Settlement and tidal zonation patterns of closely related oysters within the genus <scp><i>Saccostrea</i></scp> in a subtropical Australian estuary—implications for restoration

Oyster reefs are considered functionally extinct in many global regions, and restoration efforts are accelerating. Currently, restoration on Australia's eastern coast is focused on the Sydney rock oyster ( Saccostrea glomerata ); however, multiple species of Saccostrea are found across this region. To understand the intertidal zonation patterns of co‐occurring Saccostrea species, we surveyed wild assemblages of adult oysters at different intertidal elevations in the subtropical Noosa River estuary using molecular methods for species identification. We then conducted an oyster gardening experiment across three tidal elevations to assess spat recruitment patterns and determine whether oyster gardening is a viable method of seeding shell for restoration. Adult S. glomerata were more abundant in the upper intertidal zone, whereas adults of the related (but as yet unnamed) Saccostrea lineage G were found almost exclusively in the lower intertidal zone. Spat recruitment was highest in the intertidal zone and lowest in the subtidal zone, indicating that subtidal restoration targeting Saccostrea species may be unsuccessful in this region. We concluded that lineage G is likely to be reef‐building; however, restoration programs will likely rely on in situ recruitment for this species until knowledge relating to spawning triggers can inform reliable hatchery production. Furthermore, we show that oyster gardening can be a viable method of seeding shell when systems are not recruitment limited. This method will likely be useful for multispecies restoration across the tropics, where there are many co‐occurring and often morphologically indistinguishable reef‐building species, providing benefits that far outweigh those of single‐species approaches.