1. Live Shrimp and Crabs Are Not Red
If you look at live shrimp or crabs, you will not see red. Their colors vary by species and individual, ranging from dark blue to green or brownish tones [6]. Because descriptions differ across sources—some say blue, others green or purple-brown—it is safest to describe them simply as "dark colors."
The same shrimp that looks dark and blue turns red after boiling. A common assumption is that the heat "makes" the color. But is that really what happens? To understand this, we must look at what is inside the shell before any heat is applied.
2. Astaxanthin, the Red Pigment Already in the Shell
The source of the red color in shrimp and crabs is a pigment called astaxanthin. This belongs to a group of pigments known as carotenoids. When astaxanthin exists freely on its own, it has an orange-red color [1].
This means the red color is not newly created by boiling. It was present in the shell while the animal was alive. Shrimp and crabs cannot make this pigment themselves; it is said they absorb it from their food and store it in their bodies [7]. The red color in salmon eggs and salmon flesh is also due to this same pigment. However, since this food explanation comes mainly from commentary articles, and some reports suggest certain species may convert it internally, we can only be certain that the animals possess the pigment in their bodies.
If the red pigment is there from the start, why don’t live shrimp look red? The answer lies in how the pigment is held.
3. Why Lobster Shells Look Blue: Astaxanthin Bound to Crustacyanin
The astaxanthin pigment is bound to a protein complex called crustacyanin. It has been reported that this binding is the reason lobster shells appear blue [2].
Why does the same pigment change color when bound? The reason lies in the shape of the pigment and its surrounding environment. Inside the protein, the pigment is held in a nearly flat shape, and its electronic environment changes. This shifts the light it absorbs, changing the color we see [1]. Think of it like pressing down on a spring with your hand. However, this is just a simple analogy; in reality, both the shape and the electronic environment affect the color.
In lobster research, unbound pigment is orange-red, but when bound to the protein, it becomes blue-gray [2]. Note that these exact values may not apply to all shrimp and crab species, so treat this as an example from lobsters. If the protein is simply holding the pigment, releasing its grip should reveal the red color.
4. How Heat Frees the Pigment from the Protein
When you boil shrimp, the protein loses its 3D structure due to heat. This process is called denaturation. The denatured crustacyanin releases the pigment it was holding. As the pigment is released from the denatured protein, the orange-red color appears [3].
A summary of a Japanese research grant project on tiger prawns confirms this explanation. It states that crustacyanin shows its original color as a complex of apoprotein and astaxanthin, but changes to red when astaxanthin is released due to denaturation [4]. A commentary from a Japanese food news source also explains that heating changes the protein, breaking the bond with the pigment, and revealing the pigment’s true red color [6].
Hard-boiled eggs are another example of protein denaturation. In the case of shrimp, the appearance of the red pigment is more visually obvious than the hardening of the meat. Once boiled, shrimp do not turn blue again when cooled. Denatured proteins generally do not return to their original state easily.
5. Do Octopuses Turn Red the Same Way as Shrimp and Crabs?
Octopuses also turn red when boiled, but it is thought that the mechanism is different from that of shrimp and crabs. One explanation states that the red color in octopuses is not due to astaxanthin [7].
However, primary sources for this specific point have not been confirmed, and the detailed reason is not fully understood. Therefore, we can only say that even though the result—"turning red when boiled"—is the same, the underlying process may not be identical.
6. Unanswered Questions About Shrimp and Crab Colors
It has not been verified in the available materials what purpose the dark colors serve while the animal is alive. You may see explanations saying it is "to protect from predators," but no research source confirming this was found. So, we can only note that such an explanation exists.
Details of the mechanism behind the color change are still being studied. In 2015, a study reported that astaxanthin behaving like an acid is important for the binding [5]. However, since the full paper has not been read here, the fine details of the mechanism cannot be introduced.
7. How to Investigate Shrimp Color at Home
If you want to explore this further, you can try observing it at home. Buy raw shrimp with shells and observe their color before boiling. Compare the colors of the head and shell after boiling. Because hot water and fire are dangerous, leave the heating to an adult.
You can also visit an aquarium or fish market to see the colors of live shrimp and crabs. You might notice that the way the dark colors appear differs by species.
For official information in Japanese, search for "crustacean body color" or "crustacyanin" to find summaries of research grant projects.
SAFETY NOTE: Heating involves fire and hot water. Always do this activity with an adult.