With 60-mile-per-hour pincers that create noisy shockwaves capable of stunning or killing their foes, it's no wonder snapping shrimp are sometimes compared to pistols.
But being quick on the draw is not the most interesting thing about these see-through crustaceans, several species of which make their home within sponges on coral reefs.
You see, snapping shrimp are the only animals in the ocean known to practice eusociality—or the division of labor and collective raising of young most commonly seen in insects such as honeybees and some mammals, like naked mole rats.
Because these so-called pistol shrimp are the only marine creatures known to practice eusociality, scientists are extremely interested in what makes them tick. (Read how snapping shrimp are like "dinner bells" for one species of whale.)
In recent years, scientists have learned shrimp colonies are constantly at war: Males with extra-large pincers patrol the colony's perimeter like soldiers on guard duty, protecting the egg-laying queens. And hidden within each colony's center are juveniles, which adults protect and feed until they're ready to serve.
"When they grow up, they’ll stick around and take care of the next generation," says Sally Bornbusch, an evolutionary anthropologist and Ph.D. student at Duke University.
Now, in a new study, Bornbusch and colleagues have discovered an intriguing trade-off between the queens' ability to reproduce and defend themselves.
For the study, Bornsbusch and colleagues examined more than 300 snapping shrimp queens collected from sponges over the course of 25 years from coral reefs in Panama, Belize, the Bahamas, Jamaica, and Florida.
The team discovered that in snapping shrimp colonies with just one queen, that queen tends to lay more eggs and have a smaller pincer.
Likewise, in shrimp species where there are multiple queens within one colony, those queens sometimes battle with their nestmates over limited resources. And Bornbusch found that queens of these species tend to produce fewer eggs but possess larger claws.
While measuring eggs vs. claw size may seem trivial, understanding these trade-offs could help scientists understand why so few animals have evolved eusocial behaviors and what is gained by those that did. (See the amazing spiral hives of the Australian stingless bee.)
"In theory, you shouldn't be able to do a lot of both," says Bornbusch, whose study was published March 14 in the journal PLOS ONE. "You should have to trade-off your energy investment between either reproduction or defense."
But biologist Anthony Zera is not convinced that Bornbusch has discovered an evolutionary trade-off.
Particularly in a laboratory environment, "trade-offs are in fact very difficult to measure—much more so than many researchers realize," says Zera, who studies these sorts of evolutionary relationships at the University of Nebraska-Lincoln.
Zera says it’s important to note that the new study only investigated six species, and that it did not find data for this trade-off in all of them.
For her part, Bornbusch says she wanted to include more species in the study, but there are only around seven or eight species of eusocial snapping shrimp known to exist. (See pictures of deep-sea, heat-sensing shrimp.)
Next, Bornbusch wants to investigate how long these shrimp live, how new colonies are formed, and how the queens suppress reproduction in the other colonies' females.
Unfortunately, time may be running out—snapping shrimp seem to be disappearing, possibly due to the same reasons corals are dying worldwide.
A decade ago, these animals would have been found dominating these reef sponge habitats, says Bornbusch, but many are now locally extinct.
We worry about losing coral reefs because of all the beautiful fish, she says, "but it's also influencing these tiny organisms that are actually really important to science."