The description and histopathology of Leptorhynchoides polycristatus n. sp. (Acanthocephala: Rhadinorhynchidae) from sturgeons, Acipenser spp. (Actinopterygii: Acipenseridae) in the Caspian Sea, Iran, with emendation of the generic diagnosis.

Of the three known species of Leptorhynchoides Kostylew 1924, two are reported from North American fishes: Leptorhynchoides aphredoderi Buckner and Buckner 1976 and Leptorhynchoides thecatus (Linton 1891) Kostylew 1924. The third species, Leptorhynchoides plagicephalus (Westrumb 1821) Kostylew 1924, is commonly found in the Caspian and Black Sea from at least four species of sturgeons including Acipenser stellatus Pallas 1771 and the Acipenser nudiventris Lovetzsky 1828 from which Leptorhynchoides polycristatus n. sp. was found. No taxonomic work has been reported for L. plagicephalus for the last 90 years. L. polycristatus n. sp. can be readily confused with L. plagicephalus because of many superficial similarities. Such similarities include the general shape of the trunk, proboscis, and organ systems. However, L. polycristatus is clearly distinguished from the other three species primarily by having (1) 19-20 proboscis hooks per row; (2) the shortest hooks are anterior and the longest at the middle; the opposite is true in L. plagicephalus; (3) with a cuticular collar enveloping the base of the proboscis hooks; (4) the surface of its proboscis hooks is ribbed; (5) with a broad collar of multiple rectangular cuticular crests encircling the anterior end of the trunk; this is the only member of Leptorhynchoides with such a structure; (6) with many large ovoid uninucleated cells in the subcuticular layer of the trunk; (7) with paired glandular clusters near the male reproductive opening and of suction cup-like sensory structures on the bursa; (8) with dorsoventral ligament across the vagina; (9) cement glands are in a cluster of eight arranged in two horizontal tiers of four glands each; (10) with female gonopore near terminal; (11) with structures interpreted as possible microtriches on the surface of the trunk; (12) and with thinner eggs. L. polycristatus caused extensive histopathological damage to host intestinal layers. The armed proboscis invades and attaches to the host mucosa causing villi compression and necrosis of the epithelial lining with subsequent hemorrhaging and granulocyte migration. No encapsulation of the acanthocephalan is visible, and the worm can migrate deep into the smooth muscle layers of the muscularis extrema. The presence of L. polycristatus in the lumen of the host intestine obstructs and damages the absorbing surface of the host affecting the nutritional potential. Dead, necrotic host epithelial tissue and remnants of villi and crypts are visible.