If you've ever watched a nature program on coral reefs, you've probably seen a cleaner shrimp. These animals set up cleaning stations on the reefs where passing fish let the shrimp pluck off parasites. It's a classic symbiotic relationship: the fish have fewer parasites, while the shrimp get a meal and don't get made into one. Beneath this G-rated facade, however, one of the species of cleaner shrimp engages in a sex life that's calculating and murderous.

Lysmata amboinensis is what's technically known as a "protandric simultaneous hermaphrodite." That means that, as they mature, the animals only have male sexual organs. Once they reach a certain stage in development, however, they add female organs to their repertoire, with both being active simultaneously. They do not, however, mate with themselves, preferring a serial monogamy in which they fertilize a single partner and, in turn, get fertilized by them.

Now, this sort of mating could take place among any number of partners, but some researchers suspect there were two factors driving the animals towards forming pairs. For one, it simplifies the energetic cost of mating; the animals only have to make enough sperm for one partner, and don't have to waste energy overproducing for some unpredictable number of additional mates. The other issue is that larger groups open the door for selfish feeding behavior, where the shrimp move beyond eating parasites and start plucking pieces off their clients, causing the fish to avoid a cleaning station and harming the entire group.

In fact, they found that the pressures to create a stable pair of shrimp was so intense, that any larger group became positively murderous. When they put larger groups of shrimp in a single container, the animals reduced their numbers down to a pair. "Exactly one individual in each triplet and exactly two individuals in each quartet were killed in aggressive interactions, resulting in group sizes of two individuals," the authors report.

The killings took place after one member in the large group had moulted, or shed its hard outer shell in order to grow. In fact, the shrimp seem to know that moulting creates this risk, since the animals in the larger groups put off moulting (and hence sacrifice growth) until one of them finally shed its shell and was dispatched.

Presumably, this sort of lethal interaction doesn't happen much in the wild, since an odd animal out would recognize its peril and move on to a new location. Still, it's a rather extreme way to keep a visitor from overstaying its welcome and mating with your partner.

Frontiers in Zoology, 2011. DOI: 10.1186/1742-9994-8-30 (About DOIs).