Reduced dermal photosensitivity in juvenile sea lampreys (Petromyzon marinus) reflects life-history-dependent changes in habitat and behaviour

Abstract

This study tested the hypothesis that sea lampreys (Petromyzon marinus L., 1758) undergo a reduction in the photosensitivity of photoreceptors in the tail after metamorphosing from burrow-dwelling, filter-feeding larval sea lampreys (ammocoetes) into open-water, parasitic juvenile phase animals that attach themselves to and feed on the blood of marine and freshwater fishes. Using a photo-illumination apparatus, ammocoetes and juvenile sea lampreys were exposed to white light at an intensity of 10 lx and the photokinetic response (time to movement) was measured in individual animals. The median latency of the response of juvenile lampreys was 5.4 times longer (i.e., slower response time) than observed in ammocoetes, and only 61% of the juvenile animals responded to the light compared with 95% of ammocoetes. It is concluded that the greater photosensitivity of tail photoreceptors in ammocoetes helps ensure that the animals are totally concealed from potential predators while burrowing. Decreased photosensitivity following metamorphosis is likely related to the transition to an open water, parasitic life style in which tail photoreception would have little selective advantage.