The world's most promiscuous… snail

• 12:00 24 March 2010 by Michael Marshall
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Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world.

Species: Littorina saxatilis

Habitat: shorelines of the North Atlantic, and San Francisco Bay

It's a question all females have to address in one way or another: how many males should they allow to fertilise their eggs?

If the males aren't going to help with caring for the young, there may be no point in a female limiting herself to just one mate. And indeed in many species, the females allow several males to fertilise them. This helps ensure that at least some of her offspring are likely to have "good" genes.

But a few species go beyond the usual definition of the word "several". On the latest evidence, it seems the champion may be the rough periwinkle; that's the name of the animal, not my description. She will be fertilised by around 20 males for every clutch she produces.

Polyandrous periwinkle

Rough periwinkles are sea snails, less than 2 centimetres across, which exist in a range of colours. They are divided into several subspecies, but exactly how and why they are so divided is controversial.

The snails live on shorelines, generally in intertidal zones that are alternately submerged and exposed as the tides come in and out. The colonies are often densely packed, with hundreds of individuals slowly oozing across a single square metre of wet rock.

Males track females by following their mucus trails, and will attempt to mate with pretty much any snail they encounter, regardless of whether it is the correct sex or even the same species. They mate with males just as often as they do with females – though they do give up such copulations sooner.

By contrast, the females are quite passive. When a male attempts to copulate, he parks himself on her right side and stays there for 20 minutes or more while he transfers sperm into her through his penis.

What's a snail to do? The female hardly ever resists, but she seems not to do anything to encourage him either. In fact, she barely seems to notice his presence, and may continue blithely moving around while he is copulating with her, effectively giving him a piggyback.

Baby snails

Female rough periwinkles give birth to live young, half a millimetre across, which are already in their shells. The babies are known as "crawlaways", because as soon as they emerge from their mother's brood pouch they are able to… well, you can work it out.

Kerstin Johannesson of the University of Gothenburg in Sweden and colleagues analysed the genes of four female rough periwinkles, along with the 280 offspring they produced over the course of two-and-a-half months – an average of 70 offspring per female.

On average, each clutch of 70 offspring had 19 fathers between them. Larger clutches had more. This makes the female rough periwinkle one of the most polyandrous animals known.

Why does she bother? This is a far greater number of fathers than she needs to produce genetically diverse – and therefore mostly healthy – offspring, especially since she is able to bear many clutches over the course of her life.

Johannesson's team suggests that she may simply be avoiding the trouble involved in rejecting potential suitors. To get rid of an unwanted male she would have to retreat into her shell, weakening her grip on the rock and risking being swept into the sea.

But carrying a male around on her back also puts her at risk of being dislodged, so the female, it seems, is stuck between a rock and a hard place. Promiscuity was never less fun.

Journal reference: PLoS ONE, DOI: 10.1371/journal.pone.0009640