Change of expression levels of distinct transcripts in gill 7 as response of Carcinus maenas to long-term exposure (11 weeks) to hypercapnia analysed by quantitative real-time polymerase chain reaction. Bars represent the median log2-ratio and median deviation (error bars) of the respective gene. Numbers and abbreviations refer to primers and genes as used in the qRT-PCR (see Additional File 1 Table S2 for primer sequences and full names). No statistical significance was detected (sign-test with p < 0.05), but several transcripts exhibit a tendency to be up- or down-regulated, e.g. chloride associated transporters (no. 16, 18, 21).