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For Better or for Worse

September 10, 2013
by Danielle Lim
Mantis shrimp

A Berkeley biologist investigates why mantis shrimp stick together.

Mantis shrimp are known in the scientific community for their ocular prowess—they can see more colors than a butterfly. Some scientists study the shrimp’s raptorial appendages, which can generate as much force as a 22-caliber rifle. Others study their intricate fighting rituals. But Molly Wright, a former graduate student in Berkeley’s Integrative Biology department, focuses on their sex lives.

Only 8 of some 500 known mantis shrimp species are socially monogamous. Wright studied two of these, Pullosquilla litoralis and P. thomassini, in the tropical waters of Moorea. These two mantis shrimp usually mate in pairs, with both parents caring for offspring. But Wright found something even more unusual about these creatures: They don’t necessarily stay together for the kids.

In species of birds and mammals—more common subjects for social monogamy studies—biparental care is often cited as the reason why animals tend to pair up. That means Wright would expect to find larger egg clutches among paired mantis shrimp. But Wright discovered no difference in the weight of egg clutches between single and paired shrimp parents. So if biparental care doesn’t help the eggs, what is driving these shrimps to go steady?

Wright thinks predation could be the answer. She observed that more pairs lived closer to the coral heads, where there are more fish—both predators and prey for the shrimp—while the singles tended to live farther away. This means the pairs had a better chance for food, but were also surrounded by more potential danger. That’s where pairing up becomes an advantage.

“When individuals are paired, they don’t have to leave their burrow as often to look for food or mates. They greatly reduce their exposure to predation,” Wright says. Pairs also store food in their burrows and share foraging duties, further decreasing the need to venture outside solo and potentially be eaten.

Wright’s findings are intriguing because predation is not often thought of as shaping mating behaviors, and this may suggest that environmental influences play a stronger role than scientists previously assumed. But she still cannot conclusively say what is making these shrimp settle down.

They have “a lot of cool mating behaviors,” she says. “They are really complex animals, and I think we basically added a little piece to the puzzle of understanding how their reproduction works.”

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