The variable platyfish (Xiphophorus variatus) is endemic to theGulf of Mexico drainages of southern Tamaulipas to northern Veracruz,Mexico (Miller et al., 2005). The species is popular in the aquariumtrade and has a history of introduction globally. Occurrences of thespecies are known from South America (Magalhaes and Jacobi, 2012;Jimenez, 2005), North America, Asia, Africa (Global BiodiversityInformation Facility, http://data.gbif.org), and Puerto Rico (Fishbase,http://fishbase.org). In the United States it is reported to haveoccurred, at least for some duration, in Montana (Holton, 1990),California (Swift et al., 1993; Moyle, 2002), Colorado (Zuckerman andBehnke, 1986), Florida (Burgess et al., 1977), Hawaii (Eldredge, 2000),and Arizona (Minckley and Deacon, 1968).

Here we provide the first report of establishment of this species in Texas, where it has persisted for at least a decade in Waller Creek in the city of Austin (Travis County). Our identification of X. variatus is based on examination of 595 preserved specimens (284 males and 311 females), all deposited in the Texas Natural History Collections of the University of Texas at Austin (TNHC 30280, 30287, 30297, 30577, 33240, 41420, 41429, 41433, 41436, 41438, 41443, 41666, 47535, 47537, 49848, 49853, 52321, 52330, 55720, 55721; and frozen tissue samples). The structure of the gonopodium of 10 mature males examined matches the description by Rosen (1960), and most specimens display diagnostic characters mentioned by Miller et al. (2005). This population exhibits several tail spot patterns typical of the species that have been linked to temperature tolerance (Culumber et al., 2014), including "wild-type," "crescent," "cut crescent," and "upper cut" as described by Rosen (1960) and Borowsky (1984). However, hybrids are common in this genus (Cui et al., 2013), and Balon (2004) notes that the aquarium trade is dominated by hybrids of X. variatus, X. hellerii, and X. maculatus. Since it is likely that this population was established from aquarium stock, we suspect this population to include genes from X. maculatus and X. hellerii.

Waller Creek (main stem length = 11 km, watershed area = 10.7[km.sup.2]) is an entirely urban stream flowing from north to souththrough the city of Austin, passing through the University of Texascampus and downtown to its mouth at Lady Bird Lake, an impoundment ofthe Colorado River (Fig. 1). Waller Creek is typical of Central Texasstreams in being susceptible to times of very low flow and suddenflooding. At the most downstream gage, 3.6 km above its mouth, flowsfrom 1956--1980 averaged 0.10 [m.sup.3]/s, annual minima averaged 0.01[m.sup.3]/s, and annual maxima averaged 46.79 [m.sup.3]/s (United StatesGeological Survey, http://nwis.waterdata.usgs.gov/tx/ nwis).



Xiphophorus variatus was first collected in Waller Creek on 6 August 2004, 3.7 km above Lady Bird Lake and likely occurred there before this date, though we found no record of collections in the literature nor museum specimen databases (Hendrickson and Cohen, 2012; Fishes of Texas Database, http://www.fishesoftexas.org) that confirm this. The last survey of the creek, finding its absence, was conducted by Edwards (1976) in 1976, nearly three decades prior to ours, thus making time of introduction difficult to estimate precisely. Our recent (2004-2014) 25 collections, made at 15 sites throughout https://www.youtube.com/watch?v=XmtUUwAY1p0 the watershed using 3-m-wide, 0.48-cm delta-mesh seines, found the species in 20 of those collecting events at 10 sites, ranging from approximately 450 m-9.3 km above the mouth. Those collections found the species apparently absent in Hemphill Branch (a tributary entering from the west ~3.7 km above Lady Bird Lake) and the lowermost 450 m of Waller Creek, but in some reaches, including small impounded areas approximately 2.3 km above the mouth (Waterloo Park), it was the dominant fish species.

Other nonnative fishes have been documented from Waller Creek. Edwards' (1976) extensive survey of Waller Creek reported Gambusia geiseri, a species endemic to San Marcos and Comal Springs in Central Texas, as the only fish species known to be directly introduced by humans. We did not record Gambusia geiseri in our surveys, but did find other wide-ranging nonnatives previously reported by Edwards (1976) from Waller Creek: Herichthys cyanoguttatus, Astyanax mexicanus, Lepomis auritus, and Carassius auratus. Given the 10 or more years that X. variatus has persisted in Waller Creek, we anticipated that it might have also established in other area streams. The Fishes of Texas database (Hendrickson and Cohen, 2012) revealed that, aside from a thorough survey of nearby Barton Creek between 2008 and 2009 (Labay et al., 2011), which found no evidence of X. variatus, other watersheds adjacent to Waller Creek had been poorly sampled in recent years. We therefore collected at one site along the north shore of Lady Bird Lake approximately 1.9 km east of the confluence of Waller Creek and at nine sites within the adjacent Shoal Creek drainage, which parallels Waller Creek and enters Lady Bird Lake 1 km west of Waller Creek. Shoal Creek is physically similar to Waller Creek and runs through a similar geology and human landscape. Using the same gear we used in Waller Creek and sampling vegetated habitats like those the species appears to prefer in Waller Creek, we failed to find X. variatus, indicating that the species is likely restricted to Waller Creek at this time.

We are unsure why X. variatus appears, at this time, to be restricted to Waller Creek. However, Tatarenkov et al. (2010) measured very short movements in X. hellerii in Belize, almost always

[FIGURE 1 OMITTED]

Waller Creek is somewhat different from neighboring Shoal Creek in its hydrology, having far more underground storm water inputs (Fig. 1), which could increase wintertime creek temperatures. For example, on 7 December 2013, we observed 14[degrees]C water coming out of a storm water pipe into 7[degrees]C creek water, 1.8 km above the mouth of Waller Creek, and found X. variatus and several other native species in the discharged water, apparently avoiding the colder creek water. Waller Creek also has undocumented discharges from leaky city water pipes (Scoggins, 2002), numerous permitted discharges, and naturally occurring springs, all of which could affect stream temperatures at various temporal and spatial scales. Relatively high thermal buffering, from all of these sources, may explain the species' persistence in Waller Creek.

Many factors could limit success and spread of nascent introduced populations of nonnatives. However, intolerance to cold is commonly cited as a limiting factor for nonnative aquarium fishes in temperate regions, including X. maculatus in Brazil (Prodocimo and Freire, 2001), Pygocentrus natteri in the United States (Bennett et al., 1997), Oreochromis in the United States (Green et al., 2012), and various tropical species introduced and persisting only in warm springs in Canada (Crossman, 1984). Limitation by Austin's relative cold would not be surprising for X. variatus given that near the northern edge of its native range (Miller et al., 2005) at Ciudad Victoria, Tamaulipas, Mexico, winter air temperatures average 17[degrees]C with average daily lows of 11[degrees]C. Average winter air temperature in Austin is 11[degrees]C and average daily lows are 5[degrees]C (Weatherbase, http://www.weatherbase. com). However, other species native to northern Mexico and extreme South Texas, not far from the native range of X. variatus, are established in Central Texas, including Astyanax mexicanus, Herichthys cyanoguttatus, Poecilia latipinna, and Xiphophorus hellerii (Hubbs et al., 2008; Hendrickson and Cohen, 2012).

Edwards (1977) noted that Astyanax mexicanus in Waller Creek moved during winter towards the mouth and into Lady Bird Lake where thermal buffering provides some protection from cold winters, and returned in spring to upstream areas. Xiphophorus variatus does not appear to display this behavior. It is clearly overwintering in upstream reaches of the creek; we collected it in winter on more than one occasion and in high densities as far as 9.2 km above the mouth at sites that are also above small dams that would prevent upstream migration at most flows.

We found little reliable thermal tolerance information for X. variatus. Borowsky reports them occurring in water temperatures of 17[degrees]C (Borowsky, 1984) and 18-24[degrees]C (Borowsky, 1990) in winter and spring at a spring-dominated site in the species' native range, but presumably the species can tolerate lower temperatures. Dawes (2001) puts the species' lower limit at 16[degrees]C, and Hoedman (1974) reports it tolerates temperatures as low as 15[degrees]C.

From 2004-2012, after the first observation of X. variatus, 53 winter (December-February) spot water temperature recordings by City of Austin's Watershed Protection Department at nine sites on Waller Creek were observed at various times of the day (City of Austin, http://www.ci.austin.tx.us/wrequery/query_form.cfm). Those sporadic water temperatures ranged from 7.7-19.3[degrees]C, averaging 13.3[degrees]C. Air temperature, which is recorded frequently and regularly at nearby Camp Mabry (National Oceanic and Atmospheric Administration, http://www.ncdc.noaa.gov/cdo-web/search), 2.8 km southwest of the uppermost collection of Xiphophorus in Waller Creek, averaged 5.88C during the same period. The winter of 2010 (December 2010-February 2011) was one of the coldest on record (average 5.4[degrees]C) and included a 13-day period averaging-3.2[degrees]C with a single day low of-8.3[degrees]C. Unfortunately, water temperatures from Waller Creek are not available for this period of extreme cold, and there are not enough water temperature data available to produce https://twitter.com/AirstoneVeneer an estimate of water temperatures for that unusually cold period by correlating temporally overlapping air and water temperatures.

Our series of in situ observations of the species in 2013, 9.2 km above the mouth of Waller Creek, were associated with that winter's first cold front bringing below-freezing air temperatures. On 26 November (1230h) we observed one individual swimming slowly and tightly associated with the substrate in 12[degrees]C water. On 28 November (1030h) we observed none swimming, but collected 10 specimens from leaf litter where the temperature was 10[degrees]C. When placed in a container with stream water, all exhibited little to no swimming and four had lost equilibrium, orienting on their side or belly up. After that observation the weather warmed and on 2 December (1630h) we saw individuals swimming in 20[degrees]C water. On 7 December (0945h), after another cold front, we collected specimens (n = 9) from 7[degrees]C water in leaf litter, which, when placed in a container in the same water, all demonstrated at least some degree of loss of equilibrium (LOE). On 11 December (0930h) we again found and collected specimens (n = 5) from 7[degrees]C water in leaf litter, that, when placed in a container of 7[degrees]C stream water, displayed remarkably improved equilibrium compared to those observed 4 days prior. The population had apparently survived a period of at least 4 days at or near 78C, and their improved equilibrium suggests that they acclimated to the lower temperature. Attempts to locate dead individuals during these observations found only one, covered in white fungus, but also one live with white fungus covering its posterior half. We found both of these in the first observation (26 November), suggesting perhaps some mortality and morbidity associated with the initial onset of cold.

This population likely includes genes from X. maculatus and/or X. hellerii that may enhance cold tolerance. Xiphophorus hellerii occurs in the wild at higher altitudes than X. variatus (Miller et al., 2005) and may have greater cold tolerance. Evolution of increased cold tolerance could be a possible explanation for persistence of X. variatus in Waller Creek. Heritability of cold tolerance has been shown in Oreochromis niloticus (Charo-Karisa et al., 2005), and evolutionary change of cold tolerance as fast as 2.5[degrees]C in three generations has been demonstrated in natural populations of Gasterosteus aculeatus (Barrett et al., 2010). Additionally, Culumber et al. (2014) showed genetics to be tightly linked to temperature tolerances of X. variatus and two other Xiphophorus species and their hybrids (Culumber et al., 2012).

Given that our observations of cold tolerance in this population were lower than published observations and expectations based on low air temperatures from their native range, we tested for atypical cold tolerance in this population, which could indicate evolution of cold tolerance. We performed critical thermal minimum tolerance tests on 10 X. variatus individuals displaying "wild-type" tail spotting (i.e., lacking melanophores on the caudal peduncle), collected from Waller Creek in summer 2012, and on 7 specimens from a local pet store also displaying "wild-type" morphology. All individuals were maintained at 26.7[degrees]C for 70 days prior to being placed in a testing chamber. Cold water from a 28-L Styrofoam ice-water reservoir was pumped into the test tank, a 22-L-capacity Styrofoam cooler containing 15 L of dechlorinated water in which temperature was controlled by a temperature controller, semiconductor, and thermocouple. The system was programmed to pump chilled reservoir water into the test tank to cool it at a rate of 0.3[degrees]C/min. An air stone mixed and aerated the water to prevent stratification. Pumping rate varied depending on the differential temperature between the test tank and the reservoir, and increased as that difference decreased. Temperature was lowered until the subject demonstrated clear LOE, designated as the point at which a fish was unable to control locomotion, even if gently prodded with a rod. This was characterized by either an inability to maintain a vertical, upright position (i.e., floating on its side) or an inability to maintain position against the weak current produced by an air stone (Turner, 1984; Beitinger et al., 2000).

Mean temperature at LOE for Waller Creek specimens was 11.1[degrees]C (SE = 0.27), which was not significantly different from the mean LOE of pet store specimens (11.3[degrees]C; SE = 0.40; two-tailed i-test, P = 0.61, df = 15). The similar laboratory cold tolerances results for both Waller Creek and aquarium populations suggest that the Waller Creek population has not evolved greater cold tolerance. However, this comparison is complicated by unknown hybridization history with aquarium stocks for pet store and likely Waller Creek populations.

Results are consistent with our in situ observation of the Waller Creek population experiencing LOE at 10[degrees]C. That observation was upon the onset of the first significant cold front of the year, but shortly after we observed individuals tolerating temperatures as low as 7[degrees]C, suggesting that cold tolerance is affected by preceding acclimation temperatures. Culumber et al. (2012) found significant seasonal variation in temperature tolerances in other Xiphophorus species and their hybrids, indicating that had we tested with individuals acclimated to cold perhaps we would have observed even greater cold tolerances.

We then performed additional cold tolerance tests, using the same methods, on nine individuals displaying "wild-type" tail spotting collected 10 March from the same location on Waller Creek (i.e., winter-acclimated). Initial starting temperature this time was 20[degrees]C. Mean LOE was at 7.0[degrees]C (SE = 0.18), which is significantly lower (4.1[degrees]C difference) than what we observed in the laboratory using summer-acclimated individuals (two-tailed t-test, P < 0.001, df = 17). This result indicates that for this species cold tolerance is strongly affected by prior acclimation, a finding consistent among many fish species (Beitinger et al., 2000), and we thus have no reason to doubt that the species may be able to tolerate temperatures even lower if acclimated. We strongly suspect this population has experienced temperatures lower than the 7.0[degrees]C that we observed in the field and laboratory. As Culumber et al. (2012) found in other Xiphophorus species, cold tolerance can change seasonally depending on acclimation, and thus sudden cold spells could be lethal.



Little is known about ecological impacts related to introduced populations of this species. However, in some areas of Waller Creek, it has become the dominant fish species, suggesting that it may be strongly impacting the community. It has replaced native fishes near Monterrey, Nuevo Leon, in Mexico (S. Contreras, in litt., 1976), but in Hawaii may have negligible ecological impact compared to X. maculatus and especially X. hellerii (Maciolek, 1984), which are known to adversely impact native Hawaiian odonates (Englund, 1999), Australian native fishes (Warburton and Madden, 2003; Morgan et al., 2004), and a Mexican native fish (Ruiz-Campos et al., 2002). Introduced poeciliids in general have broad impacts on native ecosystems (Courtenay and Meffe, 1989; Howe et al., 1997; Komak and Crossland, 2000), and our observations lead us to conclude that at least X. variatus is likely to persist in Waller Creek and could eventually spread to other connected habitats. The fact that this species has apparently been restricted to Waller Creek for a decade does not preclude future expansion. The phenomenon of long time lags between establishment and invasion is well documented among many nonnative taxa (Kowarik, 1995; Crooks and Soule, 1999; Crooks, 2005).We therefore recommend consistent, comprehensive stream fish community monitoring throughout Central Texas.

We thank City of Austin employees T. Jackson and R. Click for providing water temperature data and information about Waller Creek's gages. J. Rosales (Rains), T. LaDuc, and A. Best are responsible for the first observation and collection of the species in Waller Creek. Others who assisted in collections include E. Theriot, C. Esposito, T. Rosen, J. Diersing, J. Mile, J. Rivera, A. Navyn, C. Bell, and students of the University of Texas' Natural History Museum Science class. E. Mclean assisted with cold tolerance tests. This work was conducted under D.A.H.'s Scientific Research Permit SPR-0391-361 (Texas Parks and Wildlife Department) and Institutional Animal Care and Use Committee protocols AUP-2010-00102 and AUP-2010-00200.

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Submitted 28 January 2014.

Acceptance recommended by Associate Editor, Mark Pyron, 2 June 2014.

Adam E. Cohen, * Laura E. Dugan, Dean A. Hendrickson, F. Douglas Martin, Jonathan Huynh, Ben J. Labay, and Melissa J. Casarez

University of Texas at Austin, Department of Integrative Biology, Biodiversity Collections, Texas Natural History Collection, J.J. Pickle

Research Campus, 10100 Burnet Road, PRC176/R4000, Austin, TX 78758-4445

* Correspondent: acohen@austin.utexas.edu



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