The weaker sex: Male lingcod (Ophiodon elongatus) with blue color polymorphism are more burdened by parasites than are other sex–color combinations

The unusual blue color polymorphism of lingcod (Ophiodon elongatus) is the
subject of much speculation but little empirical research; ~20% of lingcod
individuals exhibit this striking blue color morph, which is discrete from
and found within the same populations as the more common brown morph. In
other species, color polymorphisms are intimately linked with
host–parasite interactions, which led us to ask whether blue coloration in
lingcod might be associated with parasitism, either as cause or effect. To
test how color and parasitism are 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. We found that male lingcod carried 1.89 times more
parasites if they were blue than if they were brown, whereas there was no
difference in parasite burden between blue and brown female lingcod. Blue
individuals of both sexes had lower hepatosomatic index (i.e., relative
liver weight) values than did brown individuals, indicating that blueness
is associated with poor body condition. 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. What remains to be determined is whether parasites induce
physiological damage that produces blueness or if both blue coloration and
parasite burden are driven by some unmeasured variable, such as
starvation. Although our study cannot discriminate between these
possibilities, our data suggest that the immune system could be involved
in the blue color polymorphism – an exciting jumping-off point for future
research to definitively identify the cause of lingcod blueness and a hint
that immunocompetence and parasitism may play a role in lingcod population
dynamics.