The River Continuum Concept (RCC) predicts that species richness increases from headwater streams to small rivers because of variation in geophysical conditions. However, this prediction has not been tested for stream fish parasite communities. If such patterns occur, then headwater fish populations should be infected by less diverse parasite communities than host populations in larger (downstream) systems, which may differentially impact evolutionary trajectory of these different host populations. We predicted that there would be a positive relationship between stream size (drainage area) and richness of resident parasite communities of Redspot Darters (Etheostoma artesiae). We collected 421 Redspot Darters from 15 reference streams (range: 1.0 – 217.5 km2) in the Sipsey Fork – Black Warrior River Basin of Alabama. We conducted necropsies and microscopically examined host skin, fins, eyes, gills, brain, muscle, urinary bladder, gall bladder, liver, spleen, kidney, gonads, mesenteries and the gastrointestinal tract for parasites, which were identified and enumerated (except myxozoa which were noted as present or absent). We tested our prediction of a significant relationship between stream size and parasite community richness at two levels, stream site (component community) and host individuals (infracommunity), using a generalized linear model with a Poisson distribution and a general linear model respectively.
Results/Conclusions
Twenty-eight species of parasites were recovered. Component community richness ranged from 5 to 15 species per site whereas mean infracommunity parasite richness ranged from 1.5 to 5.4 species per fish. There was a positive relationship between drainage area and component community richness (z = 2.596, p =0.009) and between drainage area and mean infracommunity richness (R2 = 0.616, p < 0.001). The increase in richness with increasing stream size appears to be driven by relationships between stream size and microparasites (z = 2.207, p = 0.027), parasites with fully aquatic life-cycles (z = 2.692, p = 0.007), and 2-host indirect life cycles (z = 3.097, p = 0.002). Thus, myxozoa, which are microparasites with fully aquatic 2-host life cycles, and that ranged from 0 to 4 species in small and large streams respectively, appear to be major contributors to the positive parasite richness-stream size relationship. Our results support our prediction that species richness of fish parasites increases with increasing stream size, perhaps because of longitudinal patterns of free-living host diversity, food web complexity, or increases in availability of specific intermediate hosts (e.g. snails and oligochaetes). Such parasite infection gradients may have important consequences for host fish health and evolution.