Figure 2. Dynamics of spontaneous and evoked afferent neuron spike activity is elevated in cavefish.

Extracellular recordings were made in posterior lateral line afferents where the neuromast densities and hair cell quantities were similar to resolve the differences observed in afferent activity between larval surface fish (A) and Pachón cavefish (B) between 4 and 7 dpf. The number of occurrences and median intrinsic spike rates in both surface (blue; 12.4 Hz, n = 10 fish) and Pachón (red; 18.6 Hz, n = 5 fish) fish suggests that lateral line response thresholds in cavefish are lower than those of surface fish (C). Evoked afferent activity during stimulation of a single neuromast in surface fish (n = 10, D) and cavefish (n = 12, E) and peristimulus time histograms demonstrate elevated spike rate in cavefish. Evoked spike rate was stimulus frequency dependent and pairwise comparisons reveal significantly (open circles) elevated sensitivity in cavefish at 30 and 40 Hz (p = 0.03 and p < 0.01, respectively, F). Error bars are ± standard error (SE) and ** denotes a significant difference (p < 0.01) detected by an unpaired t-test.