Scientists studied the evolutionary history of the destructive crown-of-thorns starfish, the coral predator that is capable of wiping out entire coral reefs. The study helps us understand how their populations change with time, which may potentially help us mitigate the ecological damage they inflict.
Okinawa Institute of Science & Technology Graduate University (or OIST) scientists focused on the crown-of-thorns' evolutionary history. It is a species of starfish that preys on corals and has the potential to destroy coral reefs.
The crown-of-thorns' large body is full of poisonous, spiky thorns. However, this is not the cause of their massive capability for ecological damage. The real danger lies in their ability to reproduce, having millions of eggs from only one spawning. This gives them the capacity to grow into uncontrollably huge numbers in just a short time and destroy vast coral reef areas equally rapidly.
OIST Marine Genomics Unit leader and senior study author Noriyuki Satoh recounted the massive crown-of-thorns outbreak nearly four decades ago in Okinawa, where divers removed 1.5 million individual starfishes by hand.
Together with tropical cyclones and coral bleaching, the starfish is currently becoming an increasing threat to Australia's Great Barrier Reef. Outbreaks are now more severe and more common because increasingly warmer and polluted waters are favorable to the starfish larvae's survival.
The OIST Marine Genomics Unit's latest study is published in the journal G3: Genes|Genomes|Genetics, where they determined if there is any information in the genomes of the starfish that can help illuminate the reason and method behind the outbreaks.
The study involved the collection of crown-of-thorns starfish from the coral reefs surrounding three islands of the Ryukyu Archipelago, particularly Iriomote, Miyako, and Okinawa. They sequenced the mitochondrial DNA of three starfish species: the crown-of-thorns, the northern Pacific seastar, and the blue starfish. Comparing these three, the researchers determined if there is a feature that is unique to the crown-of-thorns.
The researchers found that the crown-of-thorns' evolutionary tree supports the idea of complex regional ocean currents, as each crown-of-thorns lineage have been found in several geographic locations. This is important in predicting new outbreaks in the region of the Ryukyus. Because of this, scientists now advocate for a greater understanding of the area's ocean currents.
The crown-of-thorns' evolutionary tree has a significant difference from the two other species, in that although it is a much younger species, only diverging later than a million years ago, its tree showed how it fragmented quickly into five lineages. This data suggests the starfish experienced genetic bottlenecks often, where its population dwindled radically, then jumpstarting a new lineage afterward.
According to Professor Satoh, this implies how outbreaks of crown-of-thorns are a part of a population cycle characterized as 'boom and bust,' suggesting that crown-of-thorns populations tend to overeat coral so that they use up all food sources and begin to die off.
For future research, the Marine Genomics Unit will collaborate with researchers from Australia to analyze the populations of the crown-of-thorns in the Great Barrier Reef. This time, they aim not only to sequence mitochondrial DNA, but each starfish's entire genome, including the DNA from its nucleus.
Satoh says that they hope to ultimately understand better the starfish's population trends and the part that ocean currents play in dispersing it and causing new outbreaks. It will help them predict and hopefully mitigate these future outbreaks.
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