In recent decades, an apparent uptick in infectious diseases of marine organisms has led some disease ecologists to conclude that the oceans are facing a “rising tide” of marine parasitism. Concurrently, others warn that parasite species might be facing high rates of extinction. An understanding of whether the oceans are facing a rising tide of disease or undergoing an invisible wave of parasite extinction is critical to marine resource management.
To assess how marine parasite abundance has changed over time, we collected primary data on marine parasite dynamics over nine decades within one region using parasitological dissection of fluid-preserved fishes from natural history collections. The fluid preservation process fixes both the fish and any parasites infecting it. Using the University of Washington Fish Collection, we identified and quantified the parasites infecting English sole (Parophrys vetulus; n = 109 individuals) of Puget Sound, Washington, USA from 1930 to 2019. We used generalized linear models to assess how the abundance of each parasite species has changed over time. Using the results obtained from these models, we used meta-analysis to determine whether species-level changes were consistent within broad taxonomic groupings and transmission strategies (i.e., directly transmitted versus complex life cycle).
Results/Conclusions
We counted nearly 2,500 individual parasites. These parasites represented 23 distinct species/morphotypes and 4 groups of unidentified parasites. Twelve taxa had greater than 5% prevalence and were further examined for changes in abundance over time. Of these, nine taxa did not change in abundance over time, one trematode taxon increased over time, and the Acanthocephala taxa and a trematode taxon decreased. When examining for patterns within broad taxonomic groupings, nematodes, cestodes, trematodes, and leeches exhibited no change over time, while acanthocephalans declined significantly across the study period. Directly transmitted and complex life cycle parasites did not differ systematically from one another in the direction of their response over time. The diverging patterns among parasite taxa demonstrate that parasite responses to long-term ocean change are complex, representing a double-edged sword: a "rising tide" for some, and a loss of biodiversity for others. We hypothesize that the significant changes of the Puget Sound environment over the past 90 years have driven the observed changes in parasite abundance. Therefore, the next step in our research is to assess which environmental parameters (e.g., temperature, nutrient loading) are associated with the rise and fall of parasites of Puget Sound.