The female antennule of several cyclopid copepods of the genus Acanthocyclops Kiefer, 1927 is known to have a high degree of oligomerization in both segmentation and chaetotaxy. The present work provides information on the ontogenetic development of the antennule of three (sub-)species of Acanthocyclops accommodated in the kieferi-group, with an 11- segmented antennule in the adult female. These are, A. balcanicus bisetosus Iepure, 2001, A. milotai Iepure & Defaye, 2008, and Acanthocyclops n. sp. Iepure & Oarga (unpubl.). One developmental pattern has been observed that results in an 11-segmented antennule through several successive enlargements and the addition of segments in the proximal section of the limb as follows: the first segment adds a new segment distally and an arthrodial membrane proximally during the moult to CIII; and the second segment adds a new segment distally during the moult to CIV and proximally in the moult to CV. The same (neotenic) morphology of CV is retained in the adult. The addition of segments and the distribution of setae on each segment were compared and homologized with two subterranean representatives of the genus Diacyclops Kiefer, 1927 that share the same pattern in the adult female, i.e., the D. clandestinus (Kiefer, 1926) sensu lato and the D. languidoides (Lilljeborg, 1901) sensu lato. The position of marker elements (i.e., dorsal seta (ancestral segment XV), medial distal seta (ancestral segment XX), etc.) in various copepodid stages is used to identify homologous segments. The analyses of antennula development suggest that segment number and chaetotaxy during development are characters that are less informative for phylogenetic studies in the genera Acanthocyclops and Diacyclops.

In: Studies on Freshwater Copepoda: a Volume in Honour of Bernard Dussart