Our project aims to map coral reef connectivity across the Seychelles and the western Indian Ocean. What exactly do we mean by this? One of the ways hard corals reproduce is by releasing their eggs and sperm into the water column then when a coral egg and sperm join together as an embryo, they develop into a coral larva, called a planula. Planulae float in the ocean, some for days and some for weeks, before dropping to the ocean floor. How far they go and where they end up depends on a lot of things, but especially on ocean currents.
We want to know which sites in the Seychelles are ideally situated within the current system to maximise the chance that coral planulae will arrive at another reef. The reefs that do this we are calling keystone reefs, meaning they are extremely important for the survival of corals in the region, especially now as climate change is impacting reef survival worldwide.
To investigate this we are using a multidisciplinary approach, this means we are using different scientific methods and combining these to get the best understanding possible. One method we have used is genetics; by collecting samples from different reefs across Seychelles we can sequence the DNA from each sample and compare how similar/different they are. We can see which reefs are highly connected with others and which reefs are more isolated and look at gene flow between populations. For example, between the inner granitic islands to the Amirantes and then to the outer islands of Seychelles. Right now, we are analysing this data and starting to process some results but because the data files dealing with whole DNA are so large, this takes a long time!
The second method we are using is via oceanographic current modelling, this is an entirely computational approach that feeds data derived from satellites into a mathematical model and builds in other important factors to more accurately predict the way a planulae larva would be transported from a reef site. This model is in the process of being run on what is called a supercomputer because the model will simulate ocean currents across the whole western Indian Ocean for 25 years and this takes a huge amount of time and computing power. The results from a snapshot in time of this model are already showing some very interesting and exciting results. We released 10,000 virtual larvae from different reef sites in Seychelles to see where these larvae went. The visual simulations showed that for Aldabra’s corals, larvae are being taken west and arriving along the east African coast, this could mean that Aldabra is a keystone reef for coral reefs in Africa. The coral larvae released from Mahe on the other hand swept around the inner granitics and towards the east-north-east into areas where there are few other reefs. This shows there is a potential large variation in coral connectivity between sites in the Seychelles. We can use the results from the genetic studies to check the accuracy of these predictions coming from the model. We would now predict that coral from east Africa will be more closely related to Aldabra’s coral then Mahe is to Aldabra.
However, these results present a very small fraction of the main results and could change when modelled across a 25-year span and compared with the genetics, so we will see in the coming months what exciting finding we have.
Contributed by April Burt