| Summary |
Investigating organismal response to invasions is increasingly important given human-mediated global change. Estuarine organisms face numerous biotic and abiotic factors that influence their ability to respond to invaders like parasites. When confronted with parasitic invaders, hosts can either develop resistance or expand distributions beyond the limits of the invader's tolerance. One such invader of US Atlantic estuaries is the Rhizocephalan (parasitic barnacle) Loxothylacus panopaei, which infects native panopeid crabs including the flatback mud crab Eurypanopeus depressus. Given that successful development of L. panopaei larvae is dependent on salinities >10 PSU, E. depressus may have a parasite refuge in lower salinity waters. However, in past field studies in North Carolina estuaries, the crab has not been detected at sites <10 PSU, and so it is unclear whether E. depressus would be able to tolerate salinities that would allow it to exploit a parasite refugia in low salinity waters. In Chapter 1, I aimed to answer this question by testing low salinity tolerance in adult E. depressus over a three-week exposure period. I examined mortality and righting response after exposure to salinity treatments (n=7) from fresh to moderate salinities (0-10 PSU) in E. depressus sourced from two Mid-Atlantic estuarine sites. I found higher mortality and slower righting response in 0, 0.2 and 0.5 PSU, but high survival in salinities between 3-10 PSU, where past field surveys have not detected the crab. This investigation helps us determine the low salinity tolerance range of E. depressus in a laboratory setting, allowing us to resolve whether salinity is a key barrier to the species' ability to exploit parasite refugia in Atlantic estuaries. Though survival was relatively high except in near fresh salinities, trends for slower righting response times compared to the control salinity suggests some level of sub-lethal stress that may impact the crab's survival and competitive abilities. Future research should examine multiple stressors (e.g., salinity, competition, predation) on crab survival. In addition, parasite infection prevalence and diversity can provide insight into the population dynamics of species inhabiting estuaries, as parasites may affect a species' ability to reproduce, survive, and even compete with other species. Chapter 2 of my thesis focused on identifying parasite infection prevalence and diversity in E. depressus compared to a co-occurring crab, Rhithropanopeus harrisii. I examined 2,022 crabs from 2016-2022 from 5 sites in which the crabs co-occur in the Pamlico and Neuse Estuaries, North Carolina for parasite composition, prevalence, and diversity. During my parasite surveys, I found two parasitic castrators, L. panopaei and Entoniscid isopods (possibly Cryptocancrion brevibrachium or Cancrion carolinus), as the major determinants of parasite prevalence in the two crab species. Total infection prevalence was found to be significantly higher in R. harrisii than E. depressus, as was Entoniscid prevalence, but L. panopaei prevalence did not significantly differ between the species. To our knowledge, our study is the first comprehensive investigation comparing parasite diversity in R. harrisii and E. depressus in North Carolina Estuaries. |
