An experimental test of mate choice for red carotenoid coloration in the marine copepod Tigriopus californicus

Colorful ornaments are hypothesized to have evolved in response to sexual selection for honest signals of individual quality that provide information about potential mates. Red carotenoid coloration is common in diverse groups, and in some vertebrate taxa, red coloration is a sexually selected trait whereby mates with the reddest ornaments are preferred. Despite being widespread among invertebrates, whether red carotenoid coloration is assessed during mate choice in these taxa is unclear. The marine copepod Tigriopus californicus displays red coloration from the accumulation of the carotenoid astaxanthin. Previous research on copepods has shown that astaxanthin provides protection from UV radiation and xenobiotic exposure and that carotenoid production is sensitive to external stressors. Because of the condition dependency of the red coloration, we hypothesized that Tigriopus would use it as a criterion during mate choice. To test this hypothesis, we conducted trials in which males chose between females that were wild-type red (carotenoid-rich algae diet) or white (carotenoid-deficient yeast diet). To control for dietary differences and to isolate the effect of carotenoid coloration, we also presented males with restored-red females fed a carotenoid-supplemented yeast diet. We found that wild-type red females were weakly preferred over white females. After controlling for diet, however, we found that restored-red females were avoided. Our observations do not support the hypothesis that male copepods prefer the carotenoid coloration of females during mate choice. We hypothesize that algal-derived compounds other than carotenoids play a role in mate choice. Red coloration in copepods appears to be a condition-dependent trait that is not assessed during mating.