Caudal Glands in Nematodes: Morphology, Evolutionary Shifts and Functional Implications

The presence of three caudal glands one dorsal and two subventral with distinct terminal outlets is an apomorphic feature supporting the monophyly of Nematoda. These glans primarily secrete adhesive substances for attachment but have evolved additional roles in locomotion and feeding in certain lineages. Comparative analysis reveals three or two separate gland outlets in 77 species across six ordinal taxa, while vestigial glands without external openings suggest possible internal functions. A single, common gland opening (spinneret), likely arising through independent tail-end invagination, has evolved approximately 32 times across nematode lineages. Additionally, a reversal in gland arrangement (one ventral and two subdorsal) occurred independently in about four instances. Variation in gland number, position (incaudal, suprarectal, and precaudal), and structure is documented, with shifts from three to four or six gland cells and reductions to two in some taxa. Approximately 80 independent losses of caudal glands across nematode phylogeny are detected, including 30 in Mononchida and around 40% in marine species. In clades with terrestrial and limnic representatives (e.g. Dorylaimida, Mononchida) it appears that the glands were lost in the terrestrial environment and aquatic habitats were invaded secondarily. Potential homologies between caudal glands and phasmids are discussed, along with implications for tail morphology to gland structure and function.