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Parasites of two closely related Poeciliid species across a salinity gradient on the island of Trinidad: implications for geographic range limits

Date

2018

Authors

Robison, Porsche, author
Ghalambor, Cameron, advisor
McGrew, Ashley K., committee member
Schaffer, Paula, committee member
Peers, Graham, committee member

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Abstract

Parasite communities can vary greatly both within and amongst host populations. Many factors may be responsible for this variation in parasite diversity, and parasite-host relationships are of great ecological importance as parasites can alter host behavior, impact population demography, and drive co-evolutionary dynamics. One long-standing ecological question is how these parasite-host interactions shape mutual geographic distributions, which are also impacted by various abiotic and biotic factors. However, few studies have investigated how parasite communities change across environmental gradients and different host species, or how parasite abundance changes within and outside the host geographic range. The island of Trinidad provides a model system that can be used to address these questions. On this island, the tropical fish Poecilia reticulata inhabits mountainous and lowland freshwater streams but avoids brackish waters. A close relative, Poecilia picta inhabits both lowland freshwater and brackish water streams. To date, no study has investigated how internal parasite communities vary across this salinity gradient or between these two closely related host species with overlapping geographic ranges. In lab studies, P. reticulata has been shown to be physiologically tolerant of brackish water, suggesting some other environmental factor may limit their dispersal and range expansion into brackish waters. Here we investigated how internal parasite diversity changes between 1) natural P. reticulata and P. picta populations in freshwater, 2) natural populations of P. picta in fresh and brackish water sites, and3) P. reticulata, experimentally exposed to fresh and brackish water conditions. We found the prevalence of digenean trematode metacercariae to be 100% across three different river systems for both host species, however, mean metacercarial abundance differed significantly by river. Based on morphological differences in the metacercariae, we identified three distinct morphospecies. All three morphospecies were found in freshwater P. reticulata and P. picta. However, mean abundance of parasites varied across the two host species with P. reticulata harboring more parasites, on average, compared to P. picta. All three morphospecies were also found in P. picta in brackish water, but the total mean metacercarial abundance of P. picta was found to be increased in brackish compared to freshwater sites. Collectively, these results suggest that the three morphospecies utilize both hosts and they are limited in their geographic distribution by salinity. Still, the internal abundance of parasites varies between the two hosts depending on the salinity. We tested whether these same parasites may be limiting the distribution of P. reticulata to freshwater, by experimentally exposing P. reticulata populations to field-collected brackish water. We assumed field-collected brackish water contains live cercaria and conducted exposures for a period of seven days. Compared to controls exposed to field-collected freshwater, there was a significant increase in the internal metacercarial abundance, along with an increase in mortality amongst the brackish water exposed group. These results suggest that movement of P. reticulata into novel brackish environments may be inhibited by increased parasitism; however, further investigations are warranted to better understand the mechanisms that determine the geographic distributions of parasites and their hosts.

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Subject

parasites
Poecilia reticulata
range limits
digenean trematodes

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