Selective predation and habitat shift in a copepod species — support for the predation hypothesis

Summary

In studying the relevance of the predation hypothesis as an explanation for the vertical migration of zooplankton two predictions were tested:

• 1) The egg sacs of Eurytemora are sufficient to cause different predation pressure in food selection experiments with visually hunting planktivores as predators and ovigerous vs. non-ovigerous Eurytemora females as prey.

• 2) If copepods avoid predation by vertical migration, there should occur differences in the vertical distribution pattern of ovigerous and non-ovigerous females according to selective predation.

The results showed that, when ovigerous and non-ovigerous females were presented to predators, a significant preference for ovigerous females was found in high densities of prey. No preference was found in experiments with males vs. non-ovigerous females and at low copepod densities. In a field study we found that non-ovigerous females distributed throughout the water column preferred the deeper parts. Ovigerous females were also abundant in deeper waters but almost totally avoided the surface layer above 20 m. Thus our results support the predation hypothesis when the adaptive value of vertical migration is considered.

On the basis of our results we deduced testable predictions on the evolutionary effects of predation:

• 1) In heavily foraged communities there is strong coevolution among the prey to resemble each other in terms of which are critical in selective predation.

• 2) If the carrying of an egg sac is considered as parental care, it is likely that parental care occurs more seldom in environments with high predation.