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2002. 4. 25 : R
VERTICAL DISTRIBUTION PATTERNS OF PELAGIC COPEPODS
AS VIEWED FROM gPREDATION PRESSUREh HYPOTHESIS

Hokkaido University, Hakodate, Japan

As habitat of pelagic copepods, epipelagic oceanic environments are characterized by greater abundance of food supply and higher predation pressure, and both food supply and predation pressure reduce rapidly with increasing depth.We studied day/night vertical distribution patterns of copepodid stages of 6 epipelagic (Neocalanus cristatus, Metridia asymmetrica and Metridia curticauda) at Station Knot (44N, 155E) in the western subarctic Pacific down to 4000 m depth.All epipelagic species exhibited ontogenetic vertical migration (OVM) characterized by descending with the progress of copepodid stages although the degree of magnitudes varied species-specifically.(M. pacifica) out of 6 epipelagic species showed diel vertical migration behavior (DVM) ascending to upper layers at night.Mesopelagic species were characterized by the lack of OVM yet their older copepodid stages undertook DVM.Bathypelagic species underwent OVM of which pattern (ascent with the progress of development) was opposite to that of epipelagic species.No DVM behavior was recognized for bathypelagic species.From the viewpoint of gpredation pressure hypothesish, these differential vertical distribution modes of copepods living in dissimilar bathymetric strata can be interpreted as a result of life history traits toward reducing predation mortality of early (bathypelagic copepods) or late copepodid stages (epipelagic copepods).Absence of OVM in mesopelagic copepods is a life history trait falls somewhere between these two extremes (DVM behavior of their late copepodid stages are an apparent advantage for better feeding of them).This explanation appears to be consistent with the observation that the fecundity of these copepods decreases with increasing depth.