An integrative study of some species of Gonaxia Vervoort, 1993 from off New Caledonia, with the establishment of Gonaxiidae as a new family of thecate hydroids (Cnidaria: Hydrozoa) Author Galea, Horia R. Hydrozoan Research Laboratory, 405 Chemin Les Gatiers, 83170 Tourves, France. Author Maggioni, Davide 0000-0003-0508-3987 Università degli Studi di Milano-Bicocca, Dipartimento di Scienze dell’Ambiente e della Terra, Piazza della Scienza 1, 20126 Milano, Italy. davide. maggioni @ unimib. it; https: // orcid. org / 0000 - 0003 - 0508 - 3987 & Università degli Studi di Milano-Bicocca, Marine and High Education (MaRHE) Center, 12030 Faafu Magoodhoo, Republic of the Maldives davide.maggioni@unimib.it text Zootaxa 2021 2021-07-21 5004 3 401 429 journal article 10.11646/zootaxa.5004.3.1 1175-5326 5120645 0C4DFAA3-5822-456B-916B-84A7BABEB98F Family Gonaxiidae Maggioni , fam. nov. Diagnosis. Hydroids forming upright colonies of macroscopic size built up along a fascicled, simple or branched main stem. Cladia biseriate, alternating along the stem and, when present, its branches forming either simple-pinnate or multi-pinnate, planar colonies with straight stems, or pinnate cladia-bearing branches spirally arranged along a strongly geniculate stem. Division into internodes often indistinct in both stems and cladia; usually a hydrotheca per equivalent of cladial internode; on stems, two successive cladia separated by three consecutive hydrothecae, of which the proximal most is axillar; cladia borne on short stem apophyses. Hydrothecae flask-shaped to tubular, immersed for a varied degree into their corresponding internodes, free part variably projecting out- to upward, aperture distal, transverse, margin provided with three rounded cusps (two latero-adaxial, one abaxial) separated by shallow embayments, operculum composed of three triangular flaps. Gonothecae arising from the component tubes of the stems, either club-shaped and fully free above their origin, or long, tubular, partly adnate to the stem proximally and diverging distally, in all cases provided apically with a rounded, transverse aperture, broader in females than in males.