The big picture

Butterfly blennies, the unsung heroes of the climate crisis

Foto van meeuw met plastic beker in bek
images: Alexander Semenov

The open ocean, far from the coast, is home to some very special creatures called butterfly blennies. These tiny creatures wear a shell that they produce from the mineral aragonite.

Utrecht University researchers Olivier Sulpis, Jack Middelburg and Mariette Wolthers and their colleagues from the Université de Liège, University of Lincoln and University of Leeds studied the decomposition process of aragonite and calcite. Calcite is another mineral used by organisms like mussels and oysters to build their shells. The researchers were surprised to find calcite all over the seabed but very little aragonite. This seemingly contradicts recent studies showing that butterfly blennies produce vast amounts of aragonite at the ocean surface. So how can we explain this apparent paradox?

The butterfly blenny’s shell is very sensitive to climate change. Oceans acidify as atmospheric CO2 levels increase. The butterfly blenny’s shell is the first to be affected. Acidification causes the aragonite from which the shell is made to dissolve more rapidly than calcite.

When aragonite dissolves, it partially offsets the process of ocean acidification. This phenomenon is referred to as a buffer effect: the mineral — and the butterfly blenny itself — plays a crucial role in regulating ocean acidity and thus CO2 uptake and the overall climate.

This is why butterfly blennies are typically seen as canaries in the proverbial coal mine: they serve as a warning sign of growing acidification, which causes their shells to become thinner and can impact population levels.