INTRODUCTION
Little information exists on the biology, ecology and geographical distribution of the Gorgona guitarfish, Rhinobatos prahli Acero & Franke, Reference Acero and Franke1995. It has been recorded from few localities from north-western Costa Rica to northern Peru, but scarce data makes it difficult to establish whether its occurrence is continuous across this range. The poor information available on this species led to an assessment of ‘Data Deficient’ by the International Union for Conservation of Nature (IUCN, 2011), thus it is necessary to better document its distribution and abundance (Kyne, Reference Kyne2007). In this context the present report expands the distribution of the Gorgona guitarfish northwards to the Gulf of Tehuantepec in southern Mexico (Figure 1).
Fig. 1. Capture site of Rhinobatos prahli in the Gulf of Tehuantepec; grey area represents the former known distribution.
The Gorgona guitarfish belongs to the group of batoid elasmobranchs called ‘guitarras’ by Mexican fishermen. It is characterized by a brown upper surface, including both dorsal fins and the caudal fin, and with more-or-less regularly dispersed small white spots (sometimes ringed with dark-brown); a pale area on each side of the rostral cartilages; a yellow border on the lower half of the caudal fin; a pale underside (ventrum); underside of the snout with black blotches at its tip and sides; with edges of disc in light-brown; a tail with approximately 20 light-brown blotches (Figure 2); approximately 70–80 small denticles from the nape to the origin of the first dorsal fin; several small denticles on each shoulder (Acero & Franke, Reference Acero and Franke1995; Compagno, Reference Compagno and Hamlett1999; Robertson & Allen, Reference Robertson and Allen2008).
Fig. 2. Dorsal view of the female Gorgona guitarfish Rhinobatos prahli from Chipehua Bay, Gulf of Tehuantepec, Mexico.
MATERIALS AND METHODS
During September 2005, two individuals of Rhinobatos prahli were observed (one photographed) at the Chipehua fishing camp (16°02′3″N 95°22′6″W), in the western Gulf of Tehuantepec where there is an area characterized by a wide continental shelf. Chipehua Bay has a range along the coast of approximately 8 km; its coast is rocky and steep, while the bottom offshore is rocky and sandy, with depths varying from 4 to 36 m (Gentier, Reference Gentier1982). Specimens were captured by monofilament gill-nets deployed in the small-scale fishery operating from this camp. Other, more abundant guitarfish such as R. glaucostigma were also landed in the fishery. The individuals were identified using keys by Jiménez-Prado & Béarez (Reference Jiménez-Prado and Béarez2004) and Robertson & Allen (Reference Robertson and Allen2008). Total length was measured and sex was determined for both, but state of maturity could only be determined for the male. The two individuals were not preserved.
RESULTS
The two individuals differed from other species of Rhinobatos recorded from the area by presenting a brown colouring dorsally, with approximately 100 white spots (many ringed in dark-brown) more-or-less regularly dispersed over its surface.
An annotated list of published records of this species in the eastern Pacific is shown in Table 1. The female reported here measured 77 cm total length (TL), but it was not possible to determine maturity. The male measured 71 cm TL, the clasper measured 10 cm long and presented adult characteristics such as rotation, open rhipidion and calcification. Unfortunately, preservation and disposition in a scientific collection of both individuals was not possible, because the fishermen sold their entire catch, and only the female was photographed (Figure 2).
Table 1. Annotated summary of Gorgona guitarfish records from the eastern Pacific.
*, holotype; **, observations of living individuals in their natural habitat.
DISCUSSION
The individuals reported here were within the size range reported for the species from the coast of Colombia in the eastern Pacific (50–90 cm TL) (Payan et al., Reference Payan, Mejía-Falla, Navia and Lozano2010), which were estimated from underwater photographs of specimens in natural habitat, and elsewhere in the tropical eastern Pacific (Table 1). Fishermen in the Gulf of Tehuantepec group all species of guitarfish common in the area (e.g. Rhinobatos leucorhynchus and R. glaucostigma) which represented 48% of all rays captured. These observations are obtained from the sampling of sharks and rays between September 2004 and August 2006, in which the specimens of Gorgona guitarfish described here were observed, and the fishermen said that this species is uncommon in the area, so it is possible that the frequency of occurrence of R. prahli may be low, with only occasional captures.
The fishing area investigated in the Gulf of Tehuantepec shows characteristics similar to those reported by Payan et al. (Reference Payan, Mejía-Falla, Navia and Lozano2010), whose photographs showed living individuals of R. prahli in sandy and rocky environments near coral reefs at 18–24 m depth off the Colombian coast. In contrast, the holotype described from Gorgona (Island) National Natural Park, Colombia, was caught in deeper water at 70 m depth (Acero & Franke, Reference Acero and Franke1995).
Heretofore, R. prahli had not been recorded in Mexican waters (Castro-Aguirre & Espinoza-Pérez, Reference Castro-Aguirre and Espinosa-Pérez1996) and the two individuals reported here (with voucher as the photograph of the female, Figure 1) represent the first as well as the northern-most record.
ACKNOWLEDGEMENTS
We thank the fishermen of Chipehua Bay for allowing us to take photographs of the guitarfish. We thank Secretaria de Agricultura, Ganadería, Desarrollo Rural, Pesca y Alimentación (SAGARPA); and Consejo Nacional de Ciencia y Tecnología (CONACYT) for their support in the project 2003-CO1-101-/A-1 ‘Caracterización de la pesquería artesanal de los tiburones desembarcados en Salina Cruz, Oaxaca, México’. M.C.F. and O.E.S. thank CONACYT and the Instituto Politécnico Nacional (IPN) for financial support. F.G.M. thanks Estímulo al Desempeño de los Investigadores (EDI) and Comisión de Operación y Fomento de Actividades Académicas (COFFA) del IPN for fellowships.
