Tackling coral reefs' thorny problem: Crown-of-thorns starfish
Scientists decipher the evolutionary history of a coral reef predator


Date:
July 8, 2020
Source:
Okinawa Institute of Science and Technology (OIST) Graduate
University
Summary:
Researchers have revealed the evolutionary history of the
crown-of-thorns starfish -- a predator of coral that can devastate
coral reefs. Their findings shed light on how the populations of
these starfish have changed over time and could potentially help
reduce their ecological destruction.



FULL STORY ========================================================================== Researchers from the Okinawa Institute of Science and Technology
Graduate University (OIST) have revealed the evolutionary history of
the crown-of-thorns starfish -- a predator of coral that can devastate
coral reefs. Their findings shed light on how the populations of these
starfish have changed over time and could potentially help reduce their ecological destruction.


==========================================================================
A single crown-of-thorns starfish is formidable, with a large body covered
in spiky, venomous thorns. But their true danger lies in their potent reproductive ability, with female crown-of thorns starfish releasing
millions of eggs in a single spawning. This can quickly lead to plagues,
with uncontrollably large numbers of starfish rapidly destroying vast
areas of coral reef.

"Almost 40 years ago, Okinawa experienced a massive outbreak of
crown-of-thorns starfish, where over 1.5 million starfish had to be
removed by divers by hand," said Professor Noriyuki Satoh, senior author
of the student and leader of the Marine Genomics Unit at OIST.

Although outbreaks have recently become less frequent around Okinawa and
other subtropical islands in the Ryukyu Archipelago, they have become an increasingly large threat to the Great Barrier Reef in Australia, along
with coral bleaching and tropical cyclones. These starfish outbreaks
are becoming more common and more severe, as increasingly polluted and
warmer waters aid the survival of the larvae.

In 2017, the OIST Marine Genomics Unit teamed up with Australian
scientists to decode the genome of the crown-of-thorns starfish, with
their results published in Nature. Now, in their latest study published
in G3: Genes|Genomes|Genetics, the Marine Genomics Unit wanted to explore whether any information was recorded in the starfish genomes that could
shed light on how and why these outbreaks occur.

The researchers collected crown-of-thorns starfish from coral reefs
around three different islands in the Ryukyu Archipelago -- Okinawa,
Miyako and Iriomote. The scientists then sequenced the entire DNA
found in the mitochondria, comprised of over 16,000 nucleotide bases,
and used differences in the sequences between the individual starfish
to construct an evolutionary tree.



==========================================================================
The unit also performed the same analyses on two other starfish species
-- the blue starfish and the northern Pacific sea star. By comparing the crown-of- thorns starfish to these other two species, the scientists
hoped to see whether their findings revealed anything unique to the crown-of-thorns starfish.

"The blue starfish is also a coral reef predator that lives in the same
habitat as the crown-of-thorns starfish, but it doesn't produce these uncontrollable outbreaks," said Prof. Satoh. "Meanwhile, the northern
Pacific sea star is the most common starfish in Japan and lives in
colder waters around the Japanese mainland." The scientists found that
the evolutionary tree for the northern Pacific sea star showed that
the species had split into two major lineages. Starfish collected from
three different locations in the seas around the north-eastern regions
of Japan were composed of individuals from one lineage, whilst a single population in the Seto Inland Sea in south-west Japan was formed of
individuals from a second, more recent lineage.

"We believe that in a rare migration event, starfish larvae dispersed to
the Seto Inland Sea. As these two areas are so separated, no migration
occurred afterwards between the two populations, which resulted in
the species splitting into two lineages," said Prof. Satoh. "Meanwhile,
shorter range ocean currents kept individuals from the first lineage mixed between the nearby locations in the north-east of Japan." For the blue starfish, the results were more surprising. The constructed evolutionary
tree showed that the species had first split into two lineages, with
the second lineage then diverging again into two smaller subgroups. But intriguingly, individuals from the two major lineages were found in
both Okinawa and Ishigaki -- the two areas in the Ryukyus where the blue starfish was collected. This means that two distinct starfish populations
are living in the same geographic regions but are not breeding and mixing
their genes. Prof.

Satoh believes that this is strong evidence for there being two cryptic
species of blue starfish -- in other words, the starfish look the same
despite being separate, non-breeding species.



==========================================================================
The results also suggest that blue starfish migration occurs in both
directions between Okinawa and Ishigaki. This was unexpected as the
scientists had previously assumed that the powerful northeastern current flowing from Ishigaki towards Okinawa prevented starfish larvae from
being carried in the opposite direction.

"For migration to readily occur in both directions, this suggests that
the ocean currents in the Ryukyu Archipelago may be more complex that previously imagined," said Prof. Satoh.

The results from the evolutionary tree of the crown-of-thorns starfish
also supported the idea of complex ocean currents in the region, with
each crown-of- thorns starfish lineage also found in more than one
geographic location. This has important implications for predicting where
new outbreaks of crown-of- thorns starfish may occur in the Ryukyus,
with the researchers now advocating for better understanding of the
ocean currents in the area.

Overall, the evolutionary tree for the crown-of-thorns starfish looked significantly different from the other two starfish, underlying key
differences in the species' historical population dynamics. Despite
being a much younger species than the other two species, diverging less
than one million years ago, the tree showed that the starfish quickly fragmented into five small lineages.

These findings suggest that the species underwent frequent genetic
bottlenecks, where the population was reduced to just a small number of individuals, which then jumpstarted a new lineage.

"This implies that the starfish outbreaks are just one part of a larger
'boom and bust' population cycle, where if they are left to their natural devices, the starfish eat so much coral that they run out of food and
die," said Prof.

Satoh.

For their next steps, the Marine Genomics Unit is collaborating with
Australian scientists to analyze crown-of-thorns starfish from the Great Barrier Reef.

Instead of just using DNA in the mitochondria, the scientists aim to
sequence the entire genome of each starfish, including DNA in the nucleus.

"Ultimately, we hope our findings can help us understand the population
trends of the starfish better and the role of ocean currents in seeding
new outbreaks," concluded Prof. Satoh. "This could potentially help us
predict and therefore mitigate future outbreaks."

========================================================================== Story Source: Materials provided by Okinawa_Institute_of_Science_and_Technology_(OIST)
Graduate_University. Original written by Dani Ellenby. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Jun Inoue, Kanako Hisata, Nina Yasuda, Noriyuki Satoh. An
Investigation
into the Genetic History of Japanese Populations of
Three Starfish, Acanthaster planci, Linckia laevigata,
and Asterias amurensis, Based on Complete Mitochondrial DNA
Sequences. G3: Genes|Genomes|Genetics, 2020; 10 (7): 2519 DOI:
10.1534/g3.120.401155 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200708105912.htm

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