Molecular Pathways Leading to Malpigmentation in Flounder: Part Two of Three

Abnormal pigmentation, known as malpigmentation, is an undesirable trait which has been documented worldwide in hatchery-produced flatfish, including southern flounder from the Texas Parks and Wildlife Department’s marine fish enhancement program. In recent trials, we have seen an average of 30% of the flounder produced in hatcheries become almost entirely white (pseudo-albino) after their metamorphosis. As stealthy predators, flounder use their pigmentation for camouflage to prevent their prey from seeing them. That camouflage is also what protects flounder from predators.

The first installment of this series of articles (in the August issue of Texas Saltwater Fishing Magazine) described a research project at the University of Texas Marine Science Institute’s Fisheries and Mariculture Laboratory (FAML) designed to determine the causes of malpigmentation and to devise actions to reduce it in hatcheries, or to develop a method for early detection. Theoretically, malpigmentation could be caused by having pigment cells that are unable to produce the dark pigment known as melanin, or by the pigment cells not being present.

With research funding provided by the Texas Parks and Wildlife Department and the U.S. Fish and Wildlife Service, Dr. Lee Fuiman and Cynthia Faulk at FAML examined the activity of six key genes to identify the physiological pathway that might be responsible for malpigmentation and to pinpoint precisely when during development that pathway is disrupted in pseudoalbino fish. The experiments, however, had some unexpected results. Malpigmentation rates in the laboratory were always less than 15% and mostly less than 10%, much lower than in the hatchery. This made it difficult to find the differences in physiology that were responsible for malpigmentation. Nevertheless, there were some promising results, and those results needed to be confirmed by examining fish with higher rates of malpigmentation. So, Dr. Fuiman invited colleagues at the Hubbs-SeaWorld Research Institute (HSWRI) in San Diego, California, to help out. Researchers at HSWRI work with California Halibut, a close relative of Southern Flounder that has very high rates of malpigmentation in hatcheries (40% to almost 100%).

Performing the same analyses of genes on the halibut samples that they had done for flounder, the researchers got very similar results. They found one gene in particular that showed the same relationship to malpigmentation in both species. That gene plays a critical role in the early stages of development of pigment cells, and it was significantly less active in batches of flounder and halibut that had higher rates of malpigmentation. This indicates that the reason some fish are malpigmented is because many of their pigment cells do not develop. Importantly, the difference in activity of this gene occurred when the fish were five to six weeks old, which is several weeks before malpigmentation can be seen.

This new information is useful in several ways. It focuses further research on a specific physiological pathway – pigment cell development. And, it pinpoints the stage of life to study. Digging further into this physiological pathway at this stage of development is expected to provide clues to how malpigmentation can be reduced or eliminated in hatcheries. The new information also suggests that it might be possible to use gene activity for early detection of malpigmentation. Having a means to determine early on that a batch of fish will have a high rate of malpigmentation several weeks later can help hatchery managers decide whether it is worth devoting more resources to rearing that batch and releasing it.

A new research project is already underway at FAML using funding provided by the Texas Legislature to the Texas Gulf Coast Research Center at the University of Texas Marine Science Institute. That project is using carefully controlled experiments to test the effect of different hatchery practices – such as larval diets and lighting – on malpigmentation in flounder, and it is employing new and powerful molecular tools to investigate the physiological and developmental pathways. The ultimate goal of that research is to develop strategies that can be applied in TPWD hatcheries to reduce malpigmentation and improve the effectiveness of their stock enhancement efforts. Stay tuned to the next installment of the story for some of the initial findings from this exciting project.