On recognizing Paracalanus indicus Wolfenden, 1906 from P. tropicus Andronov, 1977 (Copepoda, Calanoida, Paracalanidae) off the Iranian coast.

In the present study, we re-evaluated the presence of a row of denticles on the posterior margin of the genital double-somite as one of the morphological diagnostic characters used to distinguish Paracalanus indicus Wolfenden, 1906, from P. tropicus Andronov, 1977. The results demonstrated that these denticles are present in both dorsal and lateral margins in these two species. Consequently, the previously reported absence of denticles from the dorsal surface of P. indicus can no longer be regarded as a reliable diagnostic character to separate Paracalanus indicus from P. tropicus. Molecular analyses further confirmed that the specimens collected from Chabahar Bay, Iran, belong to P. indicus. In this species, the row of denticles along the posterior margin of the genital double-somite is arranged in a clustered pattern, with the lateral denticles being larger and more distinct than the dorsal ones. In contrast, P. tropicus exhibits a continuous row of relatively larger denticles. Therefore, the distribution pattern of the denticles along the posterior margin of the genital somite is considered a reliable diagnostic character to recognize these two species, but it can hardly be observed with light microscopy. A key to the females of Paracalanus species reported from off the Iranian coasts is also provided.