Mantis Shrimp Size Up Foes' Color Spots Before Battle

Telltale markings warn overly aggressive shrimp they could be in for a tough fight.

Mantis shrimp, the lobster-like denizens of the ocean floor, are well known for two things. First, their punching power, which is forceful enough to break break glass. Second, their dazzling array of colors. One thing scientists have wanted to learn more about is how, or even if, those colors communicate information to other shrimp.

Now, research out of Tufts University has found that a pair of colored spots on the creature do just that: they tell other mantis shrimp to not even think about messing with them or their homes.

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For all of their glitter, mantis shrimp, or stomatopods, are undersea street fighters. They live in cavities created by rocks or coral. There they raise families, eat, and hide out from things that would like to eat them. With all of that at stake, they'll defend their private property fiercely when something -- in this case another shrimp -- shows an interest in their turf.

Part of that defense involves threat displays, in which the shrimp show off their meral spots, the colored dots on their appendages shown in the photo below. The researchers found that shrimp use the ultraviolet (UV) color reflectance of the dots, as well as chemical cues, to gauge how tough another mantis shrimp might be to tangle with in battle.

The Tufts team staged fights between shrimp (hastening to add that none of the crustaceans were hurt). But, before the battles began, they applied sunscreen to the meral spots of some of the combatants. That dimmed the spots' UV reflectance.

The scientists found that shrimp whose meral spots had been dimmed suddenly looked like more inviting targets: Their opponents were more willing to ratchet up their aggression against them. This suggested to the researchers that the spots serve as enhanced threat displays. They're meant to back down opponents, in turn helping aggressor shrimp realize they could be defeated if they chose to fight.

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Colors weren't the only thing the scientists blocked for the test battles. By dipping some shrimp's antennae in freshwater for a minute, they also temporarily blocked the ability of some of the shrimp to pick up on chemical signals. With no chemical sensing in their arsenal, the affected shrimp were quick to go near an opponent's refuge and they weren't as aggressive when doing so.

The researchers said chemical signals, then, may provide a tipoff to opponent size and whether or not that opponent is inside its refuge.

"Our experiments demonstrate that [mantis shrimp] use a complex signaling system that combines the UV reflectance of an important spot of color as well chemical cues to help them judge their opponent's state of aggression, fighting ability and the presence of a stomatopod in a refuge," said Amanda Franklin, a Tufts biology PhD student, in a statement.

Franklin and her colleagues' findings have been published in the journal Royal Society Open Science.

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