Morphology and Ecological Characteristics of Paracyclops Novenarius Reid, 1987: A Cyclopoid Copepod Inhabiting a Highly Contaminated Aquifer in Central-North of Mexico

This work reports the freshwater copepod Paracyclops novenarius, in a water body with high arsenic concentrations. Morphologic analysis, abundances, body size of the copepod and physical and chemical variables of the water (including arsenic concentrations) were evaluated in two different climatic seasons (rainy and dry). Morphological analysis showed that the high arsenic concentrations do not affect the morphology of P. novenarius, including all its development and adult instars. The highest abundances of this species were found in the dry season for all development stages, with values of 1.51 and 1.50, 4.46, 0.21 ind/L -1 , for nauplii, copepodites, females, and males, respectively. However, these values are lower than other aquatic systems of the region and the world, without polluting agents. The highest arsenic concentration was found in the dry season (58 mg/L -1 ) and the lowest during the rainy (54.5 mg/L -1 ). The body size of the analyzed organisms was larger in rainy compared to dry, with an average of 637.2 ± 42 µm for females and 650 ± 37 µm for males. A Mann-Whitney U test showed signicant differences in lengths by season and by arsenic concentration (U = 1284.5, U = 1284.5; p < 0.05). The results of this study could provide information for environmental impact assessments on aquatic systems.


Introduction
Arsenic (As) is a natural element present in the environment, including aquatic systems; however, in recent years, anthropogenic activities (e.g., mining, One of the critical components in the aquatic biodiversity to assess these impacts on water systems is zooplankton, and within this is the class Copepoda H. Milne-Edwards, 1840, which contributes between 55-80% of the zooplankton biomass (Kiørboe 1998 Therefore, its study turns essential to understand better the toxic process in longer-term, such as changes in morphology and population dynamics which acquires great relevance from an ecological and environmental perspective. In this work, we performed a detailed analysis of morphology, and we evaluated some ecological characteristics of the copepod Paracyclops novenarius Reid, 1987 (initially reported as Paracyclops chiltoni Thomson, 1882 by Mendoza-Chávez et al. 2021) inhabiting water polluted by arsenic as well as physical and chemical variables of the water (including arsenic concentrations) in two different climatic seasons (rainy and dry).

Material And Methods
The study area is in the city of Matehuala, San Luis Potosi, Mexico and corresponds to a shallow water body (< 2 m depth) known as "Club de Tiro", which is To identify the species and some effect on the development of individuals due to the arsenic concentration, adult females and males collected (10 as a minimum of each collection) were analyzed with light microscopy Nikon Eclipse 50i and scanning microscopy JEOL-SM-6010. Some nauplii and copepodites were included in this analysis. Suárez-Morales et al. (2020) established the procedures for material preservation, preparation, and conservation; biological material is in the reference Collection of Zooplankton of ECOSUR at Chetumal (ECOCH-Z-10508).
The abundance of individuals (individuals/L − 1 ) was estimated based on total counts performed with a stereoscopic microscope model Olympus SZ30. Body size (µm) of adult specimens (the number depending on the availability) was obtained by measuring the distance from the head to the furcal ramus, using an optical microscope model Olympus CX21 with a graduated eyepiece (Belmonte et al. 2006). Sizes (both males and females) were compared in function to the season and arsenic concentration with a Mann-Whitney U test. The sex ratio (F:M) was estimated by the relationship between the total number of females (F) and the total number of males (M) sexed (Dur et al. 2012).

Results
Normal and stable development was observed along the different instars, different arsenic concentrations in the media, and different sampled dates between the analyzed specimens (Figs. [2][3][4][5]. All naupliar stages with the typical Labrum, A1, A2, Md, and one couple of spinulose caudal seta on each side of the body were present in Nauplius II to VI (Fig. 2a-c). Antennule armed with sabre-shaped masticatory process; the maxillule is differentiated as on setose, distal lobe, and rst leg bud is differentiated in Nauplius VI (Fig. 2a-c).
As typically, the outer lateral furcal seta (seta III) is placed more proximally during Copepodite I and lateral furcal seta (II) is placed inwards; whereas dorsal seta (VII) is placed near its nal place when the copepodite grows to instar V ( Fig. 2d-g, 3a, b). Six antennular segments, as well as the rst P1-P3, were developed during CV (Fig. 3c-f).
Ornamentation of buccal and thoracic appendages corresponds to the mentioned species, including the presence of large setules on coxal, distal margin of P1-P3, and the absence of ornamentation in this distal margin on P4 (at least not identi able with light microscopy). Furthermore, features related to sexual dimorphisms, such as the ornamentation of antennal basis, Enp3P1, and Enp3P3, also corresponds with P. novenarius.
Abundances of P. novenarius (including adult males, females and different instars) and arsenic concentrations are shown in Fig. 6. Abundances for copepodites (I-V) were slightly higher in dry (1.50 ind/L − 1 ) than rainy (1.02 ind/L − 1 ), whereas nauplii varied from 0.3 (rainy) to 1.51 ind/L − 1 (dry). Abundances for adult females varied from 1.03 (rainy) to 4.46 ind/L − 1 (dry), which were superior to adult males with 0.07 and 0.21 ind/L − 1 for rainy and dry, respectively. Although the abundances were similar in both seasons, the dry season showed the highest abundances for nauplii, copepodites (I-IV) and adults female and male, which corresponds to the highest arsenic concentration (58 mg/L − 1 ). On the other hand, the lowest abundances correspond to the lowest arsenic concentration (54.5 mg/L − 1 ) (Fig. 6).
Body size was large in rainy with 637.19 ± 42 µm for females and 650 ± 37 µm for males. In dry it was 607.3 ± 28 µm and 616.5 ± 40 µm, for females and males respectively (Fig. 7). The Mann-Whitney U test demonstrated a signi cant difference in body size by season (U = 1284.5, p < 0.05, n = 87) as well as by arsenic concentration (U = 1284.5, p < 0.05, n = 87). Adult sex ratios were skewed towards the dominance of females in both seasons (rainy = 15:1, dry = 21:1) ( Table 1). The values obtained for the physical and chemical variables and arsenic concentrations are presented in Table 2. The maximum value of dissolved oxygen was found in rainy (4.0 mg/L − 1 ), whereas the lowest value was found in dry (1.6 mg/L − 1 ). The maximum temperature was found in rainy (  This species was reported for the rst time in Colombia by Reid (1987), later by Gaviria (1994), and Gaviria (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. 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 ( , this suggests that arsenic concentration could play a key role in the abundances of P. novenarius; nevertheless, further studies are necessary to con rm this. On the other hand, individuals with egg sacs were observed along the two seasons studied, re ecting 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 . 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/cm 3 ) 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 classi ed as oligohaline (Strydom et al. 2002). and for water quality (0.05 mg/L − 1 ) as well as international guidelines (EPA 1994;DOF 1994DOF , 1998

Conclusions
In this study, we reported for the rst time in central-north Mexico the neotropical copepod P. novenarius. Additionally, this species was found inhabiting high arsenic concentrations. The morphological analysis allows us to conclude that arsenic does not affect the morphology in all development stages, and apparently, it affects some ecological aspects (body size, abundance, sex ratio). Further studies are required to know in detail more speci c effects and mechanisms of action of arsenic on the life cycle of P. novenarius. Finally, knowing the probable impact of this metalloid on ecological characteristics such as abundance, sizes, distribution, and detailed morphology of plankton in a region recognized for high arsenic concentration in its aquifers, could lay the basis for using regional fauna for health analysis of continental aquatic systems in the region.

Declarations
Ethics approval and consent to participate. We collected from several freshwater ecosystems in Mexico. However, Mexican laws do not protect Zooplankton, thus, no speci c permits for this type of eld study are needed.
Consent for publication. Not applicable.
Availability of data and materials. The authors declare that the data supporting the ndings of this study are available within the article.
Competing interests. The authors declare that they have no competing interests.
Funding. Scholarship granted for unique support (CH-22478 Scholarships Unique Support) from the Potosino Institute of Scienti c and Technological Research A.C. (IPICYT). University of Quintana Roo, Cozumel.
Authors' contribution. All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by JLUC, ACM and MAGA. The rst draft of the manuscript was written by JLUC and all authors commented on previous versions of the manuscript. All authors read and approved the nal manuscript.