It seems there's a rather darker side to the ever-popular Cleaner shrimp!
The Cleaner shrimp (Lysmata amboinensis) is no stranger to the fishkeeping hobbyist, and is perhaps the most recognisable mobile invertebrate in the marine hobby, favoured for its interesting cleaning behaviour, bright colouration and for their reputation for scavenging as part of an effective clean-up crew.
In the wild Cleaner shrimp typically live in monogamous pairs, but this social harmony has some dark undertones, notably murder.
New research published in BioMed Central's open access journal Frontiers in Zoology shows that Cleaner shrimp, in any group larger than two, viciously attack and kill each other until only a single pair remains.
Lysmata amboinensis are protandric simultaneous hermaphrodites, meaning that they are born male but, as they grow they also develop female reproductive organs. However, despite being hermaphrodites they are unable to self-fertilise so two individuals are needed for breeding to occur.
Like so many other shrimp species L. amboinensis only mate during the few hours after a moult, as this is the only time that shrimp acting as females are able to mate.
Janine Wong and Prof Nico Michiels from the University of Tübingen separated L. amboinensis shrimp into aquariums in groups of two, three or four. To minimise competition for resources the shrimp within each tank were approximately the same size, had limitless access to food, and each shrimp was provided with the same volume of water and a perch.
After only 42 days researchers found that in all groups larger than pairs one or more shrimp had been attacked and killed. The shrimp were killed during the night, just after shedding their old skin, when they were less able to defend themselves.
Janine Wong explained, "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 moulting 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 moulting increased for the remaining shrimp."
For more information see the open-access paper: Control of social monogamy through aggression in a hermaphroditic shrimp, Janine WY Wong and Nico K Michiels. Frontiers in Zoology.
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