Snapping shrimps: Females more aggressive than males

Snapping shrimp Alpheus heterochaelis Snapping shrimp (Alpheus heterochaelis) shoots jets of bubbles when it snaps shut its enlarged claw

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Female snapping shrimps are more aggressive than males, frequently snapping their deadly claws at each other, a study has shown.

Researchers in the US had expected male shrimps - which brandish a larger snapping claw - to behave more aggressively.

But despite their smaller claw size, females snapped more often when defending their territory.

Findings are published in the journal Ethology.

"We're so used to seeing animals in which males have larger weapons than females: bigger antlers, bigger teeth, bigger horns... And in many of these species, males are more aggressive than females," said Dr Melissa Hughes, from the College of Charleston in South Carolina, US who led the study.

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"Males, more so than females, benefit from being aggressive and having large weapons, in species where males compete for females."

Snapping shrimps are sometimes called pistol shrimps and use their larger claw - the major chela - as a deadly weapon to kill prey and opponents.

They can kill enemies with a direct snap, and are known to stun potential food with a jet of bubbles by rapidly closing their larger claw. The snapping sound of this bubble blast collapsing gave the animals their name.

The researchers wanted to see how different sexes use their claws to defend their territory.

They collected two species of snapping shrimp, Alpheus heterochaelis and Alpheus angulosus, and studied same-sex and opposite-sex interactions in test chambers.

The shrimps were allowed to become residents in a burrow, before being faced with intruder shrimps.

The researchers filmed these interactions and counted the number of snaps - a sign of aggression - by each shrimp.

 Alpheus angulosus Female shrimps (top) have smaller snapping claws than males (bottom)

In both species, female shrimps contradicted the researchers' expectations by snapping more overall, and behaving more aggressively towards other females. Males were equally aggressive to both male and female intruders.

Dr Hughes admitted the reasons for females' more aggressive behaviour are unclear.

"Female aggression has not been studied nearly as much, and so we don't understand it nearly as well," she told BBC Nature.

"One possibility here is that females are competing with each other for males, either through direct competition fighting over males, or by defending large territories so that the distance between females is too large for one male to easily move between multiple females."

There are over 400 species of snapping shrimp. The crustaceans use their two distinct claws for different functions, and if they lose their larger claw in battle, the smaller one transforms into their snapping claw, and a new one grows where the snapper used to be.

"These are incredibly cool animals that nearly no one has ever heard of," commented Dr Hughes.

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