Record of parasitic oomycetes on neotropical copepods in aquatic environments of Northwestern Patagonia (Argentina)

Aim: Oomycetes are one of the most widespread and destructive parasitic groups in the world. The first infection of oomycetes on copepod Parabroteas sarsi Daday 1901 was recently recorded in America. Oomycetes infection on this species makes their eggs unviable and could differentially affect the survival of females. The aim of this study was to track the incidence of oomycetes in two populations of P. sarsi. Methods: Two Patagonian ponds were monitored during spring-summer of 2014-2018, measuring environmental variables and the prevalence of oomycete infections on P. sarsi populations. Taxonomic determination of the oomycetes was performed in laboratory. Sex ratios also were recorded in each survey. Results: The presence of Aphanomyces ovidestruens Gicklhorn 1923 and Pythium flevoense Van der Plaats-Niterink 1972 was detected in females of P. sarsi in both ponds. We observed a decrease in the parasite prevalence over time, from ~25% (assessed on the total number of adults) to almost disappear completely. Besides, a lower proportion of females than males were observed in samples with high prevalence of parasitism. No temperature differences were found during the sampling years and the monthly temperatures between the maximum and minimum prevalences were similar. Conclusions: Our study presents a new record of oomycetes parasitizing copepods in America. Temperature can play a key role in the dynamics of parasite species, but in our study this variable not affected the incidence of the infection. However, other variables could be affect the pattern of the infection such as the presence of predators of oomycetes spores (cladocerans). The lower females proportion when the infection was higher, could support the idea of differential sex/mortality induced by the infection. Recording and identification of parasites is a critical starting point for wildlife management, which will provide an early warning to prevent the spread of harmful parasites such as oomycetes.

Acta Limnologica Brasiliensia, 2020, vol. 32, e16 American copepods and the first record of Pythium spp. in freshwater copepods. Garcia et al. (2018) through laboratory experiments have shown that oomycetes are not lethal to females in the short-term and do not affect their susceptibility to insect predation. However, the proportion of females decreased after the spread of the infection, when the daily water temperature increased above 20 °C. Temperature play a key role in the dynamics of parasite species and can alter host susceptibility (Mitchell et al., 2005). For example, increasing temperature favoring Aphanomyces sp. growth and zoospore production (Valois, 2015). The effectiveness of the infection also depends on the host condition. In P. sarsi, the oomycete infection produces an additional cost associate with the generation of a new egg sac, and could reduce the life expectancy of females in the long-term, as has been shown in Burns (1985b) for others copepod species. On the other hand, temperature also affects physiology parameters such as metabolism level and oxygen consumption that, in turn, affect copepod's ability to resist environmental changes (Huntley & Lopez, 1992). High temperatures can accelerate metabolism and oxygen consumption, leading to excessive use of protein and a low survival rate (Li et al., 2009;Garcia & Diéguez, 2014). It is possible that a combination of these factors imposes severe restraints on P. sarsi females from spring onwards, differentially affecting their survival (Garcia et al., 2018). However, little is known regarding this aspect and further investigation is required on the subject. Decline of the P. sarsi populations could have a pronounced effect on the aquatic food webs, due to this copepod representing a fundamental trophic link between Palavras-chave: zooplâncton; parasitismo; calanóides; zonas húmidas; proporção de sexos.
The oomycetes (Philum Oomycota) are one of the most destructive and widespread group of parasites in freshwater communities, affecting plankton, fishes, amphibians and reptiles (Phillips et al., 2008). Within zooplankton, oomycetes infections are concentrated in eggs sacs carried by females. Unlike cladocerans, infections on copepods are conspicuous, altering egg size and appearance through the development of hyphae (Rossetti, 2005). The oomycetes produce the complete mortality of egg, affecting the host reproduction and population growth (Redfield & Vincent, 1979;Burns, 1985a).
Two ponds were sampled in six opportunities in total during November and December of 2014-2018. One is an urban pond located inside the city of S. C. de Bariloche (Fantasma pond), while the other pond is located at piedmont of Cerro Catedral (Cerro Catedral pond) and surrounded by well-preserved forest of Nothofagus pumillo (Table 1). These wetlands are wet meadows mainly dominated by herbaceous plants belonging to the Cyperaceae, Juncaceae and Poaceae families (Cuassolo et al., 2016). Both ponds are temporary flooded by rainfall and snowmelt (April-May) and dry frequently in early summer (December-January). Sometimes, in very wet years, they turn in semi-permanent environments (alternating years of seasonally dry versus inundated). Electrical conductivity and water temperature were measured with a YSI85 multiprobe, and pH was measured with a Hanna HI98150 probe. The maximum depth of the ponds was also recorded. Meteorological data was obtained from an automated weather monitor ( The sex ratio and prevalence of parasitism were determined by examining random samples of 50 to 200 P. sarsi adults on each survey, depending on the abundance of copepods. The Fantasma pond population was sampled four times, while Cerro Catedral pond population was sampled only twice. Oomycetes infection in females is easily recognized by the change in the colour of eggs (white and/or blue instead of dark brown) (Garcia et al., 2018). Oomycete samples were obtained from infected P. sarsi females in laboratory. Water cultures of oomycetes were placed in sterilized Petri dishes containing several halves of hemp Cannabis sativa seeds used as bait and were incubated at room temperature (15 to 20 °C). After 2 weeks, the oomycetes species were confirmed using water cultures from their monosporic isolates in cornmeal-agar medium (CMA) (Dick, 2001;Johnson et al., 2002).
The studied ponds had similar maximum sizes and depths. Conductivity and pH were higher in the Fantasma compared to Cerro Catedral pond, while water temperature was similar in both studied sites (Table 1). Both ponds presented females of P. sarsi with eggs parasitized with the oomycetes A. ovidestruens and P. flevoense.
The oomycetes prevalence recorded in Fantasma pond during 2009 was 19% of the total population of P. sarsi (53% of females were infected) (Garcia et al., 2018). In our study, the same environment showed a prevalence of 15-26% (40-66% of females were infected) in 2014, only a 0.5% in 2017 (1.0% of  the females), while there were no signs of infection in 2018 (Table 2). The parasite prevalence in Cerro Catedral pond during 2017 was similar to those recorded in Fantasma pond during 2010 and 2014, with a prevalence of 24% in the population (56% of the females), while a very low prevalence (3%) was recorded in 2018 (Table 2). Ponds in Patagonia generally experience interannual fluctuation in the rainfall patterns and local temperatures (Perotti et al., 2005). These parasites have a positive relationship between temperature and its prevalence (Valois, 2015;Garcia et al. 2018), therefore the reduction of infection could be associated with the variability of temperature. However, we did not find differences in annual temperature between the years 2014, 2017 and 2018 ( Sampling dates with presence of parasites showed an asymmetric sex proportion with males dominating the structure of the populations. In contrast, dates without infection detection, we observed dominance of females (Table 2). This result could support the idea that oomycetes generate differential mortality on females (Garcia et al., 2018). However, the reasons for the reduction of males in periods without infection are unknown, considering that in Centropagidae family the female:male ratios tends to be equal (Kiørboe, 2006).
Fantasma is a eutrophic pond that receives inputs of allochthonous organic matter from the surrounding urbanization and non-native trees, in contrast with Cerro Catedral pond that is located in a non-urbanized area surrounded by native forest, where human impact is negligible. In this sense, the spread of oomycetes and their effects on copepods appear not be associated to anthropogenic activities. The finding of oomycetes in Cerro Catedral pond corresponds to a new record of A. ovidestruens and P. flevoense parasitizing copepods, future investigations must focus in the monitoring different populations.
P. sarsi is considered the largest freshwater copepod reaching 5 mm in body size. Its body size (Garcia et al., 2013b) has facilitated the detection of parasite infection. It is probable that other smaller species of Patagonian copepods are being affected by oomycetes, but their records are ignored due to the small size of their egg sacs. Parasite identification and monitoring is a critical starting point for wildlife management (Tompkins et al., 2015). For this reason, it is necessary to continue the studies on parasitic biodiversity on aquatic environments (Steciow, 2005). The understanding of infectious processes and their causal mechanisms will be indispensable to better predict the impact and spread of harmful parasites, such as oomycetes.