Geir Helge Johnsen

The influence of food limitation on swarming behaviour in the waterflea Bosmina longispina

Abstract Bosmina longispina showed a high degree of overdispersal in nature, which drastically reduced the local amount of food within the swarms. In experiments, swarming behaviour occurred only in the light and when food was abundant. In darkness and when food was limiting, B. longispina were more evenly distributed. If swarming prey individuals benefit from a reduced predation risk, the swarming behaviour of B. longispina can be interpreted as an adaptive diurnal rhythm in the trade-off between reducing risk of predation and maximizing feeding rate. Maximal feeding seems to have priority over passive avoidance mechanisms such as swarming when the risk of starvation is high.

Size-specific protection against predation by fish in swarming waterfleas, Bosmina longispina

Abstract When feeding on non-swarming waterflea prey, sticklebacks, Gasterosteus aculeatus, selected larger items. However, when feeding on swarming prey, the fish fed mainly at the edges of the swarms and smaller items were consumed. This selection of small prey items can be explained by the fact that when attacked by a stickleback the waterfleas escaped towards the centre of the swarm, larger individuals more quickly than smaller ones, thus leaving the stickleback with smaller than average prey at the edge of the swarm.

Behavioural response of the water flea Daphnia pulex to a gradient in food concentration

Abstract Individual Daphnia pulex were allowed to distribute themselves in two connected flow-through chambers, only one of which received an input of food. The amount of inflowing food was regulated to give either a high or a low concentration. Daphnia pulex offered high food concentrations were randomly distributed between the two chambers, in spite of differences in concentration of food. However, when food concentrations were low and limiting, the animals responded to the food levels and distributed themselves to a larger extent in the food-enriched chamber.

Egg age distribution, the direct way to cladoceran birth rates

Cladoceran hatching frequency can easily be calculated from observed egg age distribution and estimated egg development time. The large timelag errors of models incorporating birth rates in the equation are avoided, and the population growth rate can be estimated directly.