Why Do Some Shrimps Turn Red When Cooked, and Others Don’t?

Author: Louis Li

Editors: Ethan Tai, Serena Tsao, Jonathan Chen

Artist: Emily Tai

Drop a gray shrimp into a boiling pot. Puff. What floats up becomes a fiery red. This is a phenomenon that many people have witnessed in their kitchen and are familiar with to an extent that we forget to ask why. So, why do shrimps turn red when tossed into boiling water?

The answer lies in biochemistry. In crustaceans like shrimps, there is a red-orange pigment called astaxanthin which is bound to a protein called crustacyanin in the normal state. This binding changes the chemical environment and alters the pigment’s ability to absorb light. Since the colors we see with our eyes is light that an object reflects—not the ones it absorbs—the shrimp’s original red-orange color now looks  blue, green, or grey.

Once the shrimp is exposed to a temperature higher than 80°C, the heat denatures the protein crustacyanin, changing its shape. This means that crustacyanin no longer suits the shape of the pigment molecule and can no longer bind to it. Under this condition, astaxanthin is freed and resumes its red-orange color, and this is exactly what happens when your mum is tossing shrimp into a boiling pot.

However, do all shrimps turn red when they are cooked? The answer is no. Some shrimps live near hydrothermal vents, which are hot springs in the deep ocean. Every time a hot spring erupts, the fluids that come out often reach temperatures exceeding 400°C. But even under a temperature much higher than the boiling point of 100°C, we do not see red shrimps swimming around the deep ocean.

There are three possible reasons. The first reason is that deep-sea shrimps do not live right on top of the vents. Instead, they stay in peripheral zones of the hydrothermal vents where temperature ranges from 20°C to 60°C. These  temperatures are insufficient to denature crustacyanin and release bound pigments like astaxanthin, which requires temperatures above 80°C. Therefore, in their natural high-temperature habitats, these organisms do not experience the "cooking" effect that leads to reddening. 

The second possibility is that shrimps living near hydrothermal vents have lower levels of or even no astaxanthin in their bodies. Shrimps and other crustaceans cannot produce astaxanthin themselves. They derive the pigment by eating zooplankton, microorganisms in the sea, and zooplankton derive the pigment by consuming algae, the original synthesizer of astaxanthin. However, since sunlight cannot reach the deep sea, few algae are growing near the hydrothermal vents. As a result, deep-sea shrimps cannot gain astaxanthin from their diet and, therefore, fail to present a reddish color.

There is also another aspect to consider. Many deep-sea shrimps live together with symbiotic bacteria on their shells. The shrimps provide a place for the bacteria to live, and the bacteria synthesize nutrients for the shrimps in return. The transportation of these nutritious chemicals requires crossing the shells and surface cells of shrimps. The simpler the surface structures of shrimps are, the easier the transportation will be. Since pigments like astaxanthin are often stored in small bubbles within cells, their presence can physically clog up space in tissues, thicken the outer layer, and slow down the movement of chemical nutrients. Therefore, shrimps in the deep sea removed pigments in evolution, assuming a pale or translucent appearance around hydrothermal vents.

In conclusion, whether a shrimp turns red reveals more than just a cooking trick. It shows how evolution, chemistry, and the environment shape living organisms. From a simple phenomenon, we see how alterations of molecules bring about significant changes to individuals, such as colors, and how the selection of a trait shows evolutionary intelligence. This is the beauty of biology: microscopic science leads to macroscopic magic.

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