Tiny crabs have evolved to use fluorescence as camouflage in coral reefs.
During a night dive in the Red Sea, Ph.D student Susanne Bähr noticed the strange phenomenon, saying: “I had been working with these crabs for some time, so this observation was particularly intriguing, and I started reading about fluoresce.
“It’s been well studied in reef fish, which have many functions. However, less is known about fluorescence in crustaceans, and crabs and shrimps are enormously diverse. So I wondered if we could find similar patterns in crustaceans as we’ve observed in fish.”
Susanne – a Marine Science student at King Abdullah University of Science and Technology (KAUST) – and colleagues collected 286 gall crabs from 14 different genera, sampling from all the known host coral general in the Red Sea and Indian Ocean.
They developed an imaging technique to identify precisely which parts of the different crabs fluoresced and how much of each body part was fluorescent.
Using this, they carried out a morphological analysis of the other species. They also built an evolutionary tree based on the crabs’ genomic sequencing data.
Gall crabs don’t just live among the corals: they have a strong symbiotic relationship. Some invertebrates hide within the branches and crannies of corals, but for gall crabs, the connection goes much deeper.
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 an example of a species living in cylindrical coral pits. “The back of the crab is sticking out a little bit. It has this striking fluorescence pattern that disrupts how the crab looks. It disguises the crab’s outline, so you 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 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.”
Understanding these broader systems is also essential 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 we need to understand how many there are, why they’re there, and what they do for reef persistence and resilience.”
