| Summary: | The deep-pelagic is the largest biome on planet Earth. Despite its size the animal life inhabiting the deep-pelagic is severely underrepresented in global marine biological records. Accordingly, many questions related to the demographic histories and taxonomic relationships of deep-pelagic fishes remain unanswered. We utilized molecular data to investigate taxonomic issues related to the family Cetomimidae (the whale fishes) and to infer the demographic histories of 13 species of deep-pelagic fishes. Family Cetomimidae has long been plagued by taxonomic issues. Even the matching of male and female cetomimids has proven difficult due to striking sexual dimorphism within the family. We constructed maximum clade credibility trees and performed bGMYC analysis to better understand whale fish taxonomy. Our Cetomimidae tree was largely in agreement with past morphological work. Areas of disagreement regarding morphological analyses included a clade comprising Cetostoma + Ditropichthys, as well as paraphyly within Gyrinomimus with respect to Cetomimus. Our bGMYC analysis revealed Cetostoma regani to be a cryptic species complex, comprised of two operational taxonomic units that diverged ~3.1 Ma ago. We identified two new putative Cetomimus species, as well. Finally, we were able to match all of our male samples to three different female species. Reconstructions of historic demography shed light on how past ecological/evolutionary events impacted the population size of a given species. By understanding the past, we can begin to understand how populations will behave in response to current and future changes to their habitat. Mitochondrial and nuclear DNA markers were sequenced for 13 low-latitude deep-pelagic fish species representing eight families. Demographic histories were reconstructed using two sets of analyses. Historic population expansions were inferred for eight species using frequency-based statistics, while our extended Bayesian skyline plots (EBSPs) detected expansions in five of those eight species. Our EBSPs provided estimated dates of expansion that ranged from 80 ky ago to 270 ka ago. All of these dates appear to coincide with periods of warm sea surface temperature (SST) at approximately 41° of latitude in the North Atlantic, the northernmost range for many low-latitude deep-pelagic fishes.
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