DICLYBOTHRIUM-ATRIATUM N-SP (MONOGENEA, DICLYBOTHRIIDAE) FROM NORTH-AMERICAN ACIPENSERID FISHES WITH OBSERVATIONS ON DICLYBOTHRIUM-ARMATUM AND DICLYBOTHRIUM-HAMULATUM

Diclybothrium atriatum n. sp. is described from North American acipens erid fishes, Acipenser fulvescens Rafinesque (lake sturgeon) and Acipe nser brevirostrum Le Sueur (shortnose sturgeon). Diclybothrium atriatu m is distinguished from other dicylbothriids by the presence of a uniq ue precvarial structure that we name the ventral atrial organ. This st ructure possesses a ventral orifice leading to a deep groove that wide ns internally as a crypt, bordered by densely staining granular bodies and more deeply situated glandular cells. Previous records of Diclybo thrium armatum Leuckart, 1835 examined in this study from North Americ an acipenserids are referable to D. atriatum. Studies on the anatomy o f this and other species of Diclybothrium Leuckart, 1835, D. armatum a nd Diclybothrium hamulatum (Simer, 1929), reveal the presence of paire d multichanelled sperm ducts that are shown to be characteristic of th e Diclybothriidae Bykhovskii and Gusev, 1950. Paired sperm ducts, whil e common in Turbellaria with multiple testes, are unusual amongst Mono genea. The endemicity of Diclybothrium atriatum and specificity for A. fulvescens and A. brevirostrum suggests that speciation of this paras ite may have occurred following the isolation of Acipenser L. spp. in central and eastern North America.