Mon, Aug 02, 2021:On Demand
Background/Question/Methods
The unusual blue color polymorphism of lingcod (Ophiodon elongatus) is the subject of much speculation but little empirical research. Having conducted some preliminary parasitological dissections of lingcod at the outset of this study, our team knew that lingcod were unusual in at least one other way: this species is infected by among the highest average burdens of metazoan parasites that we have ever encountered in examinations of dozens of fish species. We wondered whether blue coloration might somehow be associated with parasitism, either as cause or effect. To test our multiple working hypotheses about how color and parasitism might be related in this host species, we performed parasitological dissection of 89 lingcod individuals collected across more than 26 degrees of latitude from Alaska, Washington, and California, USA.
Results/Conclusions We found that blue male lingcod carried a greater number of parasites than any other sex–color combination. This was true across all of the metazoan parasite taxa we detected, suggesting that blue coloration is linked to reduced immunocompetence in males. The immune systems of male vertebrates are typically less effective than those of females, due to the immunocompromising properties of male sex hormones; this might explain why blueness is associated with elevated parasite burdens in males but not in females. Our data do not shed light on why blue coloration occurs, nor can we discriminate between high parasite burdens as a cause or effect of blueness. However, this analysis documents an association among sex, color, and parasitism in lingcod and suggests one of the following scenarios: (1) blueness itself causes high rates of parasitism (e.g., immune defenses are eroded by the release of the compounds that cause blueness and this affects males more than females because males are inherently more susceptible to parasitic infection), (2) parasitism itself causes blueness (e.g., parasitism disrupts the function of the liver or gallbladder, releasing biliverdin, which causes blueness; males have an inherently higher threshold at which biliverdin is released), or (3) whatever causes blueness also causes high rates of parasitism (e.g., starvation induces liver or gallbladder damage, which releases biliverdin, causing blueness; starvation also weakens immune defenses, which affects males more than females because males are inherently more susceptible to parasitic infection).
Results/Conclusions We found that blue male lingcod carried a greater number of parasites than any other sex–color combination. This was true across all of the metazoan parasite taxa we detected, suggesting that blue coloration is linked to reduced immunocompetence in males. The immune systems of male vertebrates are typically less effective than those of females, due to the immunocompromising properties of male sex hormones; this might explain why blueness is associated with elevated parasite burdens in males but not in females. Our data do not shed light on why blue coloration occurs, nor can we discriminate between high parasite burdens as a cause or effect of blueness. However, this analysis documents an association among sex, color, and parasitism in lingcod and suggests one of the following scenarios: (1) blueness itself causes high rates of parasitism (e.g., immune defenses are eroded by the release of the compounds that cause blueness and this affects males more than females because males are inherently more susceptible to parasitic infection), (2) parasitism itself causes blueness (e.g., parasitism disrupts the function of the liver or gallbladder, releasing biliverdin, which causes blueness; males have an inherently higher threshold at which biliverdin is released), or (3) whatever causes blueness also causes high rates of parasitism (e.g., starvation induces liver or gallbladder damage, which releases biliverdin, causing blueness; starvation also weakens immune defenses, which affects males more than females because males are inherently more susceptible to parasitic infection).