The Mysterious Blue Crayfish

Blue crayfish, do they really exist? Of course they do – just seems like you don’t see many of them in the wild. I hadn’t either until this past December when I found numerous examples during drawdown on a local reservoir (see pic). According to an older research paper (1970) I came across, crayfish coloring is likely the result of part genetics and part diet. The pigment responsible for coloration in crustaceans is a carotenoid called astaxanthin, commonly found in salmon, shrimp, crustaceans, crayfish, microalgae, yeast, krill and trout. It usually expresses itself as the color red if unbound to a protein, or a variety of other colors if joined to a protein. There is even some interesting research that shows some birds that eat crayfish actually develop red coloring of various body parts due to the ingestion and translocation of astaxanthin.

For the study, researchers located blue colored Orconectes virilis or northern crayfish in a pair of Michigan lakes and wanted to compare them to their normally olive-green counterparts. After trapping hundreds of crayfish from the lakes over a 2 year period and analyzing catch data, they were able to learn a few interesting results.

The first was that the blue crayfish grew at the same rate as other crayfish. Another more interesting finding was that mark-recapture studies showed that the blue crayfish never made up more than 1% of the entire crayfish population. This latter finding was important because it raised the question of whether the low population might be due to selective predation of fishes on the blue variant. Many anglers have theorized that the reason you see so few blue crayfish in the wild might be due to the fact that they readily stand out and get eaten more frequently. To determine if this was the case, they captured and analyzed the stomach contents of several hundred resident trout to see what color crayfish they were eating. As it turned out, they never found a single blue crayfish in the trouts diet. This finding simply comfirmed the overall rarity of the blue color form of crayfish. In fact, they looked in other nearby lakes, but only found the blue variety in two adjoing bodies of water. This also suggested a localized population caused more likely by genetics than environment.

Lastly, when blue crayfish die, they retain their blue coloring (again – see pic). Many crayfish turn a bright red or similar hue when deceased due to the degrading of the astaxanthin-protein complex. The fact that blue crayfish stay blue might suggest a lack of astaxanthin in these specimens, or a different protein binding arrangement.