Divergence in Alewife Alosa pseudoharengus (Actinopterygii, Clupeidae), life history alters parasite communities

The Alewife, Alosa pseudoharengus, presently has two distinct life history variants, the anadromous ancestral type, and a derived landlocked freshwater type. In accordance with their variation in life history, landlocked Alewives have evolved dissimilar morphologies and ecological traits relative to the anadromous type, and as a result, their functional roles within ecosystems have changed. This study tests the prediction that hosts with an altered life history and ecosystem residency, like the landlocked Alewives would have lower rates of parasitism, which would result in a lower diversity and abundance of macroparasites compared to the anadromous variant. We compared parasite species richness and abundance between landlocked (collected from a lake in New Jersey) and anadromous Alewives (collected from river and ocean environments). The landlocked Alewives were infected with only 2 parasite species and overall parasite abundance was low in this population, while the anadromous Alewives harbored 7 parasite species, which formed distinct communities depending on host stage. Overall, this study suggests there was a loss of native parasites within landlocked Alewives and supports the idea that these populations of Alewives experience lower parasite pressure than their anadromous conspecifics.