The rare and endangered soil-dwelling purple frog (Nasikabatrachus sahyadrensis) begins its life as a tadpole in certain fast-flowing streams of the Western Ghats. Scientists have now found that the speed with which water flows down these streams is one of the main factors that determine the presence and aggregation of these tadpoles.
The tadpoles are rheophilic, which means they thrive in running water. Apart from several other body adaptations, their specialised mouthparts, which are like suckers, help them to anchor onto rocky areas in flowing water for nearly 100 days.

Behavioural insights

• To find out, researchers at the University of Delhi and the Kerala Forest Research Institute quantified four stream characteristics: water flow velocity; angle of the rocky base; water depth, and water temperature. The study, in Kerala, was restricted to 68 grids placed along 100 m in two streams at Kulamaav in Idukki district, an area the team already knew was home to the purple frog.
• Their results, published in the Journal of Asia-Pacific Biodiversity, show that the team spotted 550 tadpoles in these grids. Though found throughout the streams, the tadpoles tended to gather in large numbers only in areas with relatively higher water flow velocity. They also preferred steep, rocky slopes (65°-90° incline) and a water depth of 2-3 cm.
• The team made observations on tadpole behaviour and distribution too. The tadpoles were always active, moving even when they were attached to the rocky portions of the streams to feed on algae growing on rocks. The moment they sensed danger, they ‘escaped by immediately relaxing their hold on the rock, a behaviour that let them drift some distance downstream before re-attaching themselves to the substrate. Tadpoles in earlier stages of development stayed mostly in relatively slow-flowing portions of the stream’, while older tadpoles were found in faster currents.

Specific threats

According to the authors, these findings and observations provide a strong rationale linking the impact of dam construction to loss of tadpole habitat. The construction of dams and check dams, and levelling and narrowing of streams to expand plantations can alter stream characteristics, in turn affecting the survival of the purple frog tadpole. The damming effect can also slow down the streams feeding water to the river.