From the origin of the species to the paths of invasion
At this stage of our project, we are building a bank of biological samples of sun coral(T. tagusensis and T. coccinea) from different geographical regions, including skeletons of colonies, their tissues and DNA. This resource is a valuable source of information to advance our understanding of the sun coral, its dispersal routes and will serve as a legacy for future scientific projects.
We carefully execute a sampling plan designed to be representative and comprehensive. The collections include the length of the Brazilian coastline and international locations, in the so-called “type localities”. We covered the limits of occurrence of each species in the South and Northeast regions, in natural and artificial substrates. Collections are carried out by research divers using scuba diving. We use a duo scheme, in which one diver collects the colonies and the other records the area, helping to store the samples.
After collection, the colonies are kept in a box of seawater until a tissue sample is removed for DNA analysis. During this process, a polyp from each colony is broken with dissecting forceps and inserted into a tube with preservative solution. At the same time, each colony is stored and identified individually. Samples of polyps and colonies are kept on ice until they reach the laboratory. While the polyps undergo DNA extraction, the colonies are bleached to remove the soft tissue. The remaining (“bleached”) skeletons are photographed for later taxonomic analysis, which includes the study of morphological aspects to confirm the identity of each species.
We use advanced monitoring techniques to track and identify sun coral populations in critical areas. This gives us a detailed understanding of the distribution and density of these invasive corals.
The DNA samples isolated from the polyps in the previous stage are used to carry out genetic analyses of the sun coral. This area of research, called “population genetics”, helps us to understand how these corals have invaded new places, i.e. how they have spread and where they may have come from. The aim of this stage is to do this, and the results obtained will be relevant for assessing the risk of new dispersals and invasions of the species.
To achieve this, each DNA sample from the sun coral goes through a process in the laboratory. Initially, we used a technique called PCR (the same used in Covid-19 tests) to find the “genetic markers”, which act as signatures on the DNA that help us understand the relationships between individuals from different locations.
The DNA from the genetic markers in each colony is purified and prepared for the next stage: DNA sequencing. The resulting sequences are subjected to computational and statistical analysis to investigate the genetic diversity at each site. This is done to identify if there are genetic differences that indicate whether corals from different locations are isolated or if, on the contrary, there is a genetic flow between them. By assessing the distribution and frequency of these genetic markers in each location, we can obtain information on the origin, dispersal and adaptation of these species to new environments.
Early detection of sun coral is crucial for a quick and effective response to prevent its spread, because once established, eradication methods are not available. However, monitoring the sun coral is challenging due to the need for technical diving to verify its presence in marine areas. Monitoring artificial substrates, such as shipwrecks and oil platforms, can present even greater access difficulties.
The dispersal of the sun coral occurs mainly by means of its larvae carried by the currents. Detecting larvae in the water is an important warning sign, as they indicate the presence of the species at a stage of high risk of spreading.
One of the project’s objectives is to develop a method for the early detection of sun coral larvae, eliminating the need to dive and visually identify colonies. The method is based on detecting the larvae’s DNA in the marine environment, using a PCR technique designed specifically for sun coral DNA. In other words, the PCR test should be able to identify the sun coral’s DNA in complex environmental samples made up of various other organisms.
The first step in the early detection of sun coral is to collect environmental samples. This collection concentrates small planktonic organisms from the seawater, which, like the larvae of the sun coral, live adrift in the seawater.
We carried out two major sailing expeditions to collect samples along the coast of Brazil, from the coast of Pernambuco to Santa Catarina. We chose collection points with sun coral, shipwrecks, platforms and conservation areas in search of a comprehensive analysis of different situations. We used a special net to collect these organisms, dragging it slowly through the water for 10 minutes at a constant speed. We then washed the organisms from the net with salt water and placed them in a beaker to concentrate the sample and preserve it until it arrived at the laboratory.
After collecting the samples, the next step consists of extracting the genetic material that is still inside the cells in the environment, known as intracellular environmental DNA. This process excludes DNA from organisms that are no longer intact and would not necessarily indicate the presence of larvae.
The PCR test is then carried out using this intracellular environmental DNA to check whether or not sun coral is present in the sample of planktonic organisms.
This test will be able to detect sun coral quickly. The analysis of these environmental samples will produce the first molecular mapping of the occurrence of sun coral larvae along the coast of Brazil. This survey will help us to assess and manage risks more accurately, as the presence of larvae indicates a greater risk of dispersal of these invasive species.
We applied a hydrodynamic and particle dispersion model to simulate the dispersion of sun coral larvae in the sea. The aim is to be able to answer questions that are relevant to assessing the risk of dispersal, such as: “What is the destination of the larvae released at a given point, their trajectory and the distance they travel?”, “What is the likelihood of a given region being hit by larvae originating from a specific source?” and “What is the minimum and maximum arrival time of larvae at a given location considering release from a specific location?”
The simulations are being carried out considering more than 50 points of origin of the larvae along the Brazilian coast, including those that can act as dispersal intermediaries, such as shipwrecks and islands. In addition, shipping routes between ports and platforms in Brazil and international points, including native regions and where the sun coral is also invasive, are being analyzed.
For each point of origin of the larvae in the simulation, maps are generated showing the probability of larvae being present, as well as the minimum and maximum times of arrival at a given point.
The integration of the modeling results with the previous stages of the project forms the backbone for a comprehensive assessment of the risk of sun coral dispersal. Population genetics analyses reveal genetic patterns and provide crucial insights into the dispersal routes and origins of these invasive species.
Early detection of larvae using an innovative PCR-based method represents a significant advance, eliminating the need for diving and enabling a rapid response to prevent spread.
Finally, the application of larval dispersal models, considering different points of origin, allows the creation of probabilistic maps, supporting risk assessment with information on the probability of presence, trajectory and arrival times of larvae at different points along the coast.
The convergence of these steps reinforces the robustness of our approach, offering a solid basis for understanding and effectively managing the risks associated with sun coral dispersal.
