The cleaner shrimp is perfectly happy to live out its life in peace and tranquility with its monogamous partner. But add any more shrimp to the mix — even another happy couple — and the whole thing turns into a bloodbath.
This particular cleaner shrimp species is Lysmata amboinensis, a hermaphroditic species that lives in monogamous pairs. All the shrimp in the species actually start out as males, but over time they develop the female organs needed to reproduce. Because they can't self-fertilize, the shrimp pair off and take it in turns to incubate eggs.
It doesn't seem like this is a recipe for endless intraspecies warfare, but that's the finding of researchers at the University of Tubingen. They placed shrimp in tanks in various groups of two, three, or four. The shrimp were all basically the same size and all the tanks had unlimited food resources, meaning there was no reason for any of them to fight. And yet, after just 42 days, all the tanks with more than two shrimp had turned violent, with all the shrimp killed so that just pairs remained.
So what's going on here? Researcher Janine Wong explains:
"In the wild, monogamy is only seen for shrimp which have adopted the symbiotic 'cleaner' lifestyle. For these shrimp, competition for food is likely to be the driving force behind their monogamy (more shrimp equals less food per shrimp) and, since body size is linked to the number of eggs laid, a large group would decrease each individual's potential to produce offspring. Confirming this hypothesis we found that shrimp molting was delayed in the larger two group sizes, despite the freely available food, and that once the group size had reduced to two, the rate of molting increased for the remaining shrimp."
Via Frontiers in Zoology. Image by Janine Wong..