Clownfish mucus composition changes during host anemone symbiosis

A recent scientific paper has confirmed what many people thought all along, that Clownfish can survive the sting from host anemones because of something in their mucus. But up until now, there has been very little molecular evidence to support it.

A team of researchers decided to put that theory to the test, using pairs of Percula Clownfish and Bubble Tip anemones, and taking mucus samples before and after the clowns and anemones snuggled up. They also tried the ‘nems on two other mucus sources, from Green Chromis, and (dead,) prawns, to see what difference that made to the host anemone and firing its nematocysts, as well as specifically profiling the clownfish’s mucus lipid and glycan composition.

Experimental design showing mucus types introduced to Entacmaea quadricolor anemones (n=10) across the nine-week experimental period to assess nematocyte discharge during hosting and non-hosting periods. Each mucus type (anemonefish, damselfish, prawn) was presented to each anemone over the three-week non-hosting period. A pair of anemonefish were then added for a three week acclimation period. Mucus sampling was then repeated over three weeks for the hosting period, with ‘familiar’ anemonefish mucus introduced to the corresponding anemone.

Firstly, the study found that host anemones significantly reduced nematocyst firing when hosting familiar, acclimated clowns versus newly introduced unacclimated ones. Secondly, it found that clownfish glycan composition was noticeably different from a clown that was resident in an anemone for three weeks versus one that wasn’t and that the glycan composition reverted to non-acclimated if the clown spent more than 24 hours away from its host.

“Triggering fewer nematocysts through glycan profile alterations may be an important adaptation that has enabled anemonefish to live long-term in a sea anemone host.” say the authors. “However, the delay in mucus response indicates it is not the initial mechanism used by anemonefish to enter a host sea anemone without being stung.”

Mucus collecting procedure for *Amphiprion percula* anemonefish with and without *Entacmaea quadricolor* sea anemone presence over the 8-week experimental period, for hosted (n=12) and non-hosted (n=12) groups.

So part of the puzzle about what happens to clownfish mucus during symbiosis has been answered by this paper. After three weeks, it changes, nullifying the aggressive nature of the host anemone to the clownfish living within it. But the mucus has no nullifying effect when a new clownfish first enters an anemone, and how it gets around being stung and killed during the first encounter remains unknown, for now.

To read the full pre-print paper, its hypothesis, testing methods, and conclusions, go to https://doi.org/10.1101/2024.02.22.581653

Main image credit Nick Hobgood, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

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