Minderoo Exmouth Research Lab Spawning 2022
Work conducted at the Minderoo Exmouth Research Lab involved spawning corals in tanks, collecting and fertilizing gametes, rearing larvae for several days, and then exposing replicate larval cultures to experimental heat stress. The cultures were sampled when approximately 50% of the culture had crashed, and preserved in liquid nitrogen for whole genome sequencing analyses. The goal is to identify genetic loci that show significant allele frequency shifts in response to heat stress.
A Delicate Process
Parent coral colonies were collected from Tantabiddi Lagoon on March 18, following the full moon. These colonies were transported to the Minderoo Exmouth Research Lab. Once in holding tanks, the corals were kept at temperature and light conditions similar to that of the reef.
Prior to sampling, the lagoon from which parent colonies were taken had been under mild heat stress during the month prior to spawning. While there was no visible bleaching in the area, on average, temperatures were at least 1 degree Celsius above historical average temperatures for this time of year. To reduce their stress and ensure the success of the experiment the holding tanks temperature was reduced to the historical average for March (27 degrees).
Each night post collection, corals were observed for signs of spawning. On March 21, eleven of the twenty colonies began to show signs of setting (where the coral polyps bring egg/sperm bundles to the tips of their mouths and hold them there until the colony is ready to release) at 6:40pm and were isolated from the holding tank into individual containers. By 7:45pm all eleven were spawning, and spawning was over by 8pm.
Gamete bundles (bundles of egg/sperm) were gently removed from containers holding parent colonies, and eggs and sperm were measured. Once eggs were all counted, equal amounts of sperm and eggs from all parent colonies were mixed, from where fertilisation occurred at 9:30pm. Once fertilisation was confirmed through visual assessment of embryo cleavage, fertilised eggs were washed with filtered seawater and equally distributed between holding tanks stocked at 400 eggs per litre. These were then left to sit for 12 hours without flow and aeration for development into coral larvae.
Aeration was started 12 hours after fertilisation with gentle water flow beginning the following hour. For the next four days counts of the coral larvae in the holding tanks were completed using 12 by 10ml replicates and extrapolating stock densities from those counts.
After four days in the holding tank, settlement assays were conducted to assess if larvae were fully developed and healthy. This involved taking 6 replicates of 10 larvae and introducing them to coralline crustose algae to induce settlement. If approximately 40% settled this demonstrates larvae are actively searching for a place to settle indicating the heat stress assay could begin.