Abstract

Hydrozoans (Cnidaria) are distributed worldwide and exhibit alternating benthic polyp (hydroid) and pelagic
medusan life-history forms. Zygophylacidae are a hydrozoan family with an exclusive hydroid stage throughout their life cycle. Within Hydrozoa, they are unusual in that a relatively large proportion (50%) of species occur in deep waters, but their validity and systematic affinities have been controversial for more than a century. Here, 97 deep-sea specimens, collected by manned submersible or bottom dredging, were investigated using an integrative taxonomical approach. Molecular data concatenating the 16S, 18S and 28S rRNA genes support the validity of Zygophylacidae and their placement within Macrocolonia, instead of Lafoeida. Fourteen zygophylacids and one relevant lafoeid are described or re-described, including three newly proposed species: Zygophylax lighti sp. nov. and Z. tankahkeei sp. nov. from the South China Sea, Z. pseudosibogae sp. nov. from Portugal and five species introduced by Eberhard Stechow (1883–1959) through the re-examination of type material. Four candidate generic diagnoses are discussed for future Zygophylacidae fine systematics by integrating a taxonomic review of all 74 zygophylacids. Morphological characters such as long hydrothecae and complex protective gonothecal structures mostly present in the deep-sea zygophylacids are heuristic for future functional morphology and evolutionary studies.