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Free keywords:
Aeshna; dragonfly larvae; Pungitius; morphological defence; predation; prey capture; stickleback
Abstract:
Background: Some populations of stickleback have a reduced number and/or relative size of
spines.
Hypothesis: Macroinvertebrate predators such as dragonfly larvae cause selective pressure
against spines by capturing more stickleback with pelvic spines than stickleback that are
spineless.
Organisms: Ninespine stickleback (Pungitius pungitius) and dragonfly larvae (Aeshna
grandis).
Methods: We used 10 stickleback, five with pelvic spines and five with their pelvic spines
removed. We put them in containers with two dragonfly larvae. Every day for 4 days we
monitored how many stickleback were captured by the larvae. We repeated this experiment ten
times at two different densities of fish and predators. We also developed a model to determine
whether selection for spinelessness can be distinguished from drift.
Results: Dragonfly larvae caught as many stickleback with spines as without. The absence of
spines was not associated with a decrease in predation risk. We substituted Bayesian estimates
of the selection coefficient into quantitative genetic models of allele frequency change, and the
results of the models suggest that the selective advantage of spine loss is so small that its effects
cannot be distinguished from drift.
