Investigating the association between armour coverage and parasite infection in an estuarine population of stickleback.

Background: When threespine stickleback colonized fresh water, they repeatedly evolved reduced armour plating via changes in Eda allele frequency. This evolution is typically attributed to differential predation pressure between marine and freshwater environments. However, the chromosomal region containing Eda is associated with many other phenotypes, including schooling, antipredator behaviour, and immunity. Consequently, the evolution of armour plating may be driven by multiple selective pressures acting on Eda or linked genes.

Question: Is parasite infection associated with armour phenotype?

Hypothesis: Parasite load differs between stickleback armour plate morphs.

Organisms: An armour-polymorphic population of threespine stickleback (Gasterosteus aculeatus), and their parasites.

Field site: In June 2009 and 2012, we sampled stickleback from a single human-made salt-marsh pool in the Campbell River Estuary on Vancouver Island.

Methods: We counted macroparasites on approximately 100 fish per year and counted lateral armour plates. We used generalized linear models to test for correlations between armour morph and parasite load.

Results: Most parasite species were not associated with armour. The gill parasite Thersitina was more abundant on more fully armoured fish in both years. The nematode Eustrongylides also exhibited a marginally significant positive trend. If parasitic infections reduce stickleback fitness, this positive covariance between armour and infection would accelerate the loss of armour plating in stickleback colonizing fresh water.