. tenuis not used in the laboratory assay; 0 selectivity index value of zero; asterisk significantly different than random at α = 0.05, indicating preference if above the line or avoidance if below

Fig. 3

The relationship between recruitment rates of pocilloporids (no. per plate surface, 225 cm²) and the in situ percent cover of a preferred CCA (Titanoderma prototypum; y = 44.03x − 0.25; r = 0.83; P = 0.0002) and b avoided CCA (Porolithon onkodes and Lithophyllum insipidum) across all habitat treatments (r = −0.44; P = 0.12)

Fig. 4

Growth and mortality rates for pocilloporid corals categorized by original settlement substrate within each annual time interval since initial recruitment [year 1 (T _0.5 to T _1.5), year 2 (T _1.5 to T _2.5), year 3 (T _2.5 to T _3.5)]. Data are only from cryptic, low sedimentation habitats. a Growth rate per pocilloporid colony estimated as annual log change in diameter (mean + SE); 0 (above bars) indicates a rate of zero. b Annual mortality rate estimated as proportion of the 2004 cohort of recruits dying within each annual interval. Preferred Corals settled on the coralline algae T. prototypum, Neutral coralline algae are Hydrolithon spp. and P. conicum, Bare plate previously unoccupied space on the PVC plate, Avoided coralline algae are both P. onkodes and L. insipidum