Allee effects, spawning asynchrony, and mixed reproductive modes in a common mass spawning coral

Abstract

Understanding strategies of organisms that utilise multiple modes of reproduction presents a complex challenge for evolutionary biologists. Platygyra daedalea, a common reef-building coral with unclear reproductive boundaries between morphological species, illustrates these complexities. Here, we evaluate the contribution of these reproductive modes in the coral P. daedalea at Heron Island, on the southern Great Barrier Reef. We tagged and sequenced eighteen coral colonies, representing various degrees of spatial clustering, along a 130-m stretch of shallow reef slope. During spawning, divers collected eggs from each colony and placed them in mesh containers within the spawn slick, allowing free movement of sperm for fertilisation. High levels of spawning asynchrony were observed, potentially indicating distinct genetic clusters within the putative species, which resulted in low fertilisation success (1.5%). Notably, of those fertilised eggs, paternity assignments revealed that all resulting embryos were self-fertilised, with no cross-fertilisation occurring. The adult population showed evidence of two genetically distinct subpopulations, along with levels of spatial autocorrelation and inbreeding. This evidence supports the notion of small breeding populations within larger assemblages, density-dependent population effects, and localised recruitment. Selfing may serve as a reproductive assurance mechanism in such populations, which may be more hierarchically structured than previously thought. Given the lack of evidence for in situ outcrossed fertilisation in this natural coral population during split spawning, it appears that P. daedalea may rely on limited high-density patches of adults for successful cross-fertilisation, utilise atypical modes of reproduction when at low densities, and/or sustain its population through limited progeny.