Non-marine ostracodes from highland lakes in East-central Mexico

The ostracode fauna of the neartic-neotropical transitional zone has remained poorly known until this study. Ten ostracode species inhabit ten highland lakes (five maar lakes (phreatic/phreato-magmatic explosion origin), one volcanic-tectonic lake, three natural dams and one man-made dam) in East-central Mexico. Surface sediments from the deepest part and the littoral zone from all studied lakes were collected. Environmental variables (pH, temperature, dissolved oxygen, conductivity, TDS) were measured in situ and parallel water samples for chemical analysis were collected for habitat description. Widely distributed species in the study area (≥5 lakes) include Cypridopsis vidua, Darwinula stevensoni and Eucandona cf. patzcuaro. Limnocytherina axalapasco is an endemic species and was collected in three maar lakes and in one man-made dam. Rare species included: Chlamydotheca arcuata?, Fabaeformiscandona acuminata?, Ilyocypris gibba?, Limnocythere friabilis?, Potamocypris smaragdina? and Potamocypris unicaudata?. Highest species richness (6 spp.) was found in the large and shallow Lake Metztitlán (2.6km2, 5.5m deep), with the lake water type HCO3 >>SO4 2->Cl-Ca2+>Na+>Mg2+. The rest of studied lakes (<63m, <27km2) had not more than three species. For instance, only two ostracode species were collected in Lake Alchichica, which is the largest, deepest and most saline studied maar lake. Rev. Biol. Trop. 63 (2): 401-425. Epub 2015 June 01.

Microcrustaceans (ostracodes, cladocerans and copepods) are dominant zooplankters in aquatic ecosystems, and are key components of the food web (Dole-Olivier, Galassi, Marmonier, & Creuzé des Chattelliers, 2000;Cohen, 2003).Non-marine ostracodes (Crustacea: Ostracoda) are typically <3mm long and are commonly called "mussel-shrimps", because their soft parts are protected by two low-Mg calcite shells that form a carapace (Meisch, 2000).Non-marine ostracodes inhabit a variety of aquatic environments such as lakes, lagoons, ponds, rivers and sinkholes (Pérez et al., 2011).Ostracodes are highly sensitive to climate and environmental fluctuations, which makes them great paleobioindicators in paleolimnological studies (Cohen, 2003).Cladocerans and ostracodes are the most important groups of crustaceans in paleolimnological studies because their body segments preserve well in lacustrine sediments.Once their modern ecological requirements are known, ostracodes can be used to quantitatively reconstruct past environmental variables such as conductivity (Mischke et al., 2010), temperature, water chemical composition (Mezquita, Roca, Reed, & Wansard, 2005) and water depth (Pérez et al., 2010a(Pérez et al., , 2011)).This is only possible if species are well identified, and by establishing large local training sets (calibration data), that allows a good knowledge of the species ecological preferences (Viehberg, & Mesquita-Joanes, 2012).The performance of transfer functions based on the previous data depends as well on how reliable and consistent the species identification is.Therefore, a reliable taxonomy is a prerequisite for studies of modern and past environments (Pérez, 2010).
Central Mexico is an interesting area for ostracode studies (species richness, diversity, distribution and ecology) because it is a transitional zone between the Nearctic and the Neotropic.Besides, this region is a sensitive zone between temperate and tropical climatic regimes (Davies, Metcalfe, Caballero, & Juggins, 2002).This study reveals whether ostracode species living in these highland lakes have a Nearctic and/or Neotropical distribution and if endemic species exist.First studies on ostracodes in Central Mexico were carried out by Rio de la Loza and Craveri (1858), de Saussurre (1858), Ehrenberg (1869), in Northern Guatemala by Brehm (1932) and in the Yucatán Peninsula by Furtos (1936).The first studies of ostracodes in the Guatemalan highlands (crater lakes Ayarza, Amatitlán and Atitlán) were carried out by Pérez, Lorenschat, Brenner, Scharf, and Schwalb (2012) and Lorenschat (2009).These authors reported two unknown species of Limnocythere and Ilyocypris, respectively, which could be potential new species, and reported the lack of studies and literature regarding ostracodes from crater lakes in Mexico and Central America.
The main objective of this study was to provide a short morphological and habitat description, and a compilation of the ecological preferences and distribution for the ostracode fauna that inhabit lakes in East-central Mexico.This is a region where paleolimnological studies have been conducted, and this study will improve environmental and climatic inferences based on ostracodes.
Ostracode analysis: Ostracodes were extracted from 10mL of all sediment samples using fine brushes under an Olympus SZ stereoscope.Ostracodes with well-preserved soft parts were kept at 5°C in small eppendorf vials filled with 96% ethanol.Hard parts were stored in micropaleontological slides.For an accurate identification, well-preserved specimens were dissected following Meisch (2000).When well-preserved soft parts from adult ostracodes were not collected, we added a question mark after the species name, because identification was only based on their shell morphology and accurate identifications should include observation of both hard and soft parts.Generally it is difficult to collect well-preserved soft parts in samplings based on a limited number of surface sediment samples per lake, which is the case of calibration data sets.In this paper, we present a short description of the ostracode hard parts.Length and height were measured by taking light microscope pictures from valves and then measuring them by using the software Axio Vision Release 4.6.3.Scanning electron microscope (SEM) pictures were taken using a Jeol JSM-5600LV LCM scanning microscope of the Central Microscopy Laboratory, Institute of Physics, UNAM, Mexico.Ostracode specimens are being stored temporarily at the Department of Paleontology, Institute of Geology, UNAM, Mexico.

Highland lakes of East-Central Mexico:
Sampled highland lakes include five maar lakes, one volcanic-tectonic lake, three natural dams and one man-made dam (Table 1).The sampled altitudinal gradient ranges from 1 258 to 2 804masl.The shallowest lake is Tecocomulco (0.9m) and the deepest Alchichica (64.6m).The mean lake depth in the region is ~5.2m.Most sampled lakes display small surface areas (≤1.8km 2 ), and the largest lakes are Tecocomulco and Metztitlán with 17.7 and 29.0km 2 , respectively.Surface water temperatures ranged from 18.7 to 30.0°C, the pH from 7.7 to 10.3, the DO from 4.6 to 7.9mg/L, conductivity from 93 to 3 710mS/cm, except for Lake Alchichica that displays conductivities as much as 12 940mS/cm.The TDS concentration ranges from 0.1 to 9.0g/L.
Studied maar lakes in the Oriental basin, from 2 321-2 371masl, are small (≤1.8km 2 ), and deep (≤64.9m), and are characterized by temperatures from 18.7 to 21.9°C, with the exception of lake Tecuitlapa, which is shallow (2.5m) and displays a much higher surface water temperature (26.2°C).The pH from maar lakes fluctuated from 8.8 to 10.3 and the DO concentration ranged from 4.6 to 6.5mg/L.Electric conductivity and TDS concentration vary (756 to 12 940mS/cm and 0.5 to 9g/L, respectively).Natural dams, from 1 258 to The water chemical composition of the lakes is presented in Table 2.The dominant anions in all lakes were carbonates and bicarbonates, except for Lake Alchichica, where chloride dominated.Carbonate and bicarbonate concentrations were higher in lakes Alchichica and Tecuitlapa.Chloride concentrations were relatively high in the maar lakes La Preciosa and Quechulac.Sulfate concentrations were generally low except in Lake Alchichica.The dominant cations were magnesium in La Preciosa and Quechulac, calcium in Metztitlán, Atezca, and Zempoala and sodium-potassium in the rest of the lakes.Among all studied lakes, maar lake Alchichica displayed the highest total ionic concentration (336.25meq/L), and the highest ion concentrations, except for bicarbonate and calcium.Bicarbonate concentration was highest in the maar lake Tecuitlapa and calcium concentration was highest in the dam Metztitlán.Lake Zempoala has the lowest total ionic concentration (2.10meq/L) among studied lakes.
Freshwater ostracode fauna: A total of ten species (Table 3, Fig. 2) were collected in the studied lakes in the highlands of Eastcentral Mexico (Tables 1, 2).Ostracode classification is shown in Table 3.Seven ostracode species belong to the infraorder Cypridocopina, two species to Cytherocopina and one species to Darwinulocopina.The species assemblage of the studied lakes was composed by five cyprids, two candonids, two limnocytherids, and one darwinulid.Table 4 shows the distribution of ostracodes in East-Central Mexico.The most widely distributed species (≥5 lakes) were Cypridopsis vidua, Darwinula stevensoni and Eucandona cf.patzcuaro followed by Limnocytherina axalapasco (four lakes).Rare species (<4 lakes) included Chlamydotheca arcuata?, Fabaeformiscandona acuminata?, Ilyocypris gibba?, Limnocythere friabilis?, Potamocypris smaragdina? and Potamocypris unicaudata?.Lake Metztitlán displayed the highest species richness (n=6), while the rest of lakes, including the deep Lake Alchichica, presented less than three species.Because of the little information on the extant non-marine ostracode fauna in Mexico, especially in waterbodies of Central Mexico, a short description of the species morphology (RV: right valve; LV left valve) with shell measurements (L: length, H: height) of collected valves (Table 5), as well as their local, regional and worldwide distribution, and ecological preferences are presented below in species alphabetical order: Chlamydotheca arcuata?(Sars, 1901) (Fig. 2A, B) Identification: Furtos (1933), Smith and Delorme (2010).

Morphology:
Valves pitted and heart shaped.Ovate to subovate in dorsal view.Dorsal margin arched, and the highest carapace width in the middle.Carapaces usually with four distinct dark green transverse stripes (this study), but it can be found in shades of brown and purple (Smith, & Delorme, 2010).This species is cosmopolitan and therefore presents instraspecific variability in size, shape, surface ornamentation and color (Meisch, 2000).
Studies have shown that C. vidua is sensitive to herbicides, and it can survive for several hours frozen (Smith, & Delorme, 2010).Preferred sediment is fine to sandy (Lorenschat, 2009).

Morphology:
Valves smooth, whitish, elongated.Posterior margin rounder and broader than anterior.Dorsal margin convex, ventral margin slightly concave and almost straight.RV larger than LV and overlaps LV ventrally.Muscle scars arranged in a rosette and located in front of mid-length in adult carapaces.

Morphology:
Valves subrectangular, covered with small pits, with two transverse sulci.Dorsal margin almost straight and slightly arched over the sulci.Anterior sulcus longer than posterior.Valves display lateral nodes.Nodes in the upper part of the shell are bigger.Anterior and posterior margins of RV and LV present spinules.Front and back part of the shell rounded.Greatest height in the first half of the shell.Only valves were collected, therefore we do not compare between males and females.

Morphology:
Valves with sexual dimorphism.Surface pitted and covered with setae.Four muscle scars arranged vertically.LV overlaps RV.Marginal pore canal unbranched.Males: Anterior part of valves subrectangular.Posterior part rounder than in females.Anterior margin round, with abundant short setae.Posterior margin broad, round and covered with short setae.Ventral margin anteriorly straight, and slightly concave in the middle part of the shell.Posterior ventral margin round and widened.Females: Posterior dorsal margin slightly downward projected, dorsal margin straight.Middle part of the ventral margin slightly concave, a poor-developed flange overlaps it.Male and female can present on the middle part of their valves bumps that sometimes can turn into prominent lateral allae.
Ecological preferences: Benthic.It was collected at temperatures 19.1-20.3°C,DO 5.0-7.0mg/L,pH 7.7-9.3,conductivity 292-12 950mS/cm, and TDS 0.2-9g/L.The dominant ions in lake waters were Na + , Mg 2+ , Ca +2 , Cl -, HCO 3 2-, and SO 4 2-(Table 2).This species lives in oligotrophic lakes and tolerates high salinities.Ca and Mg seem to be the most important cations for the development of this species and DO seems to be the limiting variable for this species.It is a typical littoral species and its abundance decreases with increasing depth (Hernández et al., 2010).However, Cohuo et al. (2014) reports this species down to 64m.They found this species in alkaline waters dominated mostly by Na, Mg, Cl and HCO 3 , as well as in waters displaying temperatures 19.1-20.3°C,DO 5.0-6.5mg/L.The preferred substrate seems to be sands with low percentage of silts.

Morphology:
Valves pitted, finely reticulated.Sexual dimorphism.Greatest carapace width near the middle.Left valve overlaps right valve posteriorly.Anterior and posterior margin bearing small spines.Four muscle scars vertically arranged.Ventral margin concave.Radial pore canals simple and straight.Females: Valves small and subreniform.Greatest height in the anterior half of the valve.Anterior margin broadly rounded, posterior margin narrower than anterior.Dorsal margin slightly convex.Males: Valves subovate-subelliptical, longer, and not as high as female valves.Anterior margin round and posterior margin subrectangular and elongated.Dorsal margin almost straight.
Ecological preferences: Benthic.This species was collected at temperature 30°C, pH 8.9, DO 5.8mg/L, conductivity 505mS/ cm, and TDS 0.3g/L.The dominant ions in lake waters were Ca 2+ , Na + , Mg 2+ , HCO 3 -, SO 4 2-and Cl -(Table 5).We collected this species at a maximum water depth of 5.5m.This species inhabits freshwater lakes, however it can be found in low-energy river systems (Brouwers, 1990), and it has not been found in wetlands or springs (Smith, & Delorme, 2010).It prefers cold temperatures, and it has been found in the profundal zone of the modern Great Lakes at water depths ranging from 15 to 45m (Curry, & Yansa, 2004).However, Benson and MacDonald (1963) report that this species shows preference for shallower waters.Forester et al. (1994) uses the increase in the abundance of this species as an indicator or shore-line proximity.Curry and Yansa (2004) report that the presence of abundant specimens in relatively shallow zones of large lakes could suggest that the species is thrived in lakes that discharge meltwater or cold groundwater.Light isotopic d 18 O values (-18‰) were measured by Dennison-Budak (2010) in ostracodes valvas of L. friabilis from the Glenns Ferry Formation, indicating that this species is associated with groundwater discharge.Dennison-Budak (2010) suggests that the presence of this species indicates oligohaline environments.Machain-Castillo and Gío-Argáez ( 2004) collected this species in river mouths along the Gulf of Mexico during the rainy season suggesting that this species slightly tolerates slightly saline waters.

Morphology:
Valves smooth, covered with short, strong and backwardly directed hairs.RV shorter than LV.Pore canals found over valves, more concentrated in the ventral half.Carapace seen from the side elongated and subtriangular.RV: Dorsal margin boldly arched.The anterior margin more broadly rounded than the posterior margin.Posterior margin narrow.The greatest height in the middle.LV: Broadly rounded anterior margin and a truncate posterior margin.The posteroventral corner displays a distinct acute angle.Greatest height of LV is in front of the middle.LV encloses the RV anteriorly.Ventral margins of RV and LV slightly sinuated in the central region.
Ecological preferences: Nektobenthic.This species was collected at temperature 26.2°C, pH 10.3, DO 5.7mg/L, conductivity 3 710mS/cm, and TDS 2.4g/L.The dominant ions of lake waters were Na + , CO 3 2-, HCO 3 and Cl -(Table 2).Cohuo (2012) collected this species in waters with higher DO (8.26mg/L), lower temperature (17.7°C) and conductivity (774mS/cm).It lives in freshwater and slightly brackish habitats.It has been collected in alkaline waters (pH ≤11.1).The maximum reported water depth of this species is 2m.The preferred substrate seems to be a thin layer of mud overlaying a sandy substrate (Meisch, 2000).

DISCUSSION
Our study reveals that ten ostracode species inhabit ten highland lakes in East-Central Mexico.Species richness of non-marine ostracodes in the studied area is relatively low (≤6 spp.per lake).Similar results have been reported for other areas in Mexico and nearby regions.Caballero et al. (2013) reported six ostracode species in the crater lake Santa María del Oro in Western Mexico.Bridgwater et al. (1999a) reported nine ostracode species in modern and Holocene core samples from Lake Patzcuaro, Michoacán, Mexico.There are other paleoenvironmental studies in Central Mexico reporting the ostracode fauna.However, these reports did not study the extant fauna.Pérez et al. (2011) studied the ostracode fauna of mainly karst aquatic ecosystems from the Yucatán Peninsula and surrounding areas of Belize and Guatemala.Such karst environments are optimal for ostracode development, and therefore species abundances are expected to be high.A total of 29 species were identified and the highest species richness reported per lake was of 10 spp.(Pérez et al., 2011).Nevertheless, valve variability in the Yucatán Peninsula and in Central Mexico is high, which makes an accurate identification difficult.Therefore, such studies should carry out taxonomy-molecular analyses.Higher latitude lakes in North America display similar number of species per lake, as well.For instance, lakes in the Yukon territory, Canada display from three to eight ostracode species (Bunbury, & Gajewski, 2005).In contrast, other regions of the world are habitat a much higher number of ostracode species.The ancient Lake Ohrid, Macedonia and Albania and Lake Titicaca, Peru and Bolivia hold ca.50 ostracode species each, and approximately half of them are endemic.Lakes Baikal, Russia and Tanganyika, Africa are habitat of ca.200 species and more than the 90% of which are endemic.The higher number of species is not only attributed to the age of the lakes, but to the interaction of different environmental factors (Martens & Schön, 1999).
The studied aquatic ecosystems include maar lakes, tectonic-volcanic lakes and dams (natural and man-made).Ostracode species richness in maar lakes is low (2-3 spp.) and the ostracode species composition among studied maar lakes shows differences.For instance, the maar lakes La Preciosa, Quechulac and Alchichica share species in common (E.cf.patzcuaro and L. axalapasco) different to those from maar lakes Tecuitlapa (P.unicaudata?) and Aljojuca (C. vidua, F. acuminata?, P. smaragdina?).Lakes Tecuitlapa and Aljojuca do not have species in common, even though they are located close to one another.One reason could be that both lakes display different limnological variables as well as water chemical composition.Conductivity and sodium concentrations in the studied maar lakes were higher (≤12 940mS/cm, ≤115.50meq/L)among other measured cations, which could be a result of high evaporation in the region, and groundwater influence (Armienta et al., 2008).Chloride displayed the highest concentrations among anions (≤110.44meq/L).Crater lake Alchichica is a tropical saline oligotrophic lake (Alcocer, & Filonov, 2007), and the highest concentrations of Na + , Mg 2+ , K + , Cl -, SO 4 2-and CO 3 2were encountered in this lake.We found only two ostracode species living in lake Alchichica: Eucandona cf.patzcuaro and Limnocytherina axalapasco.Eucandona patzcuaro, was previously classified as Candona patzcuaro, but Karanovic (2012) suggests that it belongs to the genus Eucandona.Shell variability of this species in the studied lakes seems to be high, and therefore coupled taxonomy and molecular analyses should be carried out in the future to decipher if it is one or more species.
The reported salinity tolerance of this species is wide, which is uncommon in freshwater species of this genus, reason why we believe this might be another species restricted to this particular area.Future studies should focus on the revision of E. patzcuaro in Mexican lakes.
Limnocytherina axalapasco was only collected in lakes La Preciosa, Alchichica, Atlagantepec, Quechulac and Metztitlán and a recent study (Cohuo et al., 2014) suggests that this species is restricted to East-Central Mexico, and therefore is a new and endemic species for this region.Low species richness and abundances in the study area suggest that the environmental and biological conditions in the lakes are not optimal for ostracodes.However, these are only preliminary results and further studies should be carried out during different seasons to have a better overview of the ostracode fauna and adult and juvenile abundances in highland lakes from East-Central Mexico.
The natural dams Zempoala and Atezca displayed only two species, while Metztitlán displayed the highest species richness (6 spp.) among studied lakes.Lake Metztitlán, a natural dam as well, displayed the highest species richness (6 spp.), and displays a water type HCO 3 ->>SO 4 2->Cl ---Ca 2+ >Na + >Mg 2+ .Ostracode shell is mainly build of calcite and therefore Ca 2+ content in lake waters is especially important (Keyser, & Walter, 2004).The highest Ca 2+ concentration (≤4.04meq/L) among studied lake waters was determined in Lake Metztitlán, which could partly explain the higher number of ostracode species.Smith (1993) reported that ostracodes of lakes in the USA prefer waters dominated by Ca 2+ , SO 4 2-and HCO 3 -, similar to our results.Additionally, Lake Metztitlán is a large and shallow natural dam (29.0km 2 , 3m deep), with a larger macrophyte cover than in other smaller lakes.This habitat provides shelter, protection against predation and higher food availability.Chlamydotheca arcuata? was only collected in the volcanic-tectonic lake Tecocomulco.Lake Tecocomulco, a Ramsar site (Ramsar, 2013), is another large and shallow lake, that only displayed two species.The lake displays a higher altitude (2 535masl).Water temperatures in this lake are much colder (21.2°C) than in other lakes, and the lake water type is HCO 3 ->>Cl ---Na + >>Ca 2+ -Mg 2+ .Such environmental conditions seem to be unfavorable for ostracode development.The man-made dam, lake Atlangatepec, shared more species in common with the maar lakes Quechulac, La Preciosa and Alchichica than with other studied lakes.
The relatively low species richness in the studied highland lakes could be explained mainly by the lake water composition of the studied lakes, because it is largely determined by the bedrock geology and climate.Generally, ostracodes are highly sensitive to the change in the water chemical composition (Smith, 1993).The lake waters' pH, and temperature are high (8.9, 30°C), and there is sufficient DO (5.8mg/L) for an optimal ostracode development, reproduction and colonization.However, our study shows that most highland lakes in East-Central Mexico display as much as three ostracodes species per lake, except for lake Metztitlán.Factors that could influence the presence or absence of ostracodes are the following: competition among species, depredation, food availability, water pollution and human impact, among others.
The Palaearctic zone is the zoographical zone with the highest number of the extant nonmarine ostracode species (n=702), followed by the Afrotropical (n=455), Nearctic (n=298) and the Neotropical zones (n=275).One of the reasons why few species are reported for the Neotropical zone is the few number of studies that have been conducted in the region, especially in lakes in Central America.Future studies will shed light information on the actual number of ostracode species inhabiting aquatic ecosystems as well as the number of new and endemic species.Waterbodies in Mexico are located in the Nearctic-Neotropical transitional climatic zone, suggesting the presence of species of Nearctic and/or Neotropical origin, as well as endemic species.Most of the collected ostracode species display a Nearctic distribution.There are as well species with a wide geographic distribution and have been reported for other continents.Limnocythere friabilis is a Nearctic species, while E. patzcuaro is distributed in the Nearctic and Nearctic-Neotropical transitional zone.Chlamydotheca arcuata? has been reported in the Nearctics and Neotropics.Three out of ten ostracode species are widely distributed in East-Central Mexico (five lakes), as well as in other regions of Mexico, and the world.They include: E. cf.patzcuaro, C. vidua and D. stevensoni.Eucandona patzcuaro has been already reported for Mexico (Bridgwater et al., 1999b;Maddocks et al., 2009;Garduño-Monroy et al., 2011;Caballero et al., 2013).Tressler (1954) based the description of Eucandona patzcuaro on two male specimens only.Future studies should provide a description of adult females as well.We collected abundant specimens (adult and juveniles, 90 valves/mL wet sediment).Eucandona patzcuaro is restricted to the region from Central Mexico to South and Western USA.This is supported by Cohuo (2012) and Pérez et al. (2011), who do not report this species for lower latitudes (Southern Mexico, Guatemala and Belize).Lake Alchichica was the lake with the highest salinity (8.5g/L) in our study (Alcocer, & Escobar-Briones, 2007).Eucandona cf.patzcuaro and Limnocytherina axalapasco were the only two species collected in this lake, suggesting that these species can tolerate a broad range of salinity, because they can also be found in fresher waterbodies.Interestingly, adult and juvenile specimens of E. cf.patzcuaro were collected in surface sediments (45m) from the profundal zone.Similar results have been reported by Hernández et al. (2010).Future studies should focus on studying the distribution of this species in the lake (including more sites in the profundal zone), taphonomy, as well as a continuous seasonal sampling.Cypridopsis vidua and D. stevensoni are species with a wider distribution.Both species are highly tolerant and have been reported in all continents (Meisch, 2000).Cypridopsis vidua was most abundant in Lake Aljojuca that displayed lake waters with a high pH (9.5) and dominated by Na + , Mg 2+ , HCO 3 -and CO 3 2-.Darwinula stevensoni was reported in five lakes displaying different physical and chemical variables, as well as water chemical composition, confirming the high tolerance of this species reported by Meisch (2000).
A higher number of rare species (6 spp., <five lakes) inhabit the study area.Species that have been reported for other regions in Mexico include: C. arcuata?, I. gibba? and P. smaragdina?.Two unidentified species of the genus Ilyocypris were reported by Cohuo (2012) in Tlacolula, Veracruz.It is possible that the species we collected in East-Central Mexico could be the same as the ones collected by Cohuo (2012) and the species collected in crater lakes in the highlands of Guatemala by Pérez et al. (2012).Nevertheless, further sampling campaigns should be carried out to collect wellpreserved adult soft parts for their accurate identification.Meisch (2000) reported that I. gibba is abundant from May to September.Our fieldtrips were between June and October, however, only few single valves were collected and no ostracodes with well-preserved valves.First reports for Central Mexico are F. acuminata?and L. friabilis?.Limnocytherina axalapasco is an endemic species restricted to East-Central Mexico (Cohuo et al., 2014, and this study) while F. acuminata and L. friabilis are distributed in higher latitudes.Limnocytherina axalapasco has been already reported for the region but as Limnocythere itasca (Juárez, 2005), and Limnocythere inopinata (Hernández et al., 2010).Both species are very similar but analysis and comparison of their copulatory organs allows differentiating them.
Our study shows that ostracodes inhabit crater and other highland lakes in East-Central Mexico and some species are abundant and have specific environmental requirements, suggesting their potential as paleoenvironmental indicators.Additional (paleo) bioindicators (cladocerans, chironomids, diatoms, thecamoebians, among others) should be as well taken into account.Few studies have reported their presence and abundances in highland lakes in East-Central Mexico (Caballero et al., 1999;Quiroz Castelán, Díaz Vargas, Trejo Albarrán, & Elizalde Arriaga, 2000;Caballero et al., 2003;Hernández et al., 2010).Maar lakes contain high-resolution climate archives and their little human impact in some cases makes them a unique tool for reconstructing past environment and climate change in Mexico.Future studies will attempt to use ostracodes as indicators of late Quaternary climate change in East-Central Mexico.Intensive sampling and soft part as well as molecular analysis are needed to identify new and endemic species in the area.Future studies will attempt to use ostracodes as paleoenvironmental indicators in maar lakes in East-Central Mexico.

ACKNOWLEDGMENTS
We would like to thank all participants in our field trips: Edyta Zawisza (Instituto de Geofísica, UNAM), Alexander Correa-Metrio and Esperanza Torres (Instituto de Geología, UNAM).Special thanks go to María Aurora Armienta and the staff of the Departamento de Química Analítica, Instituto de Geofísica, UNAM for the water chemical analysis.We also thank the agencies and institutions that provided financial support: CONACYT (project number 167621), National Science Foundation (NSF award number 0902864) and the Instituto de Geología, UNAM.

TABLE 2
Major ion concentrations (meq/L) of surface waters collected above the lake's deepest point (LDP) and selected littoral zone (LZ) of sampled highland lakes in East-Central Mexico.Lakes listed below are arranged in horizontal order from lowest to highest altitude (1 258-2 804 masl, Table1) Ventral margin slightly convexly rounded.Greatest H in front of the middle part of the valve.Posterior margin of RV clearly serrated (ca.19 short spines).

TABLE 4
Distribution of ostracode species living in highland lakes from East-Central Mexico.Lakes listed below are arranged in vertical order from lowest to highest species richness.Species are arranged in horizontal order by the number of lakes where they are present

TABLE 5
Biometric characteristics (length and height in mm) of adult ostracodes collected in highland lakes of East-Central Mexico