Gauchergasilus euripedesi (Copepoda, Ergasilidae) parasitizing different species of fish from two environments in southern Brazil

The parasitic copepod Gauchergasilus euripedesi (Montú, 1980) Montú & Boxshall, 2002 was described from plankton samples and specimens found in four fish species from the estuarine area of Patos Lagoon, state of Rio Grande do Sul (RS), Brazil. Later, one different fish species was reported parasitized with G. euripedesi in the same locality. Species of Astyanax Baird & Girard, 1854 (Astyanax henseli Melo & Buckup, 2006 and Astyanax lacustris (Lütken, 1875)) and Psalidodon Eigenmann, 1911 (Psalidodon eigenmanniorum (Cope, 1894) and Psalidodon aff. fasciatus (Cuvier, 1819)) were collected in two environments (Pintada Island, municipality of Porto Alegre and Itapeva Lagoon, municipality of Terra de Areia, RS) to investigate their parasites. The copepods found in the gill arches were counted, processed, mounted in permanent slides, and photographed using light microscopy, or processed for observation in scanning electron microscopy. Specimens of P. eigenmanniorum from Pintada Island, A. lacustris and P. aff. fasciatus from Itapeva Lagoon, were parasitized by G. euripedesi, with prevalences of 29.03% (A. lacustris), 10.34% (P. eigenmanniorum), and 9.68% (P. aff. fasciatus). Measurements obtained for specimens of G. euripedesi were similar to those found in the literature, except for egg sacs which were larger in the specimens examined in the present study. In addition to being the first report of G. euripedesi parasitizing species of fish (A. lacustris, P. eigenmanniorum, and P. aff. fasciatus), the results presented here also extend the known geographic distribution of the copepod species. Neotropical Biology and Conservation 16(2): 289–298 (2021) doi: 10.3897/neotropical.16.e64668 Copyright Moisés Gallas, Laura R.P. Utz. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. RESEARCH ARTICLE Moisés Gallas & Laura R.P. Utz 290

Gills arches of the fishes were removed with the aid of fine-tip scissors, placed in a jar with formalin solution 1:4,000, and shaken at least 50 times (Gallas et al. 2016) to obtain detached copepods. The material was examined under stereomicroscope and the copepods were collected and stored in 70° GL ethanol (Amato et al. 1991). Copepods were mounted in toto in Faure's mounting medium without clarification. Two specimens were prepared for scanning electron microscopy (SEM), where they were dehydrated in an ethanol series to critical point dried. The copepods were mounted on stubs, coated with carbon and gold, and then examined in a Field Emission Electron Microscope (FESEM), Inspect F50, FEI at 'Laboratório Central de Microscopia e Microanálise' (LabCEMM) at Pontifical Catholic University of Rio Grande do Sul (PUCRS), Porto Alegre, Brazil.
Measurements are shown in micrometers (µm) unless otherwise indicated. They represent the minimum and maximum values followed by mean, standard deviation, and sample size in parenthesis. In the description, the terminology follows Huys and Boxshall (1991). Photomicrographs were made using an Olympus BX50 microscope and prepared using ADOBE'S PHOTOSHOP CS2. The parameters of infestations such as prevalence, mean intensity and abundance were used according to Bush et al. (1997). Voucher copepod specimen was deposited in the 'Coleção de Crustáceos do Museu de Ciências e Tecnologia da PUCRS' (MCP), Porto Alegre, RS, Brazil.
Localities: Jacuí Delta, Pintada Island, municipality of Porto Alegre, and Itapeva Lagoon, municipality of Terra de Areia, state of Rio Grande do Sul, Brazil.

Discussion
The ergasilid copepods are described mainly based on their morphology including shape and number of segments in each appendage, segmental setation, in addition to the patterns of spines and setae (Montú 1980;Araujo and Boxshall 2001;Montú and Boxshall 2002). The specimens of G. euripedesi found in this study presented all of the traits described by Montú (1980) and Montú and Boxshall (2002) as well as presented the measurements similar to those specimens reported by the two studies. However, the specimens found in the present study presented larger egg sacs (350-780 long) than the specimens (232 long) examined by Montú (1980). These differences could be the result of incompletely developed egg sacs of the specimens analyzed by Montú (1980) or due to a variation not reported until now. Therefore, most measurements in the present study represent new data for G. euripedesi. The parameters of infestations of G. euripedesi available are presented in Table 1. In general, parameters of the host species examined in the present study are more similar to those reported for M. furnieri (Velloso and Pereira Jr. 2010), but smaller than those found in G. brasiliensis (Rassier et al. 2015). These differences do not seem to be related to the number of hosts examined, since the two most sampled hosts (M. furnieri, n = 94 and G. brasiliensis, n = 79) presented differences between them. Moreover, they also presented differences in comparison with the hosts examined in the present study (A. lacustris, n = 31; P. eigenmanniorum, n = 29; and P. aff. fasciatus, n = 31). In addition, considering the tolerance to salinity variations, G. euripedesi prefers low salinity levels found in the Patos Lagoon estuary (Velloso and Pereira Jr. 2010), thus, the infestations found in Guaíba Lake and Itapeva Lagoon should be higher than those reported by Velloso and Pereira Jr. (2010) and Rassier et al. (2015). More studies are necessary to explain the different levels of infestations of G. euripedesi. Other factors such as the availability of hosts, presence of different hosts species in the same area, characteristics of the environments, to name a few, must be considered since salinity levels do not seem to be the only or the main factor influencing infestations by G. euripedesi.
In Brazil, the ergalisid crustaceans that have been reported in A. lacustris and P. fasciatus include one species (A. bryconis) and four undetermined species (Acusicola sp., Brasergasilus sp., Ergasilus sp., and Vaigamus sp.) (Eiras et al. 2010;Luque et al. 2013;Pádua et al. 2015;Camargo et al. 2016). Until now, previous reports of fish parasitized by G. euripedesi include larvae of B. pectinata, Gobiesox sp., L. grossidens, and M. furnieri (Montú 1980), and adult specimens of G. brasiliensis and M. furnieri (Velloso and Pereira Jr. 2010;Rassier et al. 2015). In additional reports, the specimens of G. euripedesi were collected from plankton samples, since only females are known to be parasitic (Araujo and Boxshall 2001;Montú and Boxshall 2002). This is the first report of G. euripedesi parasitizing species of Astyanax (A. lacustris) and Psalidodon (P. eigenmanniorum and P. aff. fasciatus), in which all species are considered new hosts. These new reports reflect the potential high number of parasitic crustaceans that are still underestimated as pointed out by Luque et al. (2013). This study also extends the known geographic record of G. euripedesi in the lagunar system of Patos Lagoon to the Jacuí Delta, and includes a new locality record (Itapeva Lagoon). It is possible that G. euripedesi could be found in other localities throughout the territory between southern and northeast Brazil since its distribution is only known in the states of Rio Grande do Sul and Sergipe (Araujo and Boxshall 2001;Montú and Boxshall 2002).

Conclusions
The specimens of G. euripedesi presented morphology and measurements (except for large egg sacs reported here) similar to the specimens reported in previous studies. The species A. lacustris, P. eigenmanniorum, and P. aff. fasciatus are reported as new hosts for G. euripedesi, thus, increasing the list of fishes species parasitized by crustaceans in Brazil. The present study also extends the known distribution of G. euripedesi in southern Brazil and contributes to the knowledge of the biodiversity of copepod parasites in the region.