Osmotic avoidance behavior evolves at the cellular level in different nematodes. Data are represented as mean avoidance index (see Supplementary Materials and Methods in Additional file 1 for details) and error bars indicate standard error of mean (s.e.m). Presence and absence of neurons is denoted by '+' and '-', respectively. Ablated animals were tested with 2 M glycerol. For unablated and ablated conditions, n = 30 and n = 10 animals, respectively. (A) Most species showed similar responses to osmotic avoidance to Caenorhabditis elegans. Panagrellus redivivus (PS2298) showed significantly low sensitivity to both 1 M and 2 M glycerol concentrations compared with C. elegans. In all species tested, ablation of the ASH neuron resulted in failure of animals to avoid osmotic stress. Ablation of ASH neurons in P. redivivus resulted in animals not responding to the 2 M glycerol. ASH-ablated C. elegans significantly differ from ASH-ablated Pristionchus pacificus. P values were generated by ANOVA for the different species and are denoted as follows: *, P < 0.05; ***, P < 0.0001. (B) Cellular additivity of osmotic avoidance behaviors in P. pacificus. Ablation of ADL neuron had no effect on osmotic avoidance in C. elegans. ADL-ablated animals in P. pacificus had a significantly lower avoidance index than unablated animals. Ablation of ASH and ADL neurons in P. pacificus resulted in complete loss of osmotic avoidance. P values were generated using ANOVA. P values are denoted as follows: *, P < 0.05; **, P < 0.01; ***, P < 0.001.