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.
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.
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.
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.