Clownfish avoid the sting of their anemone hosts with sugary slime

The secret’s in the snot. Chemical changes in the mucus that coats a clownfish’s body can blunt the sting of its symbiotic anemone partner.

Researchers have long suspected that something special about the mucus of clownfish, also known as anemonefish, protects them from the microscopic venomous barbs of an anemone’s tentacles. But the precise mechanism remained a mystery, says Karen Burke da Silva at Flinders University in Australia.

To investigate, she and her colleagues raised orange clownfish (Amphiprion percula) and bubble-tip anemones (Entacmaea quadricolor) in the lab. Some of the fish and anemones were paired together, while others lived separately. The team took mucus samples from the fish at various times before and after they acclimated to their anemones, then put the mucus on microscope slides and pressed it onto an anemone’s tentacle.

Anemones sting by explosively firing tiny, coiled, venomous harpoons from stinging cells called nematocytes. Using a microscope, the researchers counted and compared how many nematocytes fired between the mucus treatments. They found mucus from clownfish partners – but not from unacquainted fish – reduced nematocyte firing.

To figure out why, the researchers analysed how glycans – chains of sugars that attach to proteins – and fats in the mucus changed as the clownfish acclimated to their host. Three weeks into a symbiotic partnership, the mucus’s chemical profile had shifted substantially. In particular, the concentrations of seven different types of glycans had changed. Getting rid of glycans or otherwise tweaking them may be one way to suppress the nematocytes’ firing, says Burke da Silva.

Alonso Delgado at The Ohio State University wonders if other anemone symbiotes, such as anemone shrimp (Ancylomenes magnificus), use a similar glycan method to stymie stings, or if they have evolved different strategies.

Additional strategies could also be at play in clownfish. The glycan change is slow and reverts within a day of the partners being split up. Instead, the fish may use an unknown chemical strategy in the very beginning to get initial access to the anemone.