Functional significance of pigment in larval Corynosoma constrictum Van Cleave, 1918 (Acanthocephala: Polymorphidae)

The spherical larvae of the acanthocephalan Corynosoma constrictum appear as bright orange spots beneath the nonpigmented exoskeleton of its intermediate host, Hyalella azteca. Color is an oddity among parasites. It has been hypothesized that color is due to absorption of pigment from the host's diet and that color increases the probability of transmission to the next host. Yet, published data are not convincing, alternative hypotheses have not been explored, and the function of the pigment remains unknown. My research investigates alternative explanations for integumental pigmentation in C. constrictum larvae. One alternative is that the pigment acts as a biological sunscreen, shielding larvae from harmful ultraviolet radiation. High performance liquid chromatography indicated the pigment is the carotenoid astaxanthin, a potent anti-oxidant, and is present in both amphipods and parasites. Exposure of experimentally infected amphipods to natural levels of UV light did not alter larval development or host survival. A second alternative is that the carotenoid is metabolically and nutritionally essential for developing larvae. When amphipods were fed an artificial diet lacking carotenoids, larval development was delayed and ability to infect the next host (Anas discors) was significantly reduced. Further, survivorship studies suggest that high intensity infections deplete amphipods of nutrients, often castrating females, and increasing host mortality. Protein biochemistry indicated that a carotenoid-protein complex is the most likely nutrient being taken from the amphipod by developing larvae. Results from experimental and biochemical studies are consistent with the hypothesis that an amphipod carotenoprotein is a vital nutrient for C. constrictum and is the source of the orange carotenoid. This is an important finding because very little is known about the nutrition and metabolism of larval acanthocephalans.