IT鈥橲 as if the rock is crying. Sandstone blocks near Lakes Entrance on the coast of Victoria, south-east Australia, are covered with barnacles that look like they are spilling tears. It now seems that these 鈥渢ears of the Virgin鈥�, as they are known locally, result from the first known mutualism between these crustaceans and cyanobacteria.
鈥淚t is important because it shows how organisms slowly modify their environment 鈥� even if the environment seems 鈥榓s solid as rock鈥�,鈥� says John Buckeridge at RMIT University in Melbourne, Victoria. 鈥淲hat is cute here is the relationship between the barnacle and the cyanobacteria that allows this to happen.鈥�
Buckeridge surveyed the site, where barnacles sit higher out of the water than they normally do, and analysed samples in the lab. Working with William Newman from the Scripps Institution of Oceanography in San Diego, California, he found that the barnacles are surrounded by dark-coloured cyanobacteria, which feed on the crustaceans鈥� nitrogen-rich waste. This results in a rounded eye shape, with the barnacle looking like the iris and pupil at the centre, from which the tears appear to fall.
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The cyanobacteria excrete organic acids that dissolve carbonate, a major component of the sandstone. This creates burrows that protect the barnacles from the fierce sun and from material thrown up by waves. Excess acid dribbles down the rock surface and carves out tear-like grooves. They can get to 17 centimetres long, and become populated by additional cyanobacteria (Integrative Zoology, ).
鈥淭hese associations between microbes and their hosts form a coherent biological entity鈥�
鈥淭hese associations between microbes and their hosts form a coherent biological entity, or 鈥榟olobiont鈥�,鈥� says Ezequiel Marzinelli at the University of New South Wales, Australia. 鈥淭hese components must be studied together if we are to have an understanding of biological systems.鈥�
This article appeared in print under the headline 鈥淢icrobes make barnacles weep solidifying tears鈥�