Senescence and Sexual Selection in a Pelagic Copepod

The ecology of senescence in marine zooplankton is not well known. Here we demonstrate senescence effects in the marine copepod Oithona davisae and show how sex and sexual selection accelerate the rate of ageing in the males. We show that adult mortality increases and male mating capacity and female fertility decrease with age and that the deterioration in reproductive performance is faster for males. Males have a limited mating capacity because they can fertilize < 2 females day−1 and their reproductive life span is 10 days on average. High female encounter rates in nature (>10 day−1), a rapid age-dependent decline in female fertility, and a high mortality cost of mating in males are conducive to the development of male choosiness. In our experiments males in fact show a preference for mating with young females that are 3 times more fertile than 30-day old females. We argue that this may lead to severe male-male competition for young virgin females and a trade-off that favours investment in mate finding over maintenance. In nature, mate finding leads to a further elevated mortality of males, because these swim rapidly in their search for attractive partners, further relaxing fitness benefits of maintenance investments. We show that females have a short reproductive period compared to their average longevity but virgin females stay fertile for most of their life. We interpret this as an adaptation to a shortage of males, because a long life increases the chance of fertilization and/or of finding a high quality partner. The very long post reproductive life that many females experience is thus a secondary effect of such an adaptation

Stage-specific mortality of Calanus finmarchicus, Pseudocalanus elongatus and Oithona similis on Fladen Ground, North Sea, during a spring bloom

We used Woods Population Surface Method to estimate stage-specific mortality rates for populations of Calanus finmarchicus, Pseudocalanus elongatus, and Oithona similis from abundance data obtained during the Fladen Ground Experiment (FLEX 1976) in the northern North Sea. Temperatures from the day and night depth distributions of each developmental stage were used to force temperature functions for the duration of developmental stages. In addition, we considered the influence of potential food-limitation of development, which had only minor effects on the mortality estimates. We show that the risk of mortality changes substantially over the life span of the animals, with notable differences among species. O. similis, in particular, shows negligible losses after naupliar stages 1 and 2, which may account for its numerical dominance in many ocean regions. Differences among taxa are poorly explained by allometry alone and are instead related to species- and stage-specific differences in behavior. The vertical migration behavior of late developmental stages of C. finmarchicus (i.e. their choice of habitat) might help to reduce mortality risk

Stage-specific mortality of Calanus finmarchicus, Pseudocalanus elongatus and Oithona similis on Fladen Ground, North Sea, during a spring bloom

We used Woods Population Surface Method to estimate stage-specific mortality rates for populations of Calanus finmarchicus, Pseudocalanus elongatus, and Oithona similis from abundance data obtained during the Fladen Ground Experiment (FLEX 1976) in the northern North Sea. Temperatures from the day and night depth distributions of each developmental stage were used to force temperature functions for the duration of developmental stages. In addition, we considered the influence of potential food-limitation of development, which had only minor effects on the mortality estimates. We show that the risk of mortality changes substantially over the life span of the animals, with notable differences among species. O. similis, in particular, shows negligible losses after naupliar stages 1 and 2, which may account for its numerical dominance in many ocean regions. Differences among taxa are poorly explained by allometry alone and are instead related to species- and stage-specific differences in behavior. The vertical migration behavior of late developmental stages of C. finmarchicus (i.e. their choice of habitat) might help to reduce mortality risk

Advection and zooplankton fitness

Successful adaptation to the environment by zooplankton is constrained by the agents of mortality (starvation, predation) and losses due to advection. A fitness measure which explicitly includes risk of advective loss is presented. The authors show that as horizontal current speeds and vertical shear increase, the behavioral strategies that maximize fitness of zooplankton possessing different life history strategies are affected. For a Calanus finmarchicus-type life history, fitness is maximized by did vertical migrations when advection risk in surface waters is low and by spending less time in the surface layers as flow rate increases. For a Paracalanus parvus-type life history, vertical migration is postulated to occur as advective loss increases. The geographic length scale of the habitat of residence also affects the optimal mode of habitat selection. In the Calanus case an abrupt change in the optimal vertical migration pattern is postulated as a function of length scale of habitat and rate of advection

Advection and zooplankton fitness

Successful adaptation to the environment by zooplankton is constrained by the agents of mortality (starvation, predation) and losses due to advection. A fitness measure which explicitly includes risk of advective loss is presented. The authors show that as horizontal current speeds and vertical shear increase, the behavioral strategies that maximize fitness of zooplankton possessing different life history strategies are affected. For a Calanus finmarchicus-type life history, fitness is maximized by did vertical migrations when advection risk in surface waters is low and by spending less time in the surface layers as flow rate increases. For a Paracalanus parvus-type life history, vertical migration is postulated to occur as advective loss increases. The geographic length scale of the habitat of residence also affects the optimal mode of habitat selection. In the Calanus case an abrupt change in the optimal vertical migration pattern is postulated as a function of length scale of habitat and rate of advection