Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Species: Eucypris virens
Habitat: Temporary ponds throughout Europe and central Asia, shrugging things off in a blasé manner
As anyone who has ever dropped one will know, eggs are fragile things. They are so easy to break, it's a wonder they make it to hatching point – particularly when you consider that they have probably been sat on by a chicken.
But while many birds lay fragile eggs, other species have evolved distinctly tougher ones. They have had to, because they live in harsh or even toxic environments.
The champion of tough eggs may well be a tiny crustacean called Eucypris virens. Its eggs can be frozen, irradiated, drenched in saltwater or digestive enzymes, deprived of oxygen or exposed to pesticides – with no discernible ill effects.
Asexual spread
E. virens is a kind of crustacean called an ostracod, or seed shrimp. This is another of those luckless groups of animals that hardly anyone is interested in, despite being extremely common and critical to many ecosystems. At first glance they could be mistaken for molluscs, as their bodies are encased in hard shells rather like those of clams.
Like many seed shrimp, E. virens exists in two forms: sexual and asexual. The two are subtly different in shape, and while the asexuals are found throughout Europe, the sexual forms are confined to the south, particularly Spain.
The sexual E. virens are best known for producing some of the largest sperm known in the animal kingdom. A single sperm can measure up to 1 centimetre, which is several times longer than the animal itself.
Still, asexual E. virens have spread further, possibly because they can spread faster into new environments. It only takes one asexual individual to colonise a new area, which explains how they were able to rapidly spread north after the glaciers retreated at the end of the last ice age.
While E. virens appears to be widespread, that may be an illusion. A 2010 genetic analysis suggested that what was thought to be one species might actually be at least 40, only one of which had been recognised.
Pond life
E. virens lives in temporary pools that last just weeks or months before drying out. They lay piles of dormant "resting eggs" that have evolved to survive being left high and dry, and then hatch when the water returns.
Wondering just how hardy these eggs are, Jochen Vandekerkhove of the University of Gdańsk in Poland and colleagues subjected both asexual and sexual eggs to a range of stressors and monitored how many hatched successfully.
The eggs were variously frozen to -72 °C for three weeks, exposed to ultraviolet B radiation for 10 hours, deprived of oxygen for a week, exposed to four digestive enzymes, drenched with salt water and treated with insecticide. Control eggs were stored in the dark without water.
Only the UV-B radiation had any effect, slightly delaying the hatching of asexual eggs. Sexual eggs were unaffected. Nothing else had any discernible effect.
This extreme tolerance puts E. virens in an elite group of animals that cope with a huge range of different environments. Many animals can survive being frozen or being exposed to toxins and other stressors, but it's unusual to be able to survive so many different things.
The only animals that can top E. virens are the tardigrades, or "water bears": tiny invertebrates that can handle being frozen to just above absolute zero, that's -273 ºC, and have even survived in the vacuum of space. Someone needs to send E. virens eggs into space to see if they can, too.
Journal reference: Freshwater Biology, DOI: 10.1111/fwb.12051
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Zoologger: The toughest eggs in the world
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Zoologger: The toughest eggs in the world
