Summary
Rhadinorhynchus plagioscionis n. sp. (Acanthocephala: Rhadinorhynchidae) is described from the intestine of a freshwater fish called the "pescada" (Plagioscion squamosissimus (Heckel) from Manaus, Amazonas, Brazil. The new species differs from others in the genus in that the trunk spines have bifurcated roots and free points. The distribution of these spines is also different, being from the neck region posteriorly to beyond the testes in the male, and well-posterior to the uterine bell in the female. The new species has 12 longitudinal rows of hooks on the proboscis, quincuncially arranged, with 23 and 24 hooks per row alternating. In hook number, the new species most resembles R. trachuri Harada, 1935, described from a Japonese marine fish. R. plagioscionis differs from that species, however, in having cement glands that are long and tubular (not short and flask-shaped); lemnisci that are shorter (not longer) than the proboscis sac; larger eggs and a freshwater host. The new species is the thir in the genus to be reported from a freshwater host fish in the world.