Redescription of the leptothecate hydroidHalecium macrocephalumAllman, 1877 (Cnidaria: Hydrozoa)

Hydroids are a species-rich component of the benthic fauna inhabiting deep, cold-water coral habitats. Fertile colonies of the leptothecate species Halecium macrocephalum Allman, 1877 Allman, GJ. 1877. Report on the Hydroida collected during the exploration of the Gulf Stream by L.F. de Pourtalès, assistant United States Coast Survey. Mem Mus Comp Zoöl Harvard, 5(2): 1–66. [Google Scholar] collected from cold-water coral habitats off the southeastern USA exhibited morphologically peculiar male gonophores, distinct from those described and illustrated for the holotype. We redescribe H. macrocephalum based on this new material, and consider the gonophores of the holotype to be either damaged or immature. Following re-examination of non-type material (the holotype being lost), we characterize H. macrocephalum as a strictly subtropical western North Atlantic bathyal species, occurring between North Carolina and Florida, the Straits of Florida inclusive of the Bahamas, and the eastern Gulf of Mexico including the Tortugas. This study also provides a taxonomic distinction between H. macrocephalum and its morphologically similar Southern Ocean congener, Halecium jaederholmi Vervoort, 1972.