<jats:title>Abstract</jats:title> <jats:sec> <jats:title>Background</jats:title> <jats:p>Haemocyanin is the respiratory protein of most of the Mollusca. In cephalopods and gastropods at least two distinct isoforms are differentially expressed. However, their physiological purpose is unknown. For the common cuttlefish <jats:italic>Sepia officinalis,</jats:italic> three isoforms are known so far, whereas for only two of them the complete mRNA sequences are available. In this study, we sequenced the complete mRNA of the third haemocyanin isoform and measured the relative expression of all three isoforms during embryogenesis to reveal a potential ontogenetic relevance.</jats:p> </jats:sec> <jats:sec> <jats:title>Results</jats:title> <jats:p>The cDNA of isoform 3 clearly correlates to the known <jats:italic>Sepia officinalis</jats:italic> haemocyanin subunits consisting of eight functional units and an internal duplicated functional unit d. Our molecular phylogenetic analyses reveal the third isoform representing a potentially ancestral haemocyanin isoform, and the analyses of the expression of haemocyanin type 3 reveal that haemocyanin type 3 only can be observed within eggs and during early development. Isoforms 1 and 2 are absent at these stages. After hatching, isoform 3 is downregulated, and isoform 1 and 2 are upregulated.</jats:p> </jats:sec> <jats:sec> <jats:title>Conclusions</jats:title> <jats:p>Our study clearly shows an embryonic relevance of the third isoform, which will be further discussed in the light of the changes in the physiological function of haemocyanin during ontogeny. Taken together with the fact that it could also be the isoform closest related to the common ancestor of cuttlefish haemocyanin, the phylogeny of cuttlefish haemocyanin may be recapitulated during its ontogeny.</jats:p> </jats:sec>