Behavioral responses of freshwater calanoid copepods to the presence of ultraviolet radiation: avoidance and attraction

Binary choice experiments under natural solar radiation were used to test short-term behavioral responses of freshwater calanoid copepods to ultraviolet radiation (UV). Responses of the nine species from 15 populations spanning North and South America included both UV attraction and UV avoidance, and varied among habitats, species and populations. Copepods from more transparent lakes were more attracted to UV than those from less transparent lakes. When individuals were pre-exposed in the laboratory to photosynthetically active radiation (PAR) in the presence and absence of UV, those pre-exposed to UV spent more time in the high UV environment than those exposed to PAR alone. However, these differences disappeared after 150 min, suggesting that the responses were short term and mediated in part by ambient UV conditions. Copepods represent a large proportion of the biomass of zooplankton in many aquatic ecosystems, and their ability to detect and respond behaviorally to UV may enable them to use this ubiquitous environmental cue to regulate their water column position. The use of UV as a habitat selection cue may permit copepods to exploit optimal food environments or limit overlap with less UV-tolerant competitors, predators or parasites, while minimizing damage resulting from longer term UV exposure.