| Summary: | (1) Introduction: <i>Lucina pectinata</i> is a clam found in sulfide-rich mud environments that has three hemoglobins believed to be responsible for the transport of hydrogen sulfide (HbI<sub>Lp</sub>) and oxygen (HbII<sub>Lp</sub> and HbIII<sub>Lp</sub>) to chemoautotrophic endosymbionts. The physiological roles and evolution of these globins in sulfide-rich environments are not well understood. (2) Methods: We performed bioinformatic and phylogenetic analyses with 32 homologous mollusk globin sequences. Phylogenetics suggests a first gene duplication resulting in sulfide binding and oxygen binding genes. A more recent gene duplication gave rise to the two oxygen-binding hemoglobins. Multidimensional scaling analysis of the sequence space shows evolutionary drift of HbII<sub>Lp</sub> and HbIII<sub>Lp</sub>, while HbI<sub>Lp</sub> was closer to the <i>Calyptogena</i> hemoglobins. Further corroboration is seen by conservation in the coding region of hemoglobins from <i>L. pectinata</i> compared to those from <i>Calyptogena</i>. (3) Conclusions: Presence of glutamine in position E7 in organisms living in sulfide-rich environments can be considered an adaptation to prevent loss of protein function. In HbI<sub>Lp</sub> a substitution of phenylalanine in position B10 is accountable for its unique reactivity towards H<sub>2</sub>S. It appears that HbI<sub>Lp</sub> has been changing over time, apparently not subject to functional constraints of binding oxygen, and acquired a unique function for a specialized environment.
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