Darkness as an ecological resource: the role of light in partitioning the nocturnal niche

Abstract

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.

Artificial light alters natural regimes of night-time sky brightness

“…However this migration has been observed to be suppressed in freshwater lakes above which the night-time sky is artificially brightened30. Many behaviours are often synchronized to short term variations in the illumination of the moon's disk throughout each 30 day phase period, a phenomenon which is widespread throughout the animal kingdom2932373839404142. Hence, the short term destabilization of monthly lunar sky brightness regimes by artificial lights may disrupt patterns in foraging and mating linked to these phase periods.…”

Change does not happen overnight: a case study on stream macroinvertebrates

“…Several studies have stressed the importance of light variation and the influence of day/night conditions on plants and animals by examining both physiological and ecological points of view (e.g., Gerrish et al, 2009;Gaston et al, 2013). For instance for terrestrial insects, their distribution or features can markedly differ from day to night as a survival strategy to avoid insectivorous birds (e.g., Guevara and Avilés, 2013).…”

Simultaneous field measurements of ostracod swimming behavior and background flow

“…The nonluminescent myodocopid ostracod Paravargula trifax Kornicker provides an excellent model system in which to examine zooplankton interactions with fluid dynamics because ostracods are ubiquitous in marine environments, ranging from tidal pools to the deep sea; most species are associated with benthic habitats and play an important role in benthic–pelagic coupling (Marcus and Boero 1998). Benthic species of myodocopid ostracods are known to spend daylight hours associated with sessile benthic communities or buried in the sediment, emerging at twilight in response to a critical “dark threshold” (Gerrish et al 2009) and performing transitory displays in the water column until dawn (Rivers and Morin 2008, 2009). Myodocopid ostracods are omnivores, feeding on live prey and algal material, as well as scavenging dead organisms (Vannier et al 1998).…”