Volcano plot of the oil exposed ( E ) versus the unexposed ( U ) treatments. The horizontal axis is the log2 fold change between the treatment means. The -log10 (P-value) is plotted on the vertical axis. Each sequence is represented by one point on the graph. The length of each transcript is indicated by the color of the point in reference to the color scale at the top. The transition from blue to green occurs at roughly 300bp, and from green to orange occurs at roughly 3000bp. This creates three categories with 32,527 sequences shorter than 300bp, then 83,043 sequences between 300 and 3,000bp, and finally 5,155 sequences over 3,000bp. With this many points, some occlusion is unavoidable but in general points representing longer sequences provide more statistical power than shorter sequences. Points above the dotted red line indicate sequences that pass the P < 0.01 significance threshold. The two vertical columns of points in the chart are sequences for which expression was only detected in one treatment so the log2 fold change values were artificially set to (±) 0.5 (instead of +/- infinity) as appropriate.