A new method stains coral larvae red and blue to track them as they disperse and settle on coral reefs

As the effects of anthropogenic climate change, pollution, and other human activities continue to threaten coral reefs across the world, it’s increasingly critical to understand how surviving patches of coral can “reseed” damaged areas through dispersal of coral larvae.

Every year, coral reefs produce trillions of microscopic larvae in an annual mass spawning event. These tiny larvae can travel up to 100 kilometres to neighbouring reefs, and can kick-start coral recovery following disturbance. Understanding where these coral larvae disperse throughout the reefscape is challenging, and current approaches using computer models to simulate dispersal and genetic approaches to identify parental lineages are costly and limited in scale.

Now, Australian marine ecologists have published a new method for staining coral larvae blue and red that might provide an answer to the problem.

According to a new study in PLOS Biology, the protocol is simple and rapid (taking less than 60 minutes), easily scalable to large numbers (a million to a billion larvae), nontoxic to the marine environment, and low-cost – allowing 100,000 larvae to be stained for around 1 US Dollar.

Read more: Cosmos Investigation: Potential reef “saviours” (part 2 of 3)

By allowing scientists to easily differentiate between laboratory stained and wild larvae with the naked eye, co-author Dr George Roff from the CSIRO Oceans and Atmosphere told Cosmos that the method will allow scientists to better understand larval dispersal and test the efficacy of reef restoration interventions.

“Of all the options that we have out there in our toolbox of restoration, larval re-seeding is probably one of most promising ones because you can harness the natural process of larval dispersal to seed larvae across large areas of reef, potentially at the scale of  1-10 kilometres,” Roff says.

“While there’s strong potential for success, we really don’t really have a good idea of where these larvae go once they are released onto the reef.”

What exactly are coral larvae?

Obviously, corals can’t just pick up and swim about to find a mate when they want to reproduce. Instead, Roff says, about 60% of species have evolved to release their eggs and sperm in a single mass spawning event that, for most species, coordinate once per year around a full moon.

This is occurring right now on the Great Barrier Reef, in and around the full moon on December 8.

Read more: Overcoming the barriers to reef recovery.

Fertilised eggs on the surface of the ocean take 3-10 days to metamorphose into microscopic larvae as tiny as 0.2 millimetres, which travel on the ocean currents until they find a coral reef to settle and eventually grow into adult corals.

“Mass spawning is critical for the recovery of the Great Barrier Reef following disturbance such as the recent mass coral bleaching events,” says Roff.

“We know surprisingly little about where these larvae go and how they move between reefs, and this information is key to understanding whether larval reseeding can scale to become an effective restoration tool.”

Optimising the technique

Roff and co-author Dr Christopher Doropoulos, who is also a marine ecologist with CSIRO’s Oceans and Atmosphere, collected nearly 3,000 larvae from lab-reared corals of two species from the Great Barrier Reef (Acropora spathulata and Platygyra daedalea) and incubated them with one of four natural coloured dyes at different concentrations.

They found that two dyes – neutral red and Nile blue – successfully stained coral larvae and had minimal impact on their survival and settlement. 

“Optimising the technique through a series of experiments was important to ensure the method had no long-lasting effects on larval survival and their ability to settle on coral reefs,” explains Roff.

“First, we wanted to know whether the dye could effectively stain the larvae, and then we aimed to reduce the amount of dye needed to still get a strong visual effect while minimising any negative effects on survival and settlement.”

Read more: Cosmos Investigation: “The best coral cover in 36 years” What the…?

Further testing on four more coral species – including Acropora anthocercisCoelastrea asperaDipsastraea favus, and Platygyra sinensis – successfully stained 98% of larvae using this technique.

Taking the method from the laboratory and into the field, Roff and Doropoulos captured eggs from wild spawning corals on the northern Great Barrier Reef, cultured them in larval pools in the Lizard Island lagoon and stained 10,000 larvae with Nile blue dye.

They found that the stained larvae successfully settled on tiles placed in the lagoon, providing important field-validation of the method.

Blue and red stained coral larvae
Representative images of free-swimming and newly metamorphosed larvae (Nile blue, unstained, neutral red) from Acropora anthocercis, Platygyra sinensis, Coelastrea aspera, and mixed Nile blue and neutral red stained Dipsastraea favus larvae. White scale bars = 1 mm. Credit: Doropoulos C and Roff G, 2022, PLOS Biology, CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

What are the implications for future research?

So, does the stain stick around for very long?

Coral larvae floating on top of the water
Coral larvae floating on top of the water. Credit: George Roff

Roff says that the technique can be used for tracking coral larvae in the water and as they settle down and grow on the seabed for up to about ten days.

“Once they settle in where they’re going to grow, they undergo this rapid expansion of cells: they’ll produce new polyps, start to form a skeleton, within a couple of months they’ll no longer be minute larvae but instead fully formed corals a couple centimetres in size,” he says.

“So, the dyeing technique we expect won’t last that long – with the rapid growth of new cells, it’s going to dilute the colour and make them harder to differentiate.”

So why would you need to track the larvae for such a short period of time?

While genetic techniques can be used to understand the parentage of adult corals, the new dyeing technique allows for tracking coral larvae in the very early life history stages between 0 to 10 days where the most larval mortality occurs.

“If you imagine the millions of larvae that go into the water column, the ones that actually survive to grow into adult corals is absolutely tiny, as many are outcompeted or eaten in the weeks to months following settlement,” says Roff.

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With the ability to use different coloured dyes to differentiate between groups or species, scientists are now able to differentiate between background settlement from natural wild coral larvae and understand how different species of coral interact in the reef environment.

“With this new technique we can better understand the how larvae dispersed in reseeding projects disperse across reefs, and identify how we can better optimise the method to upscale restoration efforts from small patches to entire reef slopes,” concludes Roff.

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