The Mexican representatives of the genus Trachemys remain little investigated. Recent works consist of taxonomic reviews (Legler 1990; Seidel 2002) and a simplified dichotomous key (Köhler 2003) of the southern Mexican forms. Here we report on a significant range extension of the largest slider known from Mexico, Trachemys grayi (Bocourt, 1868), the Tehuantepec slider. Until the work of Seidel (2002), this taxon had been considered a subspecies (usually of Trachemys scripta). Preliminary evidence suggests that these newly reported populations are highly endangered and in need of protection. Indeed, the status of T. grayi throughout its range along the Pacific coastal plain as far south as western El Salvador (Smith and Smith 1979; Legler 1990; Köhler 2003) remains unknown, and the species is probably facing extirpation in many places.

Adults of T. grayi can be distinguished from all other Mexican forms of Trachemys by their lack of a broadened postorbital stripe or blotch (Fig. 1). However, in small juveniles this stripe may be somewhat widened, and this may lead to confusion of T. grayi with the allopatric species Trachemys venusta or Trachemys ornata (Alvarez del Toro 1982). Known as “tortuga negra” (black turtle) owing to the subdued coloration of adults, this turtle can reach carapace lengths over 60 cm (Smith and Smith 1979), making it one of the largest freshwater turtles in the Americas.

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The confirmed presence of this large slider on the Pacific littoral of the Isthmus of Tehuantepec dates from Gray (1873) who conferred the binomen Callichelys concinna on 2 specimens from San Mateo del Mar, Oaxaca (lat 16°12′N, long 94°59′W), of which he provided a short description. Sumichrast (1882) discussed in a lengthy footnote the differing taxonomic opinions by Cope and Bocourt regarding those specimens collected by him at Tehuantepec (= Santo Domingo Tehuantepec; lat 16°17′N, long 95°25′W), siding with Cope in the belief that these turtles represented but one of various “local races” of the widely distributed Emys ornata. Smith and Smith (1979) indicated the northwestern extreme of distribution to be the vicinity of Salina Cruz, Oaxaca (lat 16°11′29″N, long 95°19′15″W). In 1990, Legler proclaimed the strong likelihood that this species was present farther northwest in southeastern Oaxaca, specifically near Majada de Villalobos (lat 16°03′40″N, long 95°24′04″W) but provided no specimen or further information.

On 2 August 2000, the junior author and Guy Clairiot visited a coastal freshwater lagoon, Laguna Cangrejo, 7 km south of Morro Mazatán (lat 16°7′0″N, long 95°27′0″W, sometimes spelled Mazatlán), Oaxaca, located a few kilometers southeast of Majada de Villalobos in the Río Tenango basin and 25 km west of Salina Cruz. Though no living turtles were observed, the exfoliated pectoral scute, 6 cm in greatest length, of a large turtle, presumably T. grayi, was found on a protruding snag, and several shell bones were found on the shore. The rugose character of the pleural bones, lacking any trace of a keel, strongly suggests that they belonged to the genus Trachemys, and the long axis of the largest pleural bone, approximately 9 cm, would indicate that it had belonged to a turtle whose total length would have exceeded 35 cm. No chelonian in the region other than T. grayi attains such dimensions; Rhinoclemmys pulcherrima and Rhinoclemmys rubida reach no more than 23 cm carapace length (Smith and Smith 1979). The narrowed anterior plastron lobe of a large Staurotypus salvinii, a species possibly present in this part of Oaxaca, would be only half as wide as the nearly complete anterior lobe of this turtle (ca. 12 cm), and the entoplastron would be laterally compressed rather than about as broad as long. There can be no confusion with any species of Kinosternon because of their even smaller size and lack of an entoplastron. Furthermore, a pleural keel is present on the only species of Kinosternon known with certainty from the area (Kinosternon scorpioides), as well as on S. salvinii. These remains strongly suggest that T. grayi still occurs in this coastal lagoon considerably west of the Salina Cruz area (Fig. 2), confirming Legler's (1990) more western record.

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In January 2006, we examined a number of freshwater turtles that had been donated to the Centro Mexicano de la Tortuga (CMT) in Mazunte, near San Antonio Tonameca, Oaxaca. Among these were 4 specimens of T. grayi, of which a juvenile and large male had been captured in the nearby coastal lagoons of Zipolito and Escobilla, respectively; another (a large female) was found several kilometers inland in the Río Tonameca at Santa Maria Tonameca; the fourth specimen, a large juvenile, lacked data. The localities of origin are clustered within 20 km of Puerto Angel, Oaxaca (lat 15°40′0″N, long 96°28′60″W), some 150 km southwest of Salina Cruz (Fig. 2).

On 26 January 2006, the junior author examined and photographed a pugnacious adult male T. grayi (Fig. 1) kept captive at the “iguanario,” La Barra del Potrero del Río Cozoaltepec, Oaxaca, a captive-breeding center for Iguana iguana. According to the proprietor, Elpidio Marcelino López-Reyes, a veterinarian, this turtle had been captured in the Cozoaltepec River in July 2004 and donated to the iguanario. The parents of López-Reyes averred that such large turtles were common in the Río Cozoaltepec until the 1960s, whereupon they all but disappeared, along with the American crocodile Crocodilus acutus. To date, the Río Cozoaltepec represents the westernmost locality known to be inhabited by this tropical slider, which is eagerly sought for its flesh (Alvarez del Toro 1982).

These new records demonstrate that the actual distribution of T. grayi embraces both the North American as well as the Central American zoogeographic region. Furthermore, the presence of this turtle in southernmost Oaxaca sheds some light on a biogeographical puzzle concerning the distribution of sliders along Mexico's southern Pacific coast. An isolated, possibly introduced population of T. ornata, or of a turtle very closely resembling that species, is known with certainty from the vicinity of Acapulco, Guerrero (Smith and Smith 1979; Legler 1990), more than 400 km northwest of Puerto Angel and more than 600 km west of Salina Cruz. Additionally, according to Smith and Smith (1979), a specimen identified as T. ornata exists in the collection of the Instituto Politecnico Nacional, Escuela de Ciencias Biolo ógicas (IPN), Mexico DF, bearing the locality “Chacahua, Oaxaca.” Relying on the official standard names approved by the US Board of Geographic Names (1956), Smith and Smith plotted this locality in the coastal portion of western Oaxaca state, in the Río Grande watershed where a series of wetlands now enjoy protection as the Parque Nacional Laguna de Chacahua (locality 1, Fig. 1) This isolated record remains the only published unequivocal Trachemys locality in the vast area between Acapulco and Salina Cruz.

However, there exists another Chacahua (lat 15°41′28″N, long 96°37′5″W) in Oaxaca state, located midway between San Antonio Tonameca and the Río Cozoaltepec. Pending actual examination of the IPN specimen, it is very tempting to conclude that this specimen is a T. grayi originating from this second locality, which also supports coastal wetlands. On 24 January 2006, the authors visited this locality with volunteers from the CMT, and although the extensive freshwater marsh there provided suitable habitat for large sliders, the only chelonians seen were 2 Kinosternon oaxacae, one living and one road-killed.

That such a large turtle as T. grayi, sought for its flesh and possibly sold until recently in local or distant markets, could long elude discovery by biologists is partially explained by the lack of a good, paved coastal highway along the Oaxaca coast until the 1960s. Numerous freshwater lagoons and estuaries exist in coastal Oaxaca, which may yet harbor this turtle. At the moment, some of these uninvestigated freshwater lagoons benefit from designation as nature reserves, including the well-visited ecotourism site at Ventanilla, which boasts a reproducing population of rather relaxed Crocodilus acutus. Surveys of the remaining coastal wetlands in Oaxaca, including the larger rivers both to the east and west of Puerto Angel, are highly desired so that measures can be taken to protect the remnant populations of this huge turtle.