Tears of the Virgin: a unique relationship between barnacle and bacterium John Buckeridge, RMIT University

It’s a weeping rock. Sandstone blocks near the Lakes Entrance holiday resort on the coast of Victoria, south-east Australia, are covered with barnacles that look like they are spilling tears.

How did those so called “Tears of the Virgin” get there?

It seems that the unique geological formation is a product of equally unique biology: the first known symbiotic relationship between crustaceans and bacteria.


“It is important because it shows how organisms slowly modify their environment – even if the environment seems ‘as solid as rock’,” says John Buckeridge at RMIT University in Melbourne, Victoria. “What is cute here is the relationship between the barnacle and the cyanobacteria that allows this to happen.”

To understand how and why the structures formed, he surveyed the site and analysed samples in the lab.

Working with Bill Newman from the Scripps Institution of Oceanography, California, he found that the barnacles (Chthamalus antennatus) are surrounded by dark-coloured microbes known as cyanobacteria, which feed on their nitrogen-rich waste. This results in the rounded eye shape from which the tears appear to fall.

The cyanobacteria also excrete organic acids that dissolve carbonate, a major component of the sandstone (see below). The resulting burrows protect the barnacles from the fierce sun and material thrown up by waves.

Excess acid dribbles down the surface of the rock and carves out grooves up to 17 centimetres long. These become populated by additional cyanobacteria, creating the characteristic tears.

Victoria sandstone cliffs play host to strange tears John Buckeridge, RMIT University

The curious-looking formations are the first known example of a mutually beneficial relationship between a crustacean and bacterium, says Buckeridge. The barnacles provide food for the bacteria, while the bacteria dig out shelters up to 15 millimetres deep for the barnacles.

Nevertheless, it is strange that the barnacles would team up with bacteria that erode the rock surface on which they live, Buckeridge says. “The really odd thing about this is how they manage to hang on.”

The barnacles appear to cement themselves in place by sequestering residual non-carbonate rock towards the inner grooves of their shells, Buckeridge says.

The tears are submerged at high tide, so the barnacles are under the water for part of the day where they eat plankton. Barnacles are more commonly found lower in the intertidal zone but on these rocks they sit higher than usual. “This relationship with cyanobacteria gives them an opportunity to exploit a slightly higher zone, where there is less predation and competition,” he says.

Perching on the rock surface above the water line is advantageous because the barnacles are less likely to fall prey to crabs and other marine organisms. However, the rock can reach 50 °C during summer, meaning the barnacles would succumb to dehydration if they couldn’t nestle into the burrows made by the cyanobacteria.

Similarly, without the barnacles, it would be harder for the cyanobacteria to find food in the hostile environment, says Buckeridge. “It’s a match made in heaven.”

The symbiotic relationship most likely started out as an opportunistic partnership, he says. No one seems to know how old the Tears of the Virgin are, or who named them, but the burrow depths suggest they have been there for several decades.

The finding is part of a wider appreciation of the importance of microbes to a whole range of hosts, from barnacles to plants to humans, says Ezequiel Marzinelli at the University of New South Wales, Australia.

“These associations between microbes and their hosts form a coherent biological entity, or ‘holobiont’,” he says. “These components must be studied together if we are to have an understanding of biological systems.”

Journal reference: Integrative Zoology, DOI: 10.1111/1749-4877.12244