Molecular Characterization and Cophylogenetic Relationships between Crassicauda spp. (Nematoda: Spirurida) and Cetaceans of the Northwest Atlantic.

The evolutionary trajectory of helminth parasites of the genus Crassicauda (Nematoda: Spirurida) is thought to be tightly linked with that of their cetacean hosts. However, due to limitations in sampling these giant (some > 3 meters) worms, their diversity and evolutionary relationships remain poorly understood. In this study, we (1) identify Crassicauda species infecting cetacean hosts from the Northwest Atlantic using the mitochondrial cytochrome c oxidase subunit 1, and nuclear internal transcribed spacer 2 and small subunit ribosomal RNA genes; (2) evaluate the phylogenetic relationships among our specimens, augmented with homologous sequences from GenBank, and (3) characterize host-parasite cophylogenetic structure using distance- and event-based methods. Phylogenetic analyses resolved 11 parasite species infecting 15 cetacean species, revealing cryptic and underreported diversity. Distance-based methods detected significant cophylogenetic signal between parasite and host phylogenies, while event-based analyses detected phylogenetic incongruence. This combination supports a complex evolutionary scenario in which Crassicauda lineages colonized and speciated alongside cetaceans, particularly pelagic species within the family Delphinidae, and diversified within odontocetes. Together, these results provide insight into the mosaic of evolutionary processes that shaped the genus Crassicauda and their cetacean hosts, with implications for monitoring health and disease in marine mammal populations.