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Gall crabs are tiny, and yet these crustaceans have evolved fluorescence to help them be concealed within hideouts they have created in the coral itself.

Fluorescence in gall crabs has been reported for the first time by KAUST researchers, who suggest that it may serve as a form of disguise.

"These crabs are everywhere," says Susanne Bähr, who led the study. "People ask how they can see them, and I say, 'Take a mask and a snorkel and go anywhere you find coral, and you'll see them.'"

The paper is in the journal Proceedings of the Royal Society B: Biological Sciences.

Gall crabs don't just live among the corals; they have a strong symbiotic relationship with them. Some invertebrates hide within the branches and crannies of corals, but for gall crabs, the connection goes much deeper.

"They settle on a coral as larvae, and then, somehow, they make the coral grow around them in very specific shapes. We don't know how they do that," says Bähr.

Female crabs then stay in this den for the rest of their lives, feeding on nutritious mucus and producing larvae, while the males rove about looking for females to mate with.

Bähr noticed that gall crabs fluoresce when on a night dive.

"I had been working with these crabs for some time, so this observation was particularly intriguing, and I started reading about fluorescence. It's been well studied in reef fish, where it has a broad array of functions. However, less is known about fluorescence in crustaceans, and yet crabs and shrimps are enormously diverse. So I wondered if we could find similar patterns in crustaceans as we've observed in fish," she explains.

Bähr and colleagues collected 286 gall crabs from 14 different genera, sampling from all the known host coral genera in the Red Sea and Indian Ocean. They developed an imaging technique to identify exactly which parts of the different crabs fluoresced, as well as how much of each body part was fluorescent. Using this, they carried out a morphological analysis of the different species. They also built an evolutionary tree based on the crabs' genomic sequencing data.

The morphological analysis showed that the fluorescence patterns on the crabs were grouped into four clusters. One cluster included non-fluorescent crabs, and another included crabs with diverse fluorescent patterns; the final two had strong fluorescence on specific body parts. The evolutionary tree identified the genus in which fluorescence first appeared—probably covering most of the body—and showed that for some specific lineages, fluorescence has been lost or reduced.

The researchers suggest that fluorescence evolved in different types of gall crabs to help camouflage them in their coral dens. Different gall crab species live in dwellings of different shapes on the coral—for example, open tunnels or enclosed galls—and the fluorescence patterns affect their visibility in their dens.

Bähr offers examples of a species that lives in cylindrical pits in the coral. "Basically, the back of the crab is sticking out a little bit. It has this really striking fluorescence pattern that disrupts how the crab looks. It disguises the outline of the crab, so you really can't see a crab shape."

Francesca Benzoni, Bähr's supervisor at KAUST, highlights the importance of better understanding coral reef ecosystems. "Gall crabs are one of the many types of invertebrates living in association with corals on tropical reefs," she says.

"Much remains to be discovered about 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."

Understanding these broader systems is also important to Bähr.

"I want to use my research to highlight the importance of these coral-associated invertebrates and their significance for coral reef ecosystems," she says. "They're generally overlooked, and it's very important for us to understand how many there are, why they're there, and what they do for reef persistence and resilience."