Metabarcoding-driven discovery of copepod parasites

Pelagic copepods are hosts to numerous protistan parasites, which can have devastating effects on host fitness. Effects of parasitism include increased mortality, behavioral modifications, sterility and death. Although both pelagic copepods and parasitism are considered important, the topic of parasitism in copepods is vastly understudied. Traditionally, the parasites have been difficult to detect in regular plankton samples due to their small size, low visibility, and often low prevalences. However, modern molecular methods like metabarcoding can increase our ability to reliably detect parasites. In this thesis, I used metabarcoding to find parasites in zooplankton samples from Oslofjorden, Norway. I also re-analyzed metabarcoding data from the BioMarKs project, which sampled water and sediments in several locations across Europe. I used this data to investigate two fundamental questions about parasites in copepods: Where are the parasites, and When are they there? In addition, I identified new DNA sequences for 5 different copepod parasites, which aided in the search for parasites in the data. I also evaluated metabarcoding as a method for studying parasitism in copepods. I used two different primer sets for the metabarcoding in this thesis, both amplifying DNA from the the V4 region of the 18S gene. One primer set was general, made to amplify all taxa equally. The other was specifically made to block metazoan sequences (anti-metazoan), in an attempt to make parasites easier to detect by overcoming the biomass differences between hosts and parasite. The use of two primer sets in this study had no obvious benefit, as most sequences were still metazoan. In addition, the 18SV4 region could not distinguish between important metazoan groups. For future studies, I recommend using the anti-metazoan primers in conjunction with primers from a different genomic region, for example COI or 28S. My main conclusions are that parasites of copepods are found everywhere you look, and that they are present year-round in Oslofjorden. Many of the parasites seem to have a seasonal variation that follows the variation of hosts, as predicted by theory. In Oslofjorden, seasonal differences in parasite occurrence are larger than spatial differences. Still, there are some differences between the station outside the Drøbak sill and those inside of the sill that I attribute to host availability. In the BioMarKs data, which spans a larger geographic area, there are large differences