You have printed the following article : Individual Variability in Diel Vertical Migration of a Marine Copepod : Why Some Individuals Remain at Depth When Others Migrate

The diel vertical migration (DVM) of the copepod Metridia paci jca was examined in Dabob Bay (47"45.0S1N. 122"49.7IfW). a fjord in Washington state. Although the population showed deep daytime residence (75175 m), a proportion of the population was found in the surface waters at night. For individuals that migrated to the surface, the mean size of the oil sac was much smaller than those that remained at depth (mean lengths of oil sac 0.25 mm for individuals collected between 0 and 25 m at night. compared with 0.43 mm for individuals from between 125 and 175 m). Similarly, the C : N ratio was lower for animals collected from near the surface, indicative of their lower lipid reserves. These results suggest that individual variability in DVM was influenced by body condition, with those animals with larger lipid stores not needing to risk coming to the surface to feed at night. The ubiquity of diel vertical migration (DVM) in planktonic systems has led to extensive investigations into this phenomenon over the last century and more. Although early studies focused on quantifying the occurrence of this behavior in different taxa, over recent decades there has been increased consideration of the ultimate factors that drive this behavior (e.g., Zaret and Suffern 1976; Dagg et al. 1998), the proximate cues that regulate the timing of vertical movements (e.g., Ringelberg 1999), and the biogeochemical implications in terms of the vertical transport of elements such as carbon and nitrogen (e.g., Longhurst and Harrison 1989). The normal pattern for DVM is for animals to ascend from depth around dusk, feed near the surface during the night, descend around dawn, and then fast at depth during the day. The generally accepted cost of these daily vertical movements is that individuals at depth during the day are separated from their near-surface phytoplankton and microzooplankton prey and so have a reduced feeding rate. This general scenario is supported by gut analysis, although occasionally material is found in the guts of migrants during the day. which may reflect that some feeding continues at depth (e.g., Dagg et al. 1998). There is now very extensive evidence that the ultimate function of normal DVM is to reduce the risk of predation from visual predators such as fish, the so-called predatorevasion hypothesis (Zaret and Suffern 1976). For example, DVM tends to be most pronounced in larger and/or more highly pigmented species, i.e., those that are more susceptible to visual predation because of their greater visibility (Hays 1995 and references therein); within species, DVM is more pronounced when planktivorous fish are more abundant (Bollens and Frost 1989); and in artificial enclosures, DVM can readily be induced by the addition of planktivorous fish (Dawidowicz et al. 1990). Similarly, when normally migrating invertebrate predators are abundant, their prey may exhibit reverse DVM (populations near the surface during the day and deeper at night), again in order to reduce their risk of predation (e.g., Neil1 1990). However, although the evidence in support of the predator evasion hypothesis is very strong, questions remain about the factors that drive individual variability in vertical movements. For example, although there may be general upward and downward movements at dawn and dusk, respectively, only a proportion of individuals may migrate on this schedule, with some of the population remaining at depth in all parts of the diel cycle (e.g., Bollens and Frost 1989, 1991; Dagg et al. 1998). There is little empirical evidence to explain what drives such individual variability. However, given the conceptual framework provided by the predator-evasion hypothesis, we can develop testable a priori predictions about potential causal factors. The predator-evasion hypothesis explains DVM behavior on the basis of the lower ambient light levels near the surface at night, which make it harder for visual predators to perceive their prey. However, because the size of animals also influences their visibility, it might be predicted that smaller individuals may be more likely to come to the surface and will spend longer at the surface than larger individuals. comparison between species and developmental stages support this argument. For example, juvenile stages of the copepod Metridin lucens spend longer near the surface than adult stages (which are larger), whereas the larger copepod, Metridio lorzgo, spends even less time at the surface each night (Hays 1995). Similarly, De Robertis et al. (2000) have recently shown this same link between the size of individuals and their residency at the surface. These same arguments for the importance of size may similarly apply within the same developmental stage for a single species-i.e., when planktivorous fish are abundant, larger individuals may not migrate to the surface because of their higher risk of predation. Because there is a cost to near-surface residence in terms of the increased risk of predation, the predator-evasion hypothesis predicts that individuals would only incur this cost when it is outweighed by the benefit of near-surface foraging. Theoretical arguments suggest that the benefit of migrating to the surface may be dependent on the body condition of individuals (Fiksen and Carlotti 1998; Sekino and Yamamura 1999): for example, for animals with large lipid reserves, there may be less benefit to near-surface foraging. and so