We just came back from checking some rocks, where we had released coral larvae 2 weeks ago. We're super excited to see many new coral polyps that successfully attached. But here's the story behind:
On Nov 2nd (just 3 weeks ago) we collected coral eggs and sperm from the water surface directly after the corals had spawned. The water surface was full of these spawn slicks and we just collected a tiny bit of it (using the typical Indonesian "Mandi" scoops). Back in the lab we washed the eggs several times in filtered (using simple 5 µm + 1 µm household tab water filters) seawater and the released the larvae into a 600 l seawater tank of the same filtered water. Every 30 min, we collected a few eggs - embryos - larvae to have a look at and photograph with the stereo microscope. Witnessing their development from egg to Planula larvae fascinated everyone likewise.
After one day, we added aeration to the tanks and kept the larvae for 5 days, monitoring their development. Once most of the larvae started swimming downwards (which means they start looking for a suitable substrate to settle) we released them to the restoration site.
The restoration site was selected based on the following:
- it used to be a reef (that presumably got destroyed because had been dominated by one species that had died off due to bleaching or disease)
- it has no threat of sedimentation
- the substrate is stable (tiny rubble cemented together by coralline algae)
- the light supply is good
- there is no major algae growth
We deployed natural rocks as settlement substrates for easier follow up monitoring (because those larvae settling on the existing coralline algae would be difficult to spot in the beginning). The rocks were secured with bamboo sticks (all natural materials!!!) in the existing mesh of cemented coralline algae. We then placed domes made from Organza fabric on top of the rocks. They sealed well to the ground with a sewed-in chain of led. 5200 larvae were released into each one of the domes using 200 ml syringes. Counting them was a nice little mathematics exercise (calculating the number based on samples).
One week later, the net domes were removed and two weeks later we went to check the settlement rate. We were excited to see many successful settlers. Each rock was carefully photographed and it is now up to Rifaldi and Joshua (whose Bachelor theses projects this is) to count. Rifaldi will go back weekly from now on and monitor survival. We will keep you posted!
Why is this promising for reef restoration?
Using the natural way of coral larvae to attach themselves facilitates new opportunities for reef restoration. The natural diversity (species and genetic) is preserved, the time effort (and, therefore, the financial need) is relatively low, it does not need much aftercare, no artificial materials will remain in the sea, and no coral fragments need to be collected from other reefs. Of course, this technique also needs careful planning and some knowhow. But we are more than happy to share our experiences with others, especially students from other places in Indonesia. We taught ourselves this technique with just the available literature and some experience here and there. We were successful with raising larvae at first attempt and with high settlement rates at third attempt. Contact us if you are interested in it so you can be successful with your first attempt! It just needs to be said that knowing the time of spawning is prerequisite. Read our previous post to find out more about that.