Shape-shifting rotifers (via Chad Arment)

Michael Marshall


In a pond in Argentina three animals are being hunted. They are tiny beasts, animals a tenth of a millimetre long, and are being menaced by monsters many times their size. The prey are rotifers, and each comes with a different complement of defensive spines.

They all look different, but the three hunted rotifers are genetically identical: not only are they from the same species, they are clones. So how did they become so distinct?

Attack the clones
A rotifer is small but intricate. Each one has a mouth lined with tiny hairs called cilia, which pulls food into its stomach and intestine. It also has a simple brain and nervous system. Most of the time they reproduce asexually, only having sex every once in a while – although one group, the bdelloid rotifers, have done without sex for 70 million years.

Keratella tropica is a herbivore, feeding mostly on algae but also snaffling the odd bacterium. It is box-shaped, twice as long as it is wide, and in its normal state it has three pairs of small spines on its front and one pair on its rear.

If predators arrive, Keratella quickly finds out that its standard complement of spines is not much of a defence. It would help if it could grow longer spines to fend them off, but it can't. "Its phenotype is fixed at birth," says rotifer biologist John Gilbert of Dartmouth College in Hanover, New Hampshire.

Instead, the mere presence of a certain predator can make it change the shape of its offspring. Occasional bouts of sex aside, Keratella's children are clones of the parent and carry exactly the same genes. How it does it is a mystery, but the animal "knows" which specific predators share its water and produces offspring with different defences accordingly. It is the only rotifer that responds selectively to different predators.

Hunted mother
The most dramatic transformation is triggered when it is hunted by another rotifer, Asplanchna brightwelli, which can be a relatively monstrous 0.7 millimetres long – seven times as long as Keratella. It is a voracious but unsophisticated predator that basically swims around randomly until it bumps into something, then tries to pull it in.

The offspring of an Asplanchna-hunted mother grow their right rear spines up to 0.1 millimetres long, four times as long as normal, while the left one disappears. They also grow two of the front pairs a bit, though the big rear spine is the key: it makes the animal too long for Asplanchna to swallow.

A second transformation occurs if Keratella has been hunted by Daphnia pulex. Sometimes clocking in at a whopping 3 millimetres long, these crustaceans are commonly known as water fleas and can take a major toll on rotifers. To deal with them, Keratella grows both its rear spines from the normal 0.02 millimetres to 0.03 millimetres, leaving the front spines unchanged.

Daphnia feeds by pulling a current of water through its body, then picking out food particles and rejecting everything else. Keratella is often rejected but gets damaged in the process. Having longer spines makes Daphnia reject it sooner, reducing the chance of fatal damage fourfold. Still, the spines are no good against Asplanchna, and Keratella faced with both enemies will always develop the enormous rear spine.

All this raises the question: why bother? If the anti-Daphnia defence is so much weaker, why not just use the anti-Asplanchna defence all the time? Perhaps the distended rear spine is more expensive to make, but Gilbert has found no evidence for a cost: anti-Asplanchna rotifers live just as long, and reproduce just as well, as their normal and anti-Daphnia counterparts.

Somehow these genetically identical animals recognise their enemies and, with no genetic mixing, create offspring with custom-made defences. What's more, as far as we can tell, it doesn't cost them anything. Talk about parenting skills.

http://www.newscientist.com/article/dn19875-zoologger-child-clones-shapeshift-to-escape-hunters.html