Morphological evidence for the systematic position of the order Muspiceida (Nematoda)

Muspiceida are tiny, highly specialised nematodes parasitic as adults in the connective and organ tissues of vertebrates. Nine genera, six monotypic, are recognised in two families. Life cycles are unknown but modes of transmission have been widely discussed in the literature and postulated as occurring by cannibalism, cutaneous penetration or during lactation or grooming. The phylogenetic affinities and systematic rank of the Muspiceida have long been in doubt, some morphological features suggesting similarities with the Secernentea, others suggesting similarities with the Adenophorea. Morphological studies using light microscopy, SEM and TEM of Muspicea borreli Sambon 1925 from wild Mus domesticus Schwartz & Schwartz 1943 (see Figueroa et al. 1986) in Australia strengthen the latter proposition. Although an onchiostyle was not observed in larval or adult M. borreli, evidence indicates that phasmids are absent in this species. What had previously been interpreted as "phasmidoid cells" in this and two other species in which both adult and larval forms were studied are in fact caudal glands, terminal in adults and sub-terminal in third-stage larvae, with no associated nervous tissue. This finding, together with previous evidence from other authors strengthens the view that the Muspiceida provide a link between the Mermithida and the Trichocephalida (=Trichinellida), and on morphological grounds are adenophorean not secernentean. This conclusion accords with recent studies of phylogenetic relationships within the Nematoda and a re-classification of the phylum based on morphological and life cycle knowledge and molecular data, in particular SSU rDNA sequences from animal and plant parasitic, and free-living taxa. It accords with the placement of the vertebrate parasitic Muspiceida, Dioctophymatida and Trichinellida (=Trichocephalida) alongside the insect parasitic Mermithida and Marimermithida, the plant-parasitic Dorylaimida and the free-living Mononchida in the subclass Dorylaimia, of the class Enoplea.