Melanistic Mysteries

The picture above is a classic example of melanism found in manta rays.

Picture obtained from MantaMatcher.

The underwater world is both a fascinating and terrifying place due to how little we truly know about the millions of species that inhabit these ecosystems. However, as technology continues to improve we are slowly gaining more information about creatures or ecosystems that have never been studied in-depth before. In my eyes any small discoveries relating to the oceans or the creatures that inhabit them are of extreme importance as it could completely re-shape how we perceive aquatic life as a whole. Even a study done on something considered trivial by most could provide the human race as a whole with key insight into how and why a particular creature came to be. Thus, if one wants to unlock the secrets that the world's oceans hold then it is imperative that we study everything about a particular organism and not just certain areas of interest. With this goal in mind, a group of researchers conducted a study on manta rays in order to fully understand the commonly overlooked processes that affect melanism in marine environments.

On land, the factors influencing melanism are quite evident as it often provides a distinct evolutionary advantage to the individuals affected with this change. For example, rock pocket mice are generally light colored and live near light colored rocks. However, populations of this species are also found near dark colored lava rocks and as one might expect, have darker coats. Ultimately, these darker coats have a selective advantage within the darkened areas as they provide the mice with a greater amount of camouflage and protection from predators. Despite the clear reasoning for changes in melanin on land, melanism in marine life is only exhibited in the two known species of manta rays and seems to have no evolutionary explanation. 

In order to shed light upon this puzzling evolutionary mystery, the researchers working on this project compiled thousands of images of manta rays from 2003 to 2018 in hopes of being able to visually identify the frequency of melanism across Indo-Pacific manta ray populations. Similarly, the group used the photo catalog in tandem with “mark and recapture modeling" to test for predation selection that may act on any manta ray color morph variants. Interestingly enough, the researchers found that in some areas 40 percent of the manta ray population had distinctive melanistic features, but in other areas there were no melanistic manta rays at all. Judging from this, one might hypothesize that these melanistic changes do indeed have a particular evolutionary advantage that is causing melanism to be selected for in particular areas. However, after tracking both normal and melanistic manta rays throughout a period of several years the research group found no significant difference between the survival rate of the two manta ray morphs. A likely explanation for the data obtained is that melanism may be a neutral trait that does not influence the fitness of the individual in any way. If this were the case, then the frequency of this gene would only be influenced by gene flow and genetic drift, which could account for the large variability in this trait across different manta ray populations. As seen in other species, melanistic manta rays could also be experiencing a form of frequency-dependent selection and the rare-male effect where females instinctively shift their preference towards less common phenotypes, which gives their offspring a reproductive advantage. Likewise, the genes that form the melanocortin system have also been linked to influencing other phenotypes, such as aggression and stress response. Thus, it may also be possible that the melanism itself is not being selected for, but rather the associated behaviors with this gene. The most likely explanation for this phenomenon stems from the fact that these manta rays may possibly be experiencing a number of different selective pressures in various geographic areas which would lead to some populations having an abnormally high frequency of melanism in comparison to other groups. Whatever the reasoning may be, it is evident that both genetic and environmental factors interact in complex ways that take significant amounts of research to fully comprehend.

As species across the globe continue to decrease substantially in numbers it is crucial that we as a collective perform an increased number of both macro and micro studies on aquatic species before they completely go extinct. The manta rays are just one example of why these studies are absolutely time sensitive. If we can devote additional resources to globally fund comprehensive studies of a similar nature, then we may be able to see how different populations of aquatic species are linked to one another. This critical information could effectively improve conservation efforts by providing researchers with never before seen studies that may help form new strategies to protect vulnerable and endangered species from extinction in one of the most globally threatened biospheres.

Posted by James Levangie (5)