Sexual reproduction and larval ecology of broadcast spawning <em>Acropora</em> reef corals at Moorea, French Polynesia

Andrew Gerard Carroll

doi:10.25918/thesis.61

This study examined patterns of sexual reproduction, embryogenesis, larval development, planulae metamorphosis and settlement competency of Acropora reef corals on outer reef slope and lagoonal reefs at Moorea, French Polynesia, during 2002 to 2004. The spatial heterogeneity of consecutive coral bleaching events in 2002 and 2003 and their impact on Acropora gametogenesis were also investigated. Seasonal changes in mean oocyte and spermary sizes for A. hyacinthus, A. cytherea, A. striata, A. lutkeni, A. nasuta, and A. retusa indicate that these species have single annual gametogenic cycles, with gametes reaching maturity in late spring. Mean oocyte diameter and polyp fecundity were considerably reduced in partially bleached and heavily bleached colonies in the majority of Acropora species sampled in May 2002, compared with non-bleached corals, and in subsequent sample periods in which these colonies were re-sampled. Therefore, coral bleaching has long-term sublethal affects on surviving corals that impairs their reproductive success. Species of Acropora primarily spawned during a single lunar period between September and November, during the 3rd/4th to 4th lunar quarter, 6-10 nights after the full moon (nAFM). Split-spawning over two consecutive lunar periods was recorded in A. retusa, A. pulchra, A. secale, A. austera, A. striata and A. polystoma in some years, which may have occurred as a response to a shift in the lunar cycle. The greatest overlap of spawning occurred in either October or November each year, which suggests that multispecific spawning characterises Acropora assemblages at Moorea. Coincident spawning periods have now been documented during these periods from the Great Barrier Reef across the Pacific to French Polynesia. Spawning occurred during a period of rising sea temperatures and increasing day length, in the months between dominant trade wind shifts, and prior to the onset of the wet season. The timing of sexual reproduction for Acropora reef corals at Moorea may therefore be influenced by rising sea temperatures or increased solar insolation during these seasonal changes. Although tidal range at Moorea is small (rarely exceeding 40 cm), peak spawning coincided with neap tides, which may enhance fertilisation success by reducing gamete dilution. Embryogenesis and larval development of A. striata, A. lutkeni and A. nasuta, were generally similar to other gamete spawning scleractinian corals. Embryos of each species exhibited pseudospiral holoblastic cleavage and gastrulation led to the formation of endoderm and ectoderm tissues. Cilia developed among the microvilli at the onset of the larval stage and larvae elongated along the polar axis to form well ciliated, azooxanthellate planulae. The appearance and abundance of cnidae during the development of A. striata, A. lutkeni and A. nasuta larvae appeared to coincide with the onset of larval settlement competency. Peak larval attachment rates were observed 3-4 and 4-5 days after spawning (DAS) for A. striata and A. retusa, respectively. This indicates that these larvae have the potential to rapidly attach once in the presence of appropriate settlement substrata; thus enhancing the likelihood of some larvae being retained on, or close to, their natal reef. However, some larvae attached at up to 10 DAS, demonstrating that larval dispersal may occur in the presence of appropriate dispersal vectors. Maximum larval settlement competency periods for A. striata and A. retusa larvae were 27-31 days, and 17-27 DAS, respectively, which may enhance the dispersal potential of some of these planulae away from Moorea. Nevertheless, the potential effective dispersal range of these larvae may not be realised, due to the low proportion of larvae remaining alive nearer the end of their settlement competency period. While this may not limit the successful settlement of some A. striata and A. retusa planulae dispersed away from Moorea, it may restrict large-scale geographic long-distance dispersal. Coral bleaching patterns at Moorea were genus specific and differences in susceptibility among major genera were generally consistent between 2002 and 2003, with Acropora showing the greatest level of susceptibility. Some genera did exhibit substantial spatial variability in bleaching intensity between years (e.g. Pocillopora and Montipora); however, this variability was only statistically significant for partially and heavily bleached Acropora. Multivariate analyses revealed that spatial patterns in the proportion of partially bleached and heavily bleached mean coral cover were highly variable at small spatial scales, both within and between bleaching events. This study provides the first detailed records of sexual reproduction and early life history of Acropora reef corals in French Polynesia. Successful reproduction is important for the maintenance and recovery of multispecies assemblages of Acropora and other scleractinian coral genera. The diminished capacity of bleached Acropora reef corals to reproduce, may contribute to spatial variation in the abundance of Acropora species on outer reef slope and lagoonal reefs at Moorea. This has important implications for assessing the plasticity of Acropora assemblage patterns through time, and estimating the resistance and resilience capacity of this important coral genus to disturbance by bleaching events.

Sexual reproduction and larval ecology of broadcast spawning Acropora reef corals at Moorea, French Polynesia

Files and links (1)

Metrics

Abstract

Details

Sexual reproduction and larval ecology of broadcast spawning Acropora reef corals at Moorea, French Polynesia

Files and links (1)

Metrics

Abstract

Details

Southern Cross University Social media