Evolution of Feeding Structures in the Marine Nematode Order Enoplida

Marine nematodes of the order Enoplida may represent the earliest lineage of nematodes and have a variety of fixed and movable feeding structures in their stomas. This study used an 18S ribosomal RNA phylogeny of the orders Enoplida and Triplonchida (subclass Enoplia) to explore the evolution of these feeding structures in light of previous hypotheses based solely on morphology. The Enoplida and Triplonchida were found to be paraphyletic, as several taxa currently classified as Triplonchida, such as Rhabdodemania, were found to be part of the Enoplida clade. The position of Rhabdodemania within Enoplida was unclear, but a close relation to Enoplidae and Thoracostomopsidae was not supported, making it unlikely that its movable odontia are homologous with the mandibles of these families. A member of Anticomidae was well-supported as the base of the clade containing Phanodermatidae, Enoplidae, and Thoracostomopsidae, suggesting that taxa with buccal rods and mandibles evolved from nematodes with unarmed stomas. The Phanodermatidae were shown to be more closely related to the Enoplidae and Thoracostomopsidae than were the Leptosomatidae, suggesting that the buccal rods of the phanoderms (rather than the mandibular ridge/odontia complex of the Leptosomatidae), may be the origin of the mandibles.