They may be among the smallest of the marine life to dwell among the coral reef ecosystem of the Red Sea, but shrinking violets they are not. In fact, when it comes to the matter of self preservation, a new study has found that gall crabs turn up the flamboyance, by dialling up the fluorescence to camouflage with their surroundings.

This is the conclusion of a new study, authored by marine researchers from the King Abdullah University of Science and Technology (KAUST), who have discovered that these tiny, coral-dwelling crabs – measuring no more than one centimetre in size – use a highly evolved fluorescence to help conceal themselves within coral hideouts.

Led by Susanne Bähr, a marine science doctoral student at KAUST, the study marks a new first for marine science, being among the only published studies to date to take a closer look at fluorescence among crustaceans – an area of study that has – until now – been largely dominated by research into reef fish and other marine life.

The inspiration for the piece of research came to Bähr during a series of night dives in the Red Sea when her curiosity was piqued after observing the fluorescent behaviours of gall crabs under the cloak of darkness.

“I had been working with these crabs for some time, so this observation was intriguing,” said Bähr. “I started reading about fluorescence.”

While the many functions of fluorescence have been studied closely in a number of reef fish species, less is known about the display among crustaceans. 

“Crabs and shrimps are very diverse, so I wondered if similar patterns occur in crustaceans as in fish,” said Bähr. 

Bähr and her team of colleagues set about collecting 286 gall crabs from 14 genera, sampling from all known host coral genera in the Red Sea and Indian Ocean. Together, they developed an imaging technique to identify which parts of the crabs fluoresced and how much. They also performed a morphological analysis and built an evolutionary tree based on genomic sequencing.

Among the observations made during the study, it was concluded that gall crabs don’t just live among coral; they have a strong symbiotic relationship with them. Some invertebrates hide in coral branches and crevices, but for gall crabs, the connection – as suggested in the report now published in the Proceedings of the Royal Society – is far deeper.

In fact, the researchers suggest that fluorescence evolved in various gall crab species to help camouflage them in their coral homes. Different species inhabit different coral structures – such as open tunnels or enclosed galls – and their fluorescence patterns influence their visibility.

Bähr gave an example of a species living in cylindrical coral pits.

“The back of the crab sticks out slightly. Its fluorescence pattern disrupts its outline, disguising its shape,” she said. 

Bähr’s supervisor at KAUST, Francesca Benzoni has highlighted the importance of better understanding coral reef ecosystems and the possible role that gall crabs have to play in the resilience of those systems both in the Red Sea and across the world.

“Gall crabs are one of the many types of invertebrates living in association with corals on tropical reefs,” said Benzoni. “Much remains to be discovered on cryptic and poorly studied reef invertebrates and their fundamental biology, ecological role, and the role they play in the resilience of coral reef ecosystems in the Red Sea and worldwide.”

For Bähr, the aim is to use her research to highlight the importance of these coral-associated invertebrates and their significance within the coral reef.

“They’re generally overlooked and we need to understand how many there are, why they’re there, and what they do for reef persistence and resilience,” she concluded.

The paper, Integrative phylogenomics sheds light on the diversity and evolution of fluorescence in coral-dwelling gall crabs – has now been published in Proceedings of the Royal Society.

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