Figure 1. The study system.

(A) Both tested molly populations originate from the South of Mexico near the city of Tapijulapa, federal state of Tabasco. Here, ancestral forms of Poecilia mexicana colonized both surface; (B, top, picture of a surface-dwelling molly) as well as cave; (B, bottom, picture of cave molly) habitats. (C) Locations of the study sites are indicated with blue (cave) and red (surface) circles along the El Azufre river that flows from left to right, i.e., out of the cave. On site, individually marked fish were transferred into a net cage and recorded with an infrared camera (D).