Darkness as an ecological resource: the role of light in partitioning the nocturnal niche
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Nocturnal behaviors that vary as a function of light intensity, either from the setting sun or the moon, are typically labeled as circadian or circalunar. Both of these terms refer to endogenous time-dependent behaviors. In contrast, the nightly reproductive and feeding behaviors of Vargula annecohenae, a bioluminescent ostracod (Arthropoda: Crustacea) fluctuate in response to light intensity, an exogenous factor that is not strictly time-dependent. We measured adult and juvenile activity of V. annecohenae throughout lunar cycles in January/February and June 2003. Overnight and nightly measurements of foraging and reproductive behavior of adult V. annecohenae indicated that activity was greatest when a critical “dark threshold” was reached and that the dark threshold for adult V. annecohenae is met when less than a third of the moon is visible or at the intensity of light 2–3 min before the start of nautical twilight when no moon is illuminated. Juvenile V. annecohenae were also nocturnally active but demonstrated little or no response to lunar illumination, remaining active even during brightly moonlit periods. In addition to light level, water velocity also influenced the behaviors of V. annecohenae, with fewer juveniles and adults actively foraging on nights when water velocity was high (>25 cm/s). Our data demonstrate that the strongest environmental factor influencing adult feeding and reproductive behaviors of V. annecohenae is the availability of time when illumination is below the critical dark threshold. This dependence on darkness for successful growth and reproduction allows us to classify darkness as a resource, in the same way that the term has been applied to time, space and temperature.
KeywordsBioluminescence Ostracod Dark threshold Lunar cycle
We thank our handling editor Charles Peterson and two anonymous reviewers whose comments helped to improve this manuscript and Suzanne Stapleton for assistance with figure revisions. We also thank Nelson G. Hairston Jr and Amy McCune for comments on project development and this manuscript. Paul Helfenstein was gracious in his assistance with the lunar model. We also are grateful for field assistance from Krystal Rypien, Michael Pipersburgh and Richie Williams and support from International Zoological Expeditions on Southwater Caye. Funding sources included the College of Agriculture and Life Sciences Cornell University, the Mario Eunaudi Fund and the S. Ann and Robert R. Morley Research Fund. This research was completed in accordance with permits received from the Belize Department of Fisheries, Belize.
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