DOI: 10.25349/d9b89t
title: High parasite diversity in the amphipod Gammarus lacustris in a subarctic lake

description: Amphipods are often key species in aquatic food webs due to their
 functional roles in the ecosystem and as intermediate hosts for
 trophically transmitted parasites. Amphipods can also host many parasite
 species, yet few studies address the entire parasite community of a
 gammarid population, precluding a more dynamic understanding of the food
 web. We set out to identify and quantify the parasite community of
 Gammarus lacustris to understand the contributions of the amphipod and its
 parasites to the Takvatn food web. We identified seven parasite taxa: a
 direct life cycle gregarine, Rotundula sp., and larval stages of two
 digenean trematode genera, two cestodes, one nematode, and one
 acanthocephalan. The larval parasites use either birds or fishes as final
 hosts. Bird parasites predominated, with trematode Plagiorchis sp. having
 the highest prevalence (69%) and mean abundance (2.7). Fish parasites were
 also common, including trematodes Crepidostomum spp., nematode Cystidicola
 farionis, and cestode Cyathocephalus truncatus (prevalences 13, 6, and 3%,
 respectively). Five parasites depend entirely on G. lacustris to complete
 their life cycle. At least 11.4% of the overall parasite diversity in the
 lake was dependent on G. lacustris, and 16% of the helminth diversity
 required or used the amphipod in their life cycles. These dependencies
 reveal that in addition to being a key prey item in subarctic lakes, G.
 lacustris is also an important host for maintaining parasite diversity in
 such ecosystems.
description: Study area and collection Takvatn
 (69<sup>o</sup>07’N, 19<sup>o</sup>05’E) is a
 subarctic, oligotrophic, and dimictic lake in northern Norway that has
 been the focus of intensive ecological and food web studies for more than
 30 years (details in Amundsen et al., 2009, 2013, 2019). The lake is
 situated 214 m above sea level with a surface area of 15
 km<sup>2</sup> and a maximum depth of ca 80 m. There is little
 macrovegetation in the lake, but the littoral zone (3-10 m depth) has
 dense beds of the grass-like macroalgae <i>Nitella</i> sp.,
 which contain the highest abundances of <i>G. lacustris</i>
 (Frainer et al., 2016). We sampled gammarids in the
 littoral zone (0-8 m depth) by dragging a benthic sled along
 <i>Nitella</i> sp. beds as described in Knudsen et al. (2001).
 Gammarids were collected from each haul, placed in buckets with lake water
 and vegetation, and brought back to the lab for dissection within 48
 hours. Individuals not dissected within 24 hours were kept cool overnight
 in the refrigerator or outdoors (at approx. 4-8
 <sup>o</sup>C). To obtain a broader range of parasite
 diversity in <i>G. lacustris</i> in the lake, gammarids were
 collected from five sites (L1-L5), including two in the vicinity of an
 important nesting area for birds (L4 and L5; Klemetsen and Knudsen, 2013).
 Sampling was carried out over three years during different times of the
 ice-free period (August and October 2012, June and September 2013, and
 August 2015). Not all sites were sampled every year.
 Dissection and parasite identification We
 blotted 474 amphipods on paper towels, measured length (eye to end of
 telson; mm) and wet weight (g). Due to potential variation in length
 measurements, we generated a length-weight regression from a subsample
 (<i>y</i> = 132.17<i>x</i> + 5.62,
 <i>R</i><sup>2</sup>=0.95) and used weight-based
 estimates of length for all analyses. We compressed whole gammarids
 between glass plates (150 mm × 100 mm × 3.5 mm) and examined them under a
 stereo microscope (Leica Wild M3, maximum magnification of 40X). Parasites
 were counted and transferred for further inspection under a compound
 microscope if needed. Parasites were identified to the nearest taxonomic
 level based on morphology, and select specimens were preserved in 95%
 ethanol for genetic analysis in a separate study (trematodes only; details
 in Soldánová et al., 2017) or formalin for further identification (all
 other parasites). Statistical analyses
 We characterized the parasitism in <i>G.
 lacustris</i> samples by calculating prevalence, mean intensity, and
 mean abundance (defined in Bush et al., 1997) and assessed parasite
 infra-community composition using the 7-set Venn diagram “Adelaide” (Dusa,
 2020). We investigated whether the infections of parasites with indirect
 life cycles varied between sampling locations and sampling period, using
 two analyses. To analyze if the abundance of <i>Plagiorchis
 </i>sp. differed between sampling locations and periods, we used a
 mixture model (zero-inflated negative bionomial generalized linear model
 (ZINB GLM); R (version 3.5.1; R Core Team, 2018), with <i>G.
 lacustris </i>size (length) as a covariate (Zuur et al., 2009). The
 ZINB GLM contains two parts; a negative binomial GLM that models parasite
 counts and a binomial GLM that models the probability of observing excess
 zeros above those of the count process (Zuur et al., 2009). Other parasite
 species were low in intensity so we used infection status (infected versus
 uninfected) rather than abundance as the binomial response variable in
 logistic regressions with the same predictor variables (sampling location,
 period, and <i>G. lacustris </i>size).