Population ecology of the red algal parasite Choreocolax polysiphoniae (Rhodomelaceae, Ceramiales) , from Nova Scotia, Canada

The red alga Choreocolax polysiphoniae is part of a complex marine symbiotum involving at least seven species from four eukaryotic phyla. The red alga C. polysiphoniae is an obligate parasite on another red alga, Vertebrata lanosa , that, in turn, is an obligate epiphyte on the ultimate host, the brown macroalga A. nodosum . For the first time, population ecology of C. polysiphoniae was examined to evaluate the distribution and abundance of the parasite. We visited 18 sites in southwestern Nova Scotia, Canada in 2017 and 2018. Parasite frequency was evaluated at each site by scoring presence (as erumpent pustules) on 20–50, 3–5 cm clumps of its obligate red algal host, V. lanosa, from the mid-intertidal zone where the ger is typically abundant. Parasite frequency on host clumps varied from 4% to 95%. Some sites were sampled two or three times, but no clear seasonal pattern of abundance emerged. At eight sites where parasite pustules were counted on each clump of V. lanosa ( n = 20 to 50), Morisita’s index of dispersion showed a non-random distribution in which abundance of C. polysiphoniae was significantly clustered at all sites. This was confirmed from three sites where parasite frequency was over 75%, but host mass showed no significant correlation with parasite number. Our results suggest that C. polysiphoniae is a potential organism for studying population ecology of a macroalgal host–parasite system in the northwest Atlantic.