Bright coloration in crayfish does not serve an evolutionary purpose

Biologists Zackary Graham from West Liberty University and Dylan Padilla Perez from Arizona State University have discovered evidence in crayfish which challenges the notion that bright coloration always serves an adaptive function. 

In their study, published in the journal Proceedings of the Royal Society B: Biological Sciences, the researchers examined the coloring and attributes of hundreds of crayfish species to understand why these bottom-dwelling creatures exhibit vibrant colors.

Evolutionary theory suggests that genetic mutations providing some advantage are preserved through generations, while non-beneficial traits requiring resources are phased out. 

However, the team aimed to understand what happens when a genetic mutation requires no resources to maintain but does not provide any apparent advantage. 

The fascination of bright coloration

“Conspicuous colors have fascinated biologists for centuries, leading to research on the evolution and functional significance of color traits,” wrote the study authors. 

“In many cases, research suggests that conspicuous colors are adaptive and serve a function in sexual or aposematic signaling.” 

“In other cases, a lack of evidence for the adaptive value of conspicuous colors garners interest from biologists, such as when organisms that live underground and are rarely exposed to the surface are nevertheless colorful.”

Bright coloration of crayfish

Crayfish, with over 300 species in North America alone, provided an ideal subject for this study. These freshwater crustaceans, which resemble small lobsters, inhabit muddy river and stream bottoms, mostly emerging at night.

Despite their murky habitats, some crayfish species display vibrant colors. Typically, bright coloring in animals serves to deter predators or attract mates, but crayfish do not seem to use their colors for either purpose. 

Graham and Padilla Perez investigated the crayfish phylogenetic tree and compared coloring among species. 

Conspicuous and cryptic colors

“Here, we define conspicuous colors as colors that from humans’ color perception stand out from a background and are unlikely to appear in natural backgrounds, such as reds, oranges, and yellows,” noted the researchers.

“By contrast, cryptic colors are colors that are less likely to stand out in natural backgrounds, such as browns, tans, greys and blacks. We acknowledge that conspicuous versus cryptic is relative and depends on the background, the light environment and the sensory system of receivers, among other factors.”

“But, despite these varying circumstances, the functional significance of conspicuous colors has been the focus of more than two centuries of evolutionary thought, with both Charles Darwin and Alfred Russel Wallace using natural selection to explore and explain the utility of puzzlingly conspicuous colors such as the peacock’s plumage or the contrasting color patterns of elapid snakes.”

Some crayfish lost their coloration

The experts discovered that bright coloration had independently evolved in over 50 crayfish species. 

Additionally, they noted a correlation between bright coloring and burrowing behavior, suggesting that many species evolved tan coloring to blend into their environment. They observed that brightly colored crayfish lost their vivid hues as they adapted to more aquatic behaviors.

“The intuitive but not evolutionarily justified assumption when presented with these results is to assume that these colors are adaptive. But contrary to this intuition, we discuss the hypothesis that coloration in crayfish is neutral,” said the researchers.

“Supporting these ideas, the small population sizes and reduced gene flow within semi-terrestrial burrowing crayfishes may lead to the fixation of color-phenotype mutations. Overall, our work brings into question the traditional view of animal coloration as a perfectly adapted phenotype.”

Neutral theory of color evolution 

The study authors noted that the theory that phenotypic traits such as coloration can evolve and persist as neutrally selected traits is not a novel idea, but finding natural systems to test such hypotheses is difficult. 

“Across animals, the overly adaptationist point of view that all coloration may be perfectly adapted or fit for the needs of the animal is not always expected.”

“To progress our understanding of the neutral theory of color evolution, proximate mechanisms must be explored. Specifically, the genetic architecture of color is widely studied, but surprisingly little research has begun on this topic within decapod crustaceans.”

“Crayfishes may serve as an ideal taxa for such genetic investigations, especially based on the knowledge that the genetic control of at least some color phenotypes is relatively simple.”

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