Blending in or stressing out? Coloration of a grass shrimp in response to ocean acidification and ocean warming

Wondering what I spent my first year of grad school doing? Read on for a quick synopsis:

I held up the plastic tank in front of the visitor’s face.

“They’re hanging out on the seagrass,” I said. “Can you see them?”

The teenager, part of the group touring our department’s aquarium room, shook his head. None of the students had had any luck finding the grass shrimp I was studying. This was great news for these small shrimp, just half the length of a matchstick, clinging to the green blades. Predators like pipefish and surf perch roam their natural habitats scanning for such a nice little snack, but effective camouflage can help protect them.

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This is a species of grass shrimp that lives in Southern California. They cling to the blades of eelgrass, a kind of marine plant that’s about the size of curling ribbon. This guy is super green, allowing him to blend in with the grass and hopefully avoid the detection of eagle-eyed pipefish.

Chromatophores, structures filled with pigments that can expand or contract, are responsible for these animals’ ability to blend in. In octopuses, these look like little dots of watercolors on crisp paper, drying up or pooling out as dictated by light sensors both in the eyes and skin. Hippolyte californiensis, the shrimp I study, instead have little starbursts of blue, white, yellow, and red splashed across their bodies, comprising an overall coloration of green or brown that matches healthy or dying seagrass.

shrimp2

This little shrimpie doesn’t have much color! Here, her chromatophores are contracted into little dots. This shrimp has been living in the lab by herself for about two months by this point, and most animals in these conditions lose their color. Normally, it’s because the food they’re getting doesn’t allow them to make all their normal pigments. However, shrimp that lived together and ate the same food stayed pretty colorful, leading me to believe the color of other shrimp may act as a signal.

Their coloration is likely one of their most important predator defenses, but how will future ocean conditions impact this strategy? Ocean acidification and ocean warming are two stressors that have been shown to mostly negatively affect marine organisms, but we don’t know how these conditions may impact processes that control coloration. To test this, I exposed shrimp to ambient seawater, reduced pH conditions, or reduced pH and increased temperature conditions for seven weeks. Near the end of that period, I imaged them under controlled conditions, then switched the white lights bathing their tanks to green lights that mimicked their seagrass habitat. Shrimp should adjust their coloration to blend in with their new environment, as preliminary tests indicated. Animals stressed by the unfavorable conditions may reallocate energy elsewhere or experience disrupted signaling pathways, which may result in an inability to change color and more predation.

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The eye of this shrimp (top left) is really important for determining its coloration. They not only use them to see what their environment looks like (and thus what they should match), but hormones that change coloration also get released from here. This shrimp has many expanded chromatophores filled with different-colored pigments.

However, no group of shrimp changed color even after ten days. My colleagues and I then wondered if these shrimp had the diversity of visual pigments necessary to determine the difference between white and green light. After shining white light on their eyes and measuring what wavelengths were reflected back, we determined that they do have visual pigments that absorb blue, red, and yellow, but mostly green light, so they should have been able to detect our change in their environmental color. While it is still not clear what induces a color change, the shrimp in stressful conditions didn’t react differently than shrimp in ambient conditions, not only in terms of coloration, but also for growth and survival. For now, it seems that their greatest worry will be the fish lurking among seagrass beds, not the changes we expect in our oceans.