Two’s company, three’s murder (Image: Jeff Rotman/Nature Picture Library/Rex Features)

Species: Lysmata amboinensis

Habitat: coral reefs in the Indian and Pacific, striking in the night

Cannibalistic and willing to fight to the bitter end under the cover of darkness: Lysmata amboinensis shrimp have a brutally romantic side. They are so hell-bent on living in pairs that when placed in groups of three or four, they attack their peers until just one couple remains alive.

These hermaphrodite cleaner shrimps start out as males and develop female reproductive organs as they grow. They can mate as a female only in the few hours after moulting their exoskeleton, but can then mate as males even while they are incubating eggs. Despite all this gender-bending prowess, however, there is one thing they cannot do: self-fertilise. Both male and female they may be, but they still need a mate to mate.


The bright orange shrimp, which have bold red and white go-faster stripes on their backs, wouldn’t normally encounter many of their kind as they move around the reefs where they live. Mostly, they just hang out on their own, occasionally pairing up to mate. If do they happen across others, they might ignore each other or sit quietly together as the afternoon sun streams through the water from above. But as soon as the sun sinks and the water goes dark, the seemingly calm crustaceans turn into killers.

Moonlit massacre

At just 6 centimetres long, L. amboinensis has a Napoleon complex to go with its size.

It scavenges parasites and dead tissue off the backs of larger fish, and faces intense competition for food resources. As a result, L. amboinensis is aggressive to the point of killing off other shrimp that threaten their livelihood.

In a study at the University of Basel in Switzerland, Janine Wong placed groups of three or four shrimps together in tanks, fed them regularly, and observed them for 42 days. During the day, the shrimps sat quietly in the tanks, mostly ignoring each other.

But at night, infrared cameras captured a very different scene. “The shrimp started to go crazy and chase one individual until they had killed it,” Wong says. “They are also cannibalistic, so they were eating the dead individual in the morning.” The murderous rampages always happened just after the victim had moulted its exoskeleton, when the crustaceans are at their most vulnerable.

Killing patterns depended on how many shrimp were in a tank, but always left one couple alive. With three shrimp in one tank, the smallest one usually got the chop, simultaneously attacked by the pair of larger animals.

In tanks with four shrimp, the shrimp that moulted first died first. The largest of the remaining trio then turned on the smallest.

The shrimp would fight to the death even when they had plenty of food, suggesting that the serial killing had nothing to do with resources and everything to do with living in pairs. “Their behaviour is sort of hardwired,” says Wong.

Fickle monogamy

“It’s surprising to find social monogamy in hermaphrodites,” she adds, explaining that in most hermaphroditic species, cheating is common because having more mating partners increases the chances of reproducing.

Though the experiments always resulted in only one pair of shrimps surviving in each tank, Wong says that an interloper could cause the happy couple to split up. Partnerships only last as long as they are the best thing on offer for both. If another, larger shrimp were to enter the tank, the smaller of the existing pair would not survive its next moulting.

Journal reference: Frontiers in Zoology, DOI: 10.1186/1742-9994-8-30

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