Octocoral co-infection as a balance between host immunity and host environment
Co-infection is the reality in natural populations, but few studies incorporate the players that matter in the wild. We integrate the environment, host demography, two parasites, and host immunity in a study of co-infection to determine the drivers of parasite interactions. Here, we use an ecologically important Caribbean sea fan octocoral, Gorgonia ventalina, that is co-infected by a copepod and a labyrinthulid protist. We first expanded upon laboratory studies by showing that immune suppression is associated with the labyrinthulid in a natural setting. Histological analyses revealed that immune cells (amoebocytes) were significantly suppressed in both labyrinthulid infections and co-infections relative to healthy sea fans, but remained unchanged in copepod infections. However, surveys of natural coral populations demonstrated a critical role for the environment and host demography in this co-infection: the prevalence of copepod infections increased with sea fan size while labyrinthulid prevalence increased with water depth. Although we predicted that immune suppression by the labyrinthulid would facilitate copepod infection, the two parasites did not co-occur in the sea fans more often than expected by chance. These results suggest that the distinct ecological drivers for each parasite overwhelm the role of host immune suppression in determining the distribution of parasites among hosts. This interplay of the environment and parasite-mediated immune suppression in sea fan co-infection provides insights into the factors underlying co-occurrence patterns in wild co-infections. Moving forward, simultaneous consideration of co-occurring parasites, host traits, and the environmental context will improve the understanding of host – parasite interactions and their consequences.
KeywordsDisease ecology Parasite interactions Gorgonia ventalina Labyrinthulid Eco-immunology
We would like to acknowledge statistical input from Erika Mudrak and Jay Barry; field survey assistance from Luis Rodriguez and Duane Sanabria; help with amoebocyte analyses from Phoebe Dawkins and Phillip Fargo; and feedback from Alison Power, Brian Lazzaro, Joleah Lamb, Morgan Eisenlord, Katherine Sirianni, and Nicholas Fletcher. The reviewers’ comments also greatly improved this manuscript. AMT was supported by the National Science Foundation Graduate Research Fellowship Program (DGE- 1650441); a Grant-in-Aid of Research from the National Academy of Sciences, administered by Sigma Xi; the Andrew W. Mellon Foundation; and the Cornell Department of Ecology and Evolutionary Biology Paul P. Feeney Fund. EW was funded by the National Science Foundation (IOS-1017510, 2010 - 2014) with logistical and partial funding from the Department of Marine Sciences, University of Puerto Rico at Mayagüez.
Author contribution statement
AMT, EW and DH conceived and designed the study. AMT and EW conducted field surveys. AMT performed data analyses and wrote the manuscript with editorial input from EW and DH. All authors gave final approval.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable institutional and/or national guidelines for the care and use of animals were followed.
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