ABSTRACTHydroids have historically been described by morphology, but recent genetic studies have highlighted numerous taxonomic problems. In the genus Antennella, two species, Antennella varians and Antennella billardi (formerly A. balei), were until recently considered synonyms. We sampled colonies of these two species in the southwestern Indian Ocean at mesophotic depths (between 30 and 150 m): A. varians on Mayotte Island and A. billardi on Réunion Island. This study uses an integrative taxonomic approach, combining morphological measurements, DNA barcoding, observations of biological characters and geographical distributions that complement current knowledge. The barcoding results confirmed the distinction between the two species with both mitochondrial and nuclear genes. Morphological measurements provided two main and robust characters to distinguish them: the length of the first pair of nematothecae and the length of their apophysis. Surprisingly, A. billardi showed synchrony of sexual and vegetative reproduction, as two different types of female gonotheca content were observed: oocytes and frustules. These results highlight the need to combine complementary approaches in order to describe biodiversity more accurately and continue to discover the peculiarities of the animal kingdom, especially in hard‐to‐reach areas, such as the mesophotic zone.