Snakes of the family Natricidae (or subfamily Natricinae) are cosmopolitan in distribution. They can be terrestrial, aquatic, or fossorial, cryptic, and are often secretive. The genus Opisthotropis (Günther 1872), holds 25 currently recognized species, aquatic, nocturnal, and typically oviparous, inhabitants of fast-flowing mountain watercourses. Opisthotropis are collectively distributed in tropical and subtropical regions of southern China, mainland Southeast Asia, the Ryukyu Archipelago of Japan, Sumatra in Indonesia, Borneo and the Philippines. Two recognized species in the Philippines are the Sabah Keelback, Opisthotropis typica (Mocquard, 1890) on Palawan Island and the Mindanao Keelback, and Opisthotropis alcalai Brown and Leviton (1994) on the western side of Mindanao Island. Relative to other natricine snakes, the Opisthotropis appears somewhat ancient (Deepak et al., 2022).
Opisthotropis alcalai was discovered in 1959 by Angel C. Alcala and represented the first genus record in the Philippines. It was described based on a single specimen. The snakewas encountered on Cugat Creek at the western portion of Dapitan Peak, Mt. Malindang Range Natural Park, Zamboanga del Norte, at around 762 meters ASL.
Forty-five years after its description, a subsequent specimen was found by Nuñeza et al. (2010) during their biodiversity research in 2004, surveying the municipalities of Calamba, Don Victoriano Chiongbian, and Oroquieta City in an elevation range of 120–1550 meters above sea level. It was found in the submontane dipterocarp forest habitat (elevation range: 900–1000 m elevation) on the north-western side of Mt. Malindang Range Natural Park (Nuñeza et al., 2010).
Maglangit et al. (2023) discovered O. alcalai in the foothills of Mt. Malindawag. Despite its secretive nature and rarity, potentially due to low population density, this record extends its known distribution to more than 95 km straight-line distance from its type locality. This record represents the northeastern distribution of O. alcalai. The specimen was located along the streams and riverine banks of Mount Pasonanca Natural Park in Zamboanga City.
The single-occurrence records of O. alcalai demonstrate the significance of microhabitat-specific refugia for rare, secretive, and enigmatic natricid snakes that often inhabit aquatic and terrestrial environments. Mindanao’s Lowland habitats (< 900 meters elevation) are among the most threatened and degraded habitats due to agricultural and urban sprawl.
Citation
Maglangit EP, Bonachita SA, Arnado MJ, Nuñeza OM. 2023. Notes on the distribution and natural history of the enigmatic and endangered Opisthotropis alcalai (Brown & Leviton, 1961) on Mindanao Island, Philippines. Herpetology Notes. 16:527-31.