Reassessment of phylogenetic relationships and divergence times of extant <i>Argopecten</i> Monterosato, 1889 species in the Eastern Pacific

ABSTRACT Argopecten is primarily distributed in the Caribbean Sea and Western Atlantic coasts but includes two species found in the Eastern Pacific: Argopecten purpuratus and Argopecten ventricosus. Previous evidence drawing upon morphological characters from both fossil and extant Argopecten species coupled with ecological information suggested a closer evolutionary relationship between A. purpuratus and the extant Atlantic species A. gibbus and A. nucleus. In previous research, the estimated divergence of Pacific and Atlantic lineages occurred around 7–6 million Ma. Molecular studies, however, proposed a different relationship, suggesting a closer evolutionary relationship between A. purpuratus and A. ventricosus than with their Atlantic counterparts. To assess this controversy, we re-evaluated the phylogenetic relationships and divergence times of A. purpuratus and A. ventricosus, using a combination of updated fossil age ranges and protein-coding gene sequences from pectinid mitogenomes. We found that the divergence between A. purpuratus and A. ventricosus may have occurred earlier than previously thought, c. 26.6 Ma, likely in the Central American Seaway. Coexistence of different marine environments influenced by the potential presence of cold and warm oceanic water masses during the late Oligocene may have caused this earlier divergence. Furthermore, the phylogenetic analysis dated the most recent common ancestor of A. purpuratus to 3.77 Ma, probably within the Southeast Pacific. The progressive decline in sea surface temperatures during the Plio-Pleistocene may have led to extant A. purpuratus tolerating cold waters. In addition, the climate variability related to the El Niño Southern Oscillation during the Pliocene and onwards likely contributed to promoting A. purpuratus adaptations to changing upwelling conditions. These adaptations include tolerance to variable temperatures, as well as to acidic and hypoxic conditions, traits that are essential for the survival of this species within the Humboldt Ecosystem.