Abstract
By definition, the mesopelagic twilight zone extends from 200 to 1000m depth. Rather than confining the twilight zone to a certain depth interval, we here propose a definition that covers absolute light intensities ranging from 10(-9) to 10(-1)mu mol quanta m(-2) s(-1). The lowest intensity of this twilight habitat corresponds to the visual threshold of lanternfishes (Myctophidae). The highest intensity corresponds to the upper light exposure of pearlsides (Maurolicus spp.), which have a unique eye adapted to higher light intensities than the lanternfishes. By this definition, the daytime twilight habitat extends deeper than 1000m in very clear oceanic water, while may even be largely located above 200m in very murky coastal waters. During moonlit nights in clear water, the twilight habitat would still extend deep into the mesopelagic depth zone, while becoming compressed toward the surface in dark nights. Large variation in night light, from 10(-3)mu mol quanta m(-2) s(-1) during moonlit nights to 10(-8)mu mol quanta m(-2) s(-1) in dark overcast nights, implies that division of light into night- and daylight is insufficient to characterize the habitats and distributional patterns of twilight organisms. Future research will benefit from in situ light measurements, during night- as well as daytime, and habitat classification based on optical properties in addition to depth. We suggest some pertinent research questions for future exploration of the twilight zone.