In the world, around 30 species and subspecies of the genus Paracyclops Claus 1893 have been recorded in different types of freshwater habitats, distributed in temperate-cold latitudes and in tropical areas in which the genus tends to present more species (Karaytug and Boxshall 1998a, b; Karaytug et al. 1998; Mercado-Salas and Suárez-Morales 2009). In Mexico, four species of Paracyclops have been inventoried (Suárez-Morales et al. 2020). At the study site, previous work reported the presence of the species P. chiltoni (Mendoza-Chávez et al. 2021); however, in this work, the detailed morphological analysis by scanning and light microscopy confirmed that it is P. novenarius.
This species was reported for the first time in Colombia by Reid (1987), later by Gaviria (1994), and Gaviria and Aranguren (2007), inhabiting artificial asbestos containers, and it is well known that this material is carcinogenic (Barrera et al. 2010). Asbestos is composed of silicate fibers; the mineral is obtained in open quarries or shallow mines (Castellano-Alvarado et al. 1960), and according to its physical characteristics, it can be composed of SiO4−. In addition, in the region where P. novenarius was reported, the presence of heavy metals such as Cu, Cr, Ni, and Zn, have been reported, which exceed the contamination limits established by the EPA (Collazos-Santos, 2014). Probably, there is a relationship between the habitat of P. novenarius, living in environments with some pollutants, but further analysis is required to understand this.
In Mexico, it has not been reported in previous works (Mercado-Salas and Suárez-Morales 2009, 2012; Suárez-Morales 2020); thus, this is the first record of this species in Mexico. Additionally, P. novenarius is living in this site that significantly exceeds the concentration of arsenic considered lethal for zooplankton (3 mg/L− 1) (Chen et al. 1999), and could be recognized as an extremophile organism due to the ability to thrive in this habitat which for other organisms might be intolerably hostile or even lethal (Rampelotto 2013; Mendoza-Chávez et al. 2021).
The anamorphic development of P. novenarius during its naupliar, copepodid and adult instars observed in the freshwater analyzed system was typical of the cyclopoids, even with the extremely high and seasonally variable arsenic concentration in the analyzed population. Some differences were found in comparison with additional freshwater Cyclopidae species whose development is known (Dahms and Fernando 1992; Ferrari 2000); for instance, the number of added segments on each appendage or the number of setulae on each appendage segment, but this appears to be more related to the recognizable morphological differences between species, even at the earliest developmental stages (Suárez-Morales et al. 2007), than the effect of the contaminant (arsenic) on the P. novenarius morphology.
Morphological differences in other zooplankton groups (Cladocera and Rotifera) have been recorded because of metals such as Cd, Cu, or Pb (Gama-Flores et al. 2007; Pérez and Hoang 2017; Xue et al. 2017; Araujo et al. 2019; Pérez-Yañez et al. 2019). But to our knowledge, no morphological effect during the development of copepods has been recorded in the presence of metals or metalloids: the analyzed population appears not to be the exception.
Laboratory studies have shown that metals and metalloids affect copepods in a minor way in comparison with cladocerans and rotifers because these are relatively more tolerant to toxic action; this could be explained due to their ability to accumulation of heavy metals in the body (Gagneten and Paggi 2009; Caumette et al. 2011; Mendoza-Chávez et al. 2021).
On average, the abundances of adults were similar to the values reported by Gagneten and Paggi (2009) for the order Cyclopoida inhabiting water polluted by heavy metals (0.033–1.844 ind/L− 1). However, these values are low in comparison with other copepods inhabiting other aquatic systems without pollutant agents (up to 1,182 ind/L− 1) (Gerten and Adrian 2002; Mitsuka and Henry 2002; Cervantes-Martínez et al. 2005; Sarma et al. 2011; Gómez-Márquez et al. 2013; Cervantes-Martínez and Gutiérrez-Aguirre 2015), this suggests that arsenic concentration could play a key role in the abundances of P. novenarius; nevertheless, further studies are necessary to confirm this. On the other hand, individuals with egg sacs were observed along the two seasons studied, reflecting a constant development of all stages. Thus, even in these high concentrations of arsenic, P. novenarius reproduces; this could explain the presence of copepodites and nauplii in the periods surveyed.
Adult female and male lengths were within the ranges (570–880 µm for females and 540–640 µm for males) reported by Reid (1987) for this species. On average, males were larger than females, which differs from the sexual dimorphism typically found in Copepoda, where males are smaller than females (Hirst and Kiørboe 2014). Statistical results showed a significant difference in body size by season and these variations could be related to factors such as temperature and food availability (Plath and Boersma 2001; Lin et al. 2013).
A significant difference in body size by arsenic concentration was found. This differs from other zooplanktonic groups such as cladocerans, where body growth was not significantly altered by arsenic (Hoang et al. 2007). However, differences in mean length between individuals by season are minimal, compared with other studies where copepods showed a more significant difference in body size but without any pollutant agent (Cervantes-Martínez et al. 2005; Belmonte et al. 2006; Cervantes-Martínez 2021).
According to Fisher's principle, sex ratio (F:M) is expected to be 1:1 in a natural environment or skewed to the sex in which the female invested least in the embryo phase (Hirst et al. 2010). In this study sex ratio was skewed to females in both rainy and dry season with 15:1 and 21:1, respectively, which agreed with adult females outnumbering males in copepods populations (Hirst and Kiørboe 2002; Kiørboe 2006).
Water temperature and dissolved oxygen are variables inversely related (Lewis 1987; Khani and Rajaee 2017; Koralay et al. 2018); nevertheless, in this study, a direct relationship was observed, these variations might be explained more by biological effects (photosynthesis-respiration) than by physical aspects (Cervantes-Martínez and Gutiérrez-Aguirre 2015). pH values were closer to the neutrality, probably due to the limestone buffer, according to Razo et al. (2004) and Grochowska (2020). The electrical conductivity recorded in this work (3247–3407 µS/cm3) is characteristic of freshwater systems in central-north Mexico due to the dominant processes of evaporation and salt precipitation (Alcocer and Escobar 1996). According to salinity, this system is classified as oligohaline (Strydom et al. 2002). In general, values of physical and chemical variables were lower than the reported in other studies (Copaja et al. 2016; Ali et al. 2016) in water bodies polluted by metals and metalloids.
Finally, arsenic concentrations were within the values (35–155 mg/L− 1) reported by Martínez-Villegas et al. (2013) and Mendoza-Chávez et al. (2021) at this site. These values exceeded the Mexican guidelines for the conservation of aquatic life (02. mg/L− 1) and for water quality (0.05 mg/L− 1) as well as international guidelines (EPA 1994; DOF 1994, 1998).