The foot also has a unique profile with respect to stress response transcripts. Firstly it has the lowest relative transcription of these pathways, and secondly, whereas oxidative stress transcription homologues dominate the other 3 tissues, the foot is dominated by dimethylarginine metabolism, a typical Eukaryotic stress response pathway, which was only found in the foot and mantle. Transcript homologues associated with glutathione redox metabolism were isolated in the digestive gland, and it is known that this system responds in this tissue to environmental pollutants. Nonmetric Multi Dimensional Scaling Diosmetin ordination of a BrayCurtis resemblance matrix of the four different tissues of M. galloprovincialis calculated from square-root transformed abundances of 144 unique metabolic subsystems demonstrated that the gill and digestive gland show the closest metabolic similarity. The foot was, as expected, the most dissimilar of the tissues. The similarity in metabolic profile between the gill and digestive gland could be indicative of their ��front-line�� position in interactions with the environment. This work describes the first assessment of the use of pyrosequencing in a mollusk. It has demonstrated the effectiveness of pyrosequencing in rapidly capturing large sections of the Mytilus transcriptome and shown that bar-coding via MID tags can increase productivity, allowing sequences from different tissues to be determined simultaneously but then specifically recovered at the bioinformatics stage. The average read length obtained was low for a typical 454-GS-flx pyrosequencing run, but this are not unprecedented for transcriptomic GS-flx output. The effectiveness of the bar-coding was Sarsasapogenin indicated by the distinctive pattern of transcripts found in each tissue and the limited distribution of some transcripts, e.g. vdg3 only in digestive gland, foot proteins isolated to that tissue, etc. Based on the low number of singletons in most samples a relatively high level of transcriptome coverage is suggested but in a situation where the average number of transcripts per cell in each tissue is unknown, this cannot be confirmed.