Thanks to Brilliant for supporting this episode of SciShow.

Go to Brilliant.org/SciShow to check out their interactive Computer Science Fundamentals course. [♪ INTRO] Imagine yourself fishing along the shores of the Pacific Ocean, and you excitedly reel in what could very well be your catch of the day, a big, hefty lingcod. But as you pull it up to the surface, you notice something very strange about this fish.

It’s a translucent, practically luminescent blue. Yes, there are a lot of brightly colored fish in the world. But for some lingcod, which are usually brown and white, we’re talking blue blue.

Not just their scales, but their flesh is vibrant blue! While we’re still not sure exactly why these fish are blue, we are beginning to unravel this mystery that’s been puzzling scientists, naturalists, and the fishing industry for some time. This bright blue phenomenon has actually been found in multiple species, including rock greenlings and cabezon, in addition to lingcods.

These fish are found in similar habitats, preferring to hang out on the seafloor. They’re all popular in recreational and commercial fishing, so the blue color is of even more interest when it might end up on someone’s dinner plate. And while it may look totally bizarre, it’s actually fairly common to find bright blue lingcod!

Research published in 2021 examined over two thousand lingcod across a huge geographic range, from Alaska to California. Most of the time, they found the usual color morph of lingcod, with brown scales and whitish flesh. But about 14% of all lingcod were at least some degree of blue, so it turns out it’s not really that rare.

And while it’s not uncommon for fish of the same species to vary in their internal and external pigments, this vibrant blue coloration is definitely an extreme example. The study also brought to light something unexpected. Of all the blue-fleshed fish, 80% were female!

But even though these strange blue fish are fairly widely known by fish enthusiasts, we still don’t understand why this happens. It seems to have something to do with their overall health, since the researchers found that the blue fish have lower concentrations of important fatty acids in their bodies. These molecules are their main internal source of energy, and are stored in their tissues.

So it could be that the blue fish are not getting enough of these vital fatty acids, leaving them to deplete their energy stores faster than brown individuals. But since fatty acid concentrations in fish are influenced by a number of factors including diet, physiology and the surrounding environment, that only moves the mystery back up a step. Why the difference in energy stores?

In another study published later in 2021, blue individuals were also found to have lower liver weights, which is also considered a sign of poor physical health. There were other signs of poor health revealed in this study too. Blue male lingcod were found to have almost twice as many parasites on them compared to their brown counterparts!

But while there appeared to be a strong connection between color and parasites in male fish, this was not the case for females, who had low numbers of parasites on them regardless of color. This may seem odd, but it actually makes sense. The authors of the study note that males have less effective immune systems, and less ability to fend off parasites.

So the blue females may still be worse off than the brown ones, but their immune systems are still beefy enough to keep parasites at bay. The researchers say this could help explain the different ratios of blue fish between the sexes too. Blue females may be more common simply because they’re toughing out the situation a bit better than the males, who are more likely to succumb to their poor condition.

Although the blue color may appear to be much more common in females, they might just be outlasting the blue males and skewing the ratios. The size of the fish, and the location where they’re found, seemed to be connected to the color differences too. Smaller females found in shallower water were more likely to be blue, implying that those sticking closer to shore may be worse off.

That said, whether it’s an environmental factor or dietary difference that’s making these shallow females blue is still unknown. But at least we have some evidence that blue lingcod don’t seem to be in the best shape overall. So we can start to explore whether their stressed out bodies are releasing compounds that cause the change in color.

Biliverdin is a pigment created through the breakdown of another molecule, heme, in the gallbladder. It circulates through the lymphatic system but spreads throughout muscle and skin tissues. In some other fish species, biliverdin has been shown to become elevated during starvation.

And since it is a blue-green color, that means starvation can give these other struggling fish the blues. That means biliverdin is a very likely culprit in lingcod as well. But that hasn’t been definitively shown yet, and in science, we cannot just assume!

Given all this information, it does at least appear that these fish are blue because they are unhealthy, and not unhealthy because they are blue. There’s still a bunch of unanswered questions surrounding these blue fish. But that's just how science works: you set out to answer one question and a bunch more show up to the party!

Oh, and you also might be surprised to hear that the blue-fleshed fish actually sell pretty well. Because maybe people enjoy a little novelty in their lives. You get a little salmon and a little…a little blue lingcod… you have yourself a gender reveal party.

And if the idea of serving up blue flesh might be unsettling to some, don’t worry: the color goes away once it’s cooked. Thanks to Brilliant for supporting this episode of SciShow. Brilliant is an online learning platform with courses in science, math, engineering, and computer science.

They may not turn blue, but Brilliant is constantly changing and improving to give you a more interactive experience. Like their newly updated Computer Science Fundamentals course, which will help you learn the core fundamentals with no prior coding experience required. Brilliant’s courses are designed for people of all knowledge levels, so you can jump in at any point and work your way to mastery, through their interactive and guided exercises.

You can start learning computer science fundamentals, or take any of Brilliant’s courses, at brilliant.org/scishow. And if you sign up using that link, you will save 20% on an annual premium subscription to Brilliant. [♪ OUTRO]