Ontogenetic and Seasonal Variation in the Diel Vertical Migration of the Copepods Metridia lucens and Metridia longa

Plankton samples collected and analyzed by the Continuous Plankton Recorder survey were used to examine the length of time spent near the surface in the North Atlantic and the North Sea by three closely related groups of zooplankton (copepodite stages 1-4 Metridia spp., copepodite stages 5-6 Metridia lucens, and copepodite stages 5-6 Metridia longa). For all three groups, the mean daily length of time spent near the surface in each month of the year covaried seasonally with day length. In addition, the amount of time spent near the surface varied significantly between the three groups, being longest for the copepods of smallest body size (C 1-C4 Metridia spp.) and shortest for the copepods of largest body size (C5-C6 M. longa). These results support the suggestion that diel vertical migration serves to reduce the risk of mortality from visually orienting predators. Diel vertical migration (DVM) is a common feature in zooplankton communities. Vertical migrants are normally found deeper during the day and shallower at night. The ubiquity of DVM has provoked considerable and extended debate as to its functional significance. It has been suggested that there may be a metabolic benefit associated with movement across a thermocline (McLaren 1974), but this metabolic advantage hypothesis is now generally refuted (Lampert 1989). Alternatively, the predator-evasion hypothesis suggests that certain species of zooplankton may be readily perceived and consumed if they are near the surface during the day. Hence, there may be a selective advantage for such species to descend during the day so that they are less available to visual predators (Zaret and Suffem 1976). There is considerable support for the predator-evasion hypothesis from both field and experimental observations. For example, DVM tends to be more marked when zooplanktivorous fish are abundant (Gliwicz 1986). Furthermore, experimental manipulations have shown rapid change in migratory behavior in previously nonmigrant populations of zooplankton when zooplanktivorous fish are introduced (Bollens and Frost 1989). Specific a priori predictions about the migration behavior of zooplankton can be made from the predatorevasion hypothesis. When nights are brief, migrating zooplankton would be expected to restrict heir near-surface foraging so that they are not near the surface during the day; similarly, when nights are longer, zooplankton would be expected to expand their near-surface foraging. The extensive epipelagic samples historically collected and anAcknowledgments I thank the Sir Alister Hardy Foundation for Ocean Science for allowing me to use the data from the Continuous Plankton Recorder survey. This paper forms part of PRIME (Plankton Reactivity in the Marine Environment), a program funded by the Natural Environment Research Council of the U.K. PRIME contribution 3. alyzed by the Continuous Plankton Recorder (CPR) survey have been used to examine this prediction for the copepod Metridia lucens (Hays et al. 1995). In accordance with the predator-evasion hypothesis, the amount of time this species spent near the surface did vary seasonally. For planktonic Crustacea, the probability of being perceived and consumed by visual predators tends to increase with body size (Brooks and Dodson 1965). From the predator-evasion hypothesis, itmay therefore be predicted that, in addition to seasonal variations, the amount of time spent near the surface should be shorter for larger species and for older (and hence larger) developmental stages. In this paper, I examine this prediction by using data from the CPR survey for different developmental stages of Metridia longa and M. lucens. These two species are frequently found in the CPR samples, so the amount of time that they spend near the surface can be calculated accurately. In addition, the two species differ markedly in body size: the typical prosome length for the adult stages is 2.5 mm for M. lucens and 4.0 mm for M. longa (cf. Sars 1903). Hence the amount of time spent near the surface should be markedly different for the two species.