A new species, Alexandercestus masoumehae n. sp., is described from the sicklefin lemon shark, Negaprion acutidens (Ru¨ppell), from the Iranian coast of the Gulf of Oman. The new species is the third in this genus, all parasitizing species of Negaprion Whitley. Based on morphological characters and scanning electron microscopy the new species is different from its congeners of in a combination of characters including total length, terminal proglottid length, cirrus sac length, and genital pore position. Compared to Alexandercestus gibsoni Ruhnke & Workman, 2013 and A. manteri Ruhnke & Workman, 2013 the specimens of the new species are smaller (7.08–8.5 mm), with a smaller terminal proglottid length towidth ratio (1:0.7–1.3 vs. 1.4–3.2:1 and 1.2–3.1:1, respectively), and have a more anterior genital pore (70–73% of proglottid length vs. 42–66% and 41–65%, respectively). In addition, A. masoumehae possesses less foliose bothridia compared with A. gibsoni, and more densely packed vitelline follicles in comparison with A.manteri. The SEM examination of the scolex in the newspecies reveals a specific pattern of microtriches adding new details in our understanding of the surface morphology of members of Alexandercestus. Finally, the new species increases the total number of described tapeworm species from N. acutidens to five and the described tapeworms in the Persian Gulf and Gulf of Oman region to 65.
