egg size and fecundity of biannually spawning corals at Scott Reef taryn foster * & James Gilmour egg size and fecundity are often used as proxies for coral reproductive success and health. the amount of energy a coral invests in reproduction reflects its environmental conditions during gametogenesis. Additionally, assuming resources for reproduction are limited, it is thought that an increase in egg size should result in a decrease in the number of eggs produced i.e. investing in many small eggs or fewer larger eggs. The biannually spawning populations of Scott Reef offer a unique opportunity to compare the egg size and polyp fecundity of corals exposed to different environmental conditions during gametogenesis, prior to spawning in autumn (March) and spring (october). in this study, we investigated the relationship between egg size and polyp fecundity within and between seven Acropora species from 2008 to 2010. We also quantified the fecundity and egg size of four Acropora species that spawn during both autumn and spring (2008-2010). We found no seasonal variability in egg size and fecundity in the species studied here, possibly as a result of a summer light regime being impacted by high cloud cover in cyclone season. there was high natural variability and no apparent trade-off between egg size and fecundity, both within and between each species. These findings challenge the assumption that egg size and fecundity are negatively correlated, or that a simple, energetically constrained trade-off exists between the two. Acropora corals are hermaphroditic broadcast spawners, releasing their gametes into the water column simultaneously for external fertilisation 1. Multi-specific synchronous coral spawning has been observed since the 1980s on the Great Barrier Reef (GBR) 2 , but coral reproduction on Western Australian (WA) reefs has only been studied in detail in more recent years, and spawning patterns vary latitudinally 3-8. Broadcast spawning on WA reefs occurs in autumn (usually March/April) and at some reefs also in spring (October/November), but participation in spring spawning decreases with increasing latitude 3,4,6,8. At the tropical Kimberley Oceanic reefs (Ashmore Reef, Scott Reef, and the Rowley Shoals) there are significant multi-specific spawning events in both autumn and spring, but the majority of species participate in autumn spawning 5,8,9. For Acropora corals, some species or colonies spawn only in autumn, some spawn only in spring, while others participate in both spawning seasons 10. For the biannual spawners, a similar proportion of colonies spawn in each season, but most colonies (98%) spawn only once per year and always during the same season 10. Thus conspecifics that spawn biannually at Scott Reef are temporally isolated and rarely interbreed. Although there is known variation in the participation of conspecific colonies in the different spawning seasons at Scott Reef 5,8,10 , it is unknown whether reproductive output differs between autumn and spring spawning populations. The environmental...
