The life cycle of the parasite Pomphorhynchus tereticollis in reference to 0+ cyprinids and the intermediate host Gammarus spp in the UK.

Abstract

Pomphorhynchus tereticollis is a recently resurrected parasite species that spans the UK and continental Europe. The parasite is the only Pomphorhynchus spp in the UK and has been researched since the early 1970’s. The species has an indirect life cycle which uses a Gammarus spp as an intermediate host and cyprinids and salmonids as final hosts although the main hosts are Squalis cephalus (S. cephalus) and Barbus barbus (B. barbus). Research on P. tereticollis has primarily focused on mature fish while there is limited research on 0+ fish and the intermediate host in the UK. Previous studies have shown that paratenic and post-cyclic transmission may occur in the life cycle of P. tereticollis although there has been little field research to demonstrate how much of an impact this may have on the parasite. As well as this, little data exists for what may influence infection within 0+ fish species. Therefore, a field study was designed to understand the relationship between P. tereticollis and 0+ fish as well as to explore the potential for post-cyclic or paratenic transmission of P. tereticollis. B. barbus, Leuciscus leuciscus (L. leuciscus), S. cephalus and Phoxinus phoxinus (P. phoxinus) were sampled from the River Teme (UK) in the summer and early autumn of 2015, 2016 and 2019. Data was collected on fish length, fish gape, gut fullness, temperature and date. A binary logistic regression was conducted to find variables that predicted infection with the parasite. Fish length, gut fullness and species where all factors that predict infection with P. tereticollis in the 0+ fish. Each species had different variables that predicted infection with fish length the only variable that predicts infection in B. barbus and only gut fullness predicting S. cephalus infection. Infection in L. leuciscus was predicted by temperature, date and gut fullness while infection in P. phoxinus was predicted by fish length, gut fullness and date. P. phoxinus is likely to aid transmission of P. tereticollis in a paratenic and post-cyclic manner due to the discovery of extra-intestinal infections and limited attachment to the alimentary tract when infections where found. A low prevalence was found in 0+ S. cephalus which suggests Gammarus spp are an insignificant part of the diet for the species. Diet is likely to be the driver for differences in infection of the different species. Gut fullness was higher in infected individuals compared to uninfected individuals in all species except S. cephalus which suggests an energy deficit caused by the parasite. This may cause a higher probability of predation in 0+ fish and facilitate the potential paratenic and post- cyclic transmission of P. terticollis. As well as this, Recent research in Switzerland and France has shown that P. tereticollis displays a preference for Gammarus fossarum (G. fossarum) compared to Gammarus pulex (G. pulex). Until recently, it was thought that G. pulex was the only fresh water Gammarus spp in southern England until G. fossarum was discovered with eDNA, morphological and standard DNA extraction techniques. Therefore, research was conducted to examine the intermediate host use of P. tereticollis. Gammarus spp were collected from four rivers in the south of England and dissected to find the prevalence. Species was observed morphologically using the location of Plumose setae. G. fossarum was present in all four rivers tested and was the most dominant species in eastern flowing rivers. Only G. fossarum carried cycstacanth infections of P. tereticollis in all sites tested. Rivers which contained the highest proportion of G. fossarum to G. pulex had the highest prevalence of P. tereticollis. The distribution of G. fossarum in the UK is likely to explain the discontinuous distribution of P. tereticollis described in previous research. The preference is likely to be due to overlap in the distributions of P. tereticollis and G. fossarum before and during previous ice ages. The misidentification of G. fossarum in the UK and the strong preference of P. tereticollis for one gammarid over another highlights the difficulties in conservation of habitats regarding cryptic species.