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We used the APEX telescope to observe spectral lines occurring at about 170GHz frequency towards 14 positions along the full extent of the filamentarySeahorse infrared dark cloud. Six spectral line transitions were detected($\geq3\sigma$) altogether, namely, SO$(N_J=4_4-3_3)$, H$^{13}$CN$(J=2-1)$,H$^{13}$CO$^+(J=2-1)$, SiO$(J=4-3)$, HN$^{13}$C$(J=2-1)$, and C$_2$H$(N=2-1)$.While SO, H$^{13}$CO$^+$, and HN$^{13}$C were detected in every source, thedetection rates for C$_2$H and H$^{13}$CN were 92.9% and 85.7%, respectively.Only one source (SMM 3) showed detectable SiO emission (7.1% detection rate).Three clumps (SMM 5, 6, and 7) showed the SO, H$^{13}$CN, H$^{13}$CO$^+$,HN$^{13}$C, and C$_2$H lines in absorption. We found three positivecorrelations among the derived molecular abundances, of which those betweenC$_2$H and HN$^{13}$C and HN$^{13}$C and H$^{13}$CO$^+$ are the mostsignificant (correlation coefficient $r\simeq0.9$). The statistically mostsignificant evolutionary trends we uncovered are the drops in the C$_2$Habundance and in the $[{\rm HN^{13}C}]/[{\rm H^{13}CN}]$ ratio as the clumpevolves from an IR dark stage to an IR bright stage and then to an HII region.The correlations we found between the different molecular abundances can beunderstood as arising from the gas-phase electron (ionisation degree) andatomic carbon abundances. The [C$_2$H] evolutionary indicator we found is inagreement with previous studies, and can be explained by the conversion ofC$_2$H to other species (e.g. CO) when the clump temperature rises, especiallyafter the ignition of a hot molecular core in the clump. The decrease of $[{\rmHN^{13}C}]/[{\rm H^{13}CN}]$ as the clump evolves is also likely to reflect theincrease in the clump temperature, which leads to an enhanced formation of HCNand its $^{13}$C isotopologue.

What did the seahorse swallow? APEX 170 GHz observations of the chemical conditions in the Seahorse infrared dark cloud