OBSERVATIONS ON THE MORPHOLOGY, SYSTEMATICS, AND BIOGEOGRAPHY OF THE GENUS TRUTTAEDACNITIS (NEMATODA, CUCULLANIDAE)

All species of Truttaedacnitis Fetter, 1974, were examined for a phylo genetic analysis. Morphological studies showed that Truttaedacnitis tr uttae (Fabricius, 1794) from North America possessed a reduced button- shaped or nipple-shaped caudal mucron that is in contrast to the sharp spike-shaped mucron commonly reported in Eurasian specimens. Histolog ical studies showed that the excretory pore in Truttaedacnitis sphaero cephala (Rudolphi, 1809) was situated near the level of the pseudobucc al capsule between the 2 parts of the ventral cephalic ridge. Cucullan us heterodonii Johnston and Mawson, 1943 is transferred to Truttaedacn itis based on the presence of prominent cephalic plates separated by s utures. An hypothesis regarding interrelationships among Truttaedacnit is spp. was developed using phylogenetic systematics. The anterior exc retory pore is a synapomorphy supporting the monophyly of Truttaedacni tis clitellarius (Ward and Magath, 1917) and T. sphaerocephala parasit izing sturgeons. The arrangement of cephalic plates indicates that Tru ttaedacnitis pybusae Anderson, 1992 and T. truttae may form a monophyl etic group and further supports the monophyly of T. clitellarius and T . sphaerocephala. Using unordered character states of different cephal ic plates produced a cladogram that suggested that species in holarcti c/circumboreal families (Acipenseridae, Salmonidae, Petromyzontidae) m ay be monophyletic. The systematics and biogeography of the various sp ecies of Truttaedacnitis and their host specificity indicate that asso ciations in this parasite genus are very ancient and of a relictual na ture, determined largely by past continental and oceanic configuration s, and a combination of vicariance and dispersal.