Scientists discover murderous shrimps

Nov 21, 2011, 13:33 GMT

Berlin - Scarlet cleaner shrimps live happily in stable partnerships, but when an unwanted rival intrudes, they rapidly turn to murder.

As soon as more than two of these coral reef dwellers are kept in an aquarium, a fight to the death begins, until just one pair remains, according to biologists working at Germany's Tuebingen University. Writing in the journal Frontiers in Zoology, the team describes how the unwanted shrimp is eliminated at night as it is changing its shell and unable to defend itself.

The biologists said creating optimal conditions for procreation was probably behind the aggressive behaviour. The less competition for food, the greater the chance the individual shrimps will attain a size that makes breeding more likely.

The scarlet cleaner shrimp (Lysmata amboinensis), which grows to six centimetres are hermaphrodites - each individual possesses both male and female sex organs. They live as monogamous pairs in the coral reefs of the Indopacific and the Red Sea.

It is known that these cleaner shrimps live on parasites taken from fish and pieces of dead skin, but little has been known to date about their social structure. Janine Wong and Nico Michiels placed these cleaner shrimps in aquaria in groups of two, three and four. Although the shrimps had unlimited food available, after 42 days, each aquarium contained just two specimens.

The biologists linked this behaviour to competition for food in the natural environment. The larger the individuals grow, the more eggs they are able to lay. This means the most favourable conditions for breeding are when there is just a single breeding pair in the area, so that these two individuals can monopolize all the food available.