Researchers from Southern Cross University, CSIRO, and the Australian Institute of Marine Science describe in a new study that was just published how advanced imaging techniques can be used to monitor baby corals.
Corals are vital habitats for many marine species. It’s important to understand how coral baby colonies attach themselves to reefs and, establish themselves, and then grow into adult corals.
This knowledge is especially important if you want to help the reefs recover after devastating events like mass bleaching or cyclones.
A coral recruit of approximately 2.1mm in size, aged six months.
Corals: The Secret Life
Corals begin their life in an annual synchronized spawning. Coral colonies release millions at once of tiny eggs and sperm. All of them rise to the surface, where they are fertilized and develop into larvae.
The ocean currents disperse millions of larvae over days or even weeks. If all goes according to plan, the larvae will eventually fall into the water and attach themselves to a coral reef. They will then grow to adult size. This is called “recruitment” of corals.
This recruitment happens naturally in healthy coral reefs. As coral reefs degrade – for example, through coral bleaching caused by climate change – fewer coral larvae are produced. This can lead to a slowdown or halt in recruitment, as well as a weakening of natural recovery.
Scientists are looking for ways to attach coral larvae to reefs and ensure they grow there. Scientists are working on ways to ensure coral larvae connect and expand on reefs.
The coral larvae are smaller than one millimeter, so the recruitment process is invisible to humans. Researchers have traditionally attached artificial plates on the reef to better understand the process. After the corals have grown, the plates will be taken to the laboratory to be examined under a magnifying glass.
The method is useful but does not mimic the natural environment of a reef. Our research can help. We brought the laboratory to the reef.
Mass bleaching and other damaging events are limiting the establishment of baby corals. Shutterstock
The reef captured in stunning 3D detail
Our new study explores the development and application of an innovative imaging approach known as underwater “macro photogrammetry.”
This technology combines photogrammetry, which measures photos taken at high resolution, with macrophotography. Photogrammetry was used to stitch together photos to create a three-dimensional model like the one below.
We placed three “targets” in the model to help the software combine the photos. You’ll notice a nail head on the left side of each target if you look closely. The nail head measures 2.8mm. This will give you a sense of scale.
The 3D animation shows a micrometre-resolution image of a reef covering 400 cm2.
Reef-scale photogrammetry is a useful tool for tracking changes in coral growth and cover over time. It does not have the resolution required to observe and identify tiny new corals.
This incredible scale is provided by macrophotography. This coupling also allows a comprehensive view of the ecosystem from the smallest to the largest processes.
Near Lizard Island, on the Great Barrier Reef, we conducted a macrophotogrammetry survey. We marked 25cm by 25cm on the reef. Then, we took hundreds of photos from different angles with high-resolution digital cameras.
The photos were processed using photogrammetry software to create 3D models of the reef in small sections at high resolution.
They were used to determine where coral larvae settle, mark their location, and measure their size. The models reveal the microstructure of the reef, including the tiny crevices where coral larvae are often found.
Models also reveal microorganisms, such as invertebrates or small turf algae, that interact with corals when they are recruited.
Over time, it is possible to conduct macro photogrammetry surveys at the same reef location. We can monitor the growth and survival of coral babies, as well as observe the changes in organisms that live near them.
Read more: Record coral cover doesn’t necessarily mean the Great Barrier Reef is in good health (despite what you may have heard)
The researchers monitor coral recruitment on a reef slope at Lizard Island. Lauren Hardiman CSIRO
Look ahead
Complementary methods may enhance the capabilities of macrophotogrammetry. Before release, coral larvae could be dyed in various colors to make them more visible as they swim and settle onto the reef. It could be captured on 3D models for a better understanding of the larval restoration effort.
Macrophotogrammetry can help us better understand why some larvae survive and settle on coral reefs while others don’t. This knowledge will support our efforts in improving the overall conservation of coral reefs.
