First verified record of Mastigodryas melanolomus (Cope, 1868) (Serpentes, Colubridae) from Isla del Coral, Nayarit, Mexico

. The occurrence of the Salmon‑bellied Racer, Mastigodryas melanolomus (Cope, 1868), on the Isla del Coral in the Pacific coast of Mexico is confirmed through the examination of squamation and the use of other morphological characters. Additionally, we conducted a morphological analysis to assess the differ‑ ences between M. melanolomus and Masticophis lineatus (Bocourt, 1890), which was previously reported on the island. The results of our analysis suggest a possible misidentification of the previous record. Further investigations and explorations will yield additional insights about whether both species occur on Isla del Coral.


INTRODUCTION
The herpetological exploration of the islands in western Mexico has significantly increased in the last two decades, leading to the discovery of several new records of frogs (De la Torre et al. 2010;Ramírez-Reyes et al. 2015;Nolasco-Luna et al. 2017), lizards (Ramírez-Reyes et al. 2015), and snakes (Nolasco-Luna et al. 2017, 2019, 2022).Advancements in molecular tools have played a crucial role in supporting taxonomic descriptions.As a result, three new species of leaf-toed geckos have been identified: Phyllodactylus cleofasensis Ramírez-Reyes, Barraza-Soltero, Nolasco-Luna, Flores-Villela & Escobedo-Galván, 2021 from Isla María Cleofas, P. isabelae (Castro-Franco & Uribe-Peña, 1992) from Islas Marietas, and P. lupitae (Castro-Franco & Uribe-Peña, 1992) from Isla del Coral.The latter island is the smallest in the state of Nayarit, Mexico, with a surface area of 0.12 km 2 and approximately 1.80 km of distance from the mainland (Figure 1).
Here, we report the first record of Salmon-bellied Racer, Mastigodryas melanolomus (Cope, 1868), from Isla del Coral.
Incidental surveys were conducted from 1200 to 1600 h.We observed and captured an individual of Matigodryas melanolomus, which was deposited in the herpetological collection of the Zoological Museum of the Facultad de Estudios Superiores Zaragoza, Universidad Nacional Autónoma de México (MZFZ), and a photograph of this specimen in situ was deposited in the Colección Nacional de Anfibios & Reptiles at Universidad Nacional Autónoma de México (IBH-RF).We identified the M. melanolomus specimen using the identification key of Zweifel (1960).Squamation details were obtained following the methodology described by Pérez-Higareda et al. (2007).We also sought consultation from two Mexican herpetologists regarding the snake's identity.However, discrepancies between Masticophis lineatus (previously recorded on the island) and Mastigodryas melanolomus were identified.Specifically, Rubén Castro-Franco (2022 pers.comm.)identified our specimen as Masticophis lineatus, while Marco A. López-Luna (2022 pers.comm.)identified it as Mastigodryas melanolomus.To establish a definitive identification of the snake from Isla del Coral, we performed a morphological analysis.Three meristic characters were measured: snout-vent length (SVL), from the tip of the snout to the anterior margin of the cloaca; head length (HL), from the tip of the snout to the anterior margin of the ear; and head width (HW), measured at the widest point of head.Morphological data of M. melanolomus (n = 5) and Masticophis lineatus (n = 13) were obtained from specimens deposited in the Smithsonian Institution, National Museum of Natural History (USNM; Washington, DC) and the American Museum of Natural History (AMNH; New York, New York).We conducted a similarity analysis using one-way ANOSIM (Clarke 1993) with untransformed data to determine differences between M. melanolomus and M. lineatus, where an R value of 1 indicates differences between groups, while an R value of 0 suggests little or no segregation into groups (Clarke and Warwick 2001).These data were visualized using non-parametric multidimensional scaling (NMDS) along two dimensions, employing Gower distance.The efficiency of NMDS was evaluated based on the stress value scale proposed by Fasham (1977).All analyses were performed using PAST v. 3.15 (Hammer et al. 2001).
The specimen was found on the ground, within a microhabitat characterized by rocks and leaf litter (Figure 2A).The snake was active at 13:30 h.Its SVL is 74 cm, the tail length is 35 cm, and the relative tail length (tail length/SVL) is 0.47.
Identification.Scutellation characteristics of the specimen (Figure 2B, C) were: 177 ventrals, 122 subcaudals, 15 rows, 9-9 supralabials, 8-8 infralabials, 1 preocular, 1 postocular, and 4th, 5th and 6th infralabial in contact with the orbit (Table 1).The specimen shows a particular characteristic in the fourth supralabial scale on both sides, on the right side the scale is divided in two horizontally, and on the left side it is incomplete (it does not touch the lip; Figure 3).Overall, the scutellation of the individual from Isla del Coral falls within the ranges reported for M. melanolomus.Furthermore, the one-way ANOSIM test confirmed a clear distinction between Mastigodryas melanolomus and Masticophis lineatus (R-value = 0.96, P < 0.01).The NMDS ordination also revealed an accurate separation between both species, revealing that the specimen from Isla del Coral falls within the morpho-space of M. melanolomus (Figure 4).

DISCUSSION
Based on our results and review, this is the first verified record of the Mastigodryas melanolomus on Isla del Coral, Nayarit, México (Rodríguez Malagón et al. 2012a, 2012b).Castro-Franco and Gaviño (1990) recorded Masticophis lineatus from Isla del Coral, which created uncertainty regarding the occurrence of and distribution overlap with Mastigodryas melanolomus.However, the morphological analysis has proved to be a valuable tool in distinguishing between species that exhibit similar morphological characteristics (e.g.Pazos-Nava et al. 2019).The verified occurrence of M. melanolomus on Isla del Coral raises ecological questions about the coexistence of both species on the island.To our knowledge, M. melanolomus and M. lineatus are sympatric in some islands, such as Isla María Madre and Isla María Magdalena, which are the two largest islands in the Islas Marías Archipelago Biosphere Reserve.Surprisingly, only one of these species has been recorded on islands with a surface area smaller than 25.1 km 2 .For instance, Mastigodryas melanolomus has been recorded on Isla María Cleofas with a surface area of 25 km 2 (Nolasco-Luna et al. 2022), whereas Masticophis lineatus was reported on Isla San Juanito with 8 km 2 (Zweifel 1960) and Islas Marietas with a surface area less than 1 km 2 (Casas-Andreu 1992).The above information suggests that island size may influence the occurrence of both species.Based on the theoretical models of niche and island biogeography, the overlapping distribution of both species in insular ecosystems could be related to increased habitat diversity on larger islands (Hortal et al. 2009).Nevertheless, this hypothesis requires further investigation in future studies.
On the other hand, there may be a misconception about the previous record of Masticophis lineatus on Isla del Coral.Unfortunately, we were unable to confirm the taxonomic identity of the specimen based on the description provided by Castro-Franco and Gaviño (1990).These authors mentioned that some reptiles were deposited in two Mexican scientific collections, but we have not found M. lineatus specimens from Isla del Coral in the database of the Colección Nacional de Anfibios y Reptiles at Universidad Nacional Autónoma de México (https://datosabiertos.unam.mx/biodiversidad/),and the other collection (Universidad Autónoma del Estado de Morelos) does not have a digital database to ascertain specimen information.In the light of our doubts, we politely wrote to Dr. Ruben Castro Franco, asking him to send us photos of the specimens reported in his paper (Castro-Franco and Gaviño 1990), with the aim to reduce any misinterpretations in the identification and occurrence of Masticophis lineatus in Isla del Coral.Unfortunately, to date, we have not seen the specimen reported by Castro-Franco and Gaviño (1990).One of the anonymous reviewers mentioned that the main difference between the genera Masticophis and Mastigodryas is the number of posterior scale rows.The above information suggests that it is important that Castro-Franco and Gaviño's specimen be found and its identity verified in order to clarify the occurrence of both species on the island.It is possible that the specimen reported here might have simply dispersed from the mainland because the island is less than 2 km off the coast.For example, Deso et al. (2021) observed Malpolon monspessulanus (Hermann, 1804) swimming several hundred meters offshore of Port-Man Bay close to some islands, which are less than 3 km from the coast.
Despite its small size, Isla del Coral, it exhibits a higher diversity compared to Isla Isabel (0.98 km 2 , isolated by 28 km of water, seven reptiles; Rodríguez Malagón et al. 2012b), Islas Marietas (0.70 km 2 , isolated by 7.9 km of water, seven reptiles; Rodríguez Malagón et al. 2012a), and Isla San Juanito (8.0 km 2 , isloted by 152 km of water, six species; Casas-Andreu 1992).This suggests that the herpetofauna of some islands, such as Isla del Coral, could still be poorly sampled; therefore, it is necessary to increase research efforts on the islands to understand the alpha diversity of insular ecosystems in western Mexico.

Figure 1 .
Figure 1.Map showing the new record of Mastigodryas melanolomus from the Isla del Coral, Nayarit, Mexico.Black circles on mainland showed the previous records of the study species (GBIF Secretariat 2024).

Figure 2 .
Figure 2. A. Dorsal view of the specimen of Mastigodryas melanolomus found on Isla del Coral, Nayarit.B. Specimen of M. melanolomus collected on Isla del Coral, Nayarit.C. Dorsal view of the head of the M. melanolomus specimen.

Figure 3 .
Figure 3. Head scutellation of Mastigodryas melanolomus, illustrating the condition of the 4th supralabial on each side of the head.A. View of the right side.B. View of the left side.

Table 1 .
Comparison of the captured snake from Isla del Coral with Mastigodryas melanolomus and Masticophis lineatus based on morphological characters.