On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species Author Galea, Horia R. Hydrozoan Research Laboratory, 405 Chemin Les Gatiers, 83170 Tourves, France. Author Maggioni, Davide Università degli Studi di Milano-Bicocca, Dipartimento di Biotecnologie e Bioscienze, Piazza della Scienza 2, 20126 Milano, Italy. & Marine Research and Higher Education (MaRHE) Center, Università degli Studi di Milano-Bicocca, 12030 Faafu Magoodhoo, Maldives. text Zootaxa 2024 2024-03-20 5428 1 1 57 http://dx.doi.org/10.11646/zootaxa.5428.1.1 journal article 10.11646/zootaxa.5428.1.1 1175-5326 10842930 041905ED-FCED-4ED5-8248-E9AA8D6271E9 Leptothecata : Sertulariidae The 16S rRNA sequence of Tridentata borneensis Billard, 1925a from Bali did not match any other sequenced sertulariids, but clustered with other Tridentata species ( Fig. 28 ) in a well-supported clade (BPP = 1, MLBS = 89). Tridentata loculosa ( Busk, 1852 ) has hydrothecae lacking the prominent, lateral, marginal cusps characteristic of the genus Tridentata , its hydranths are strikingly provided with a ligula, and its gonothecae (although transverselyringed) are closed by a watch-glass-shaped operculum ( Migotto 1996: 72 ), making it somehow distinct from members assigned to that genus so far. Additionally, the 16S rRNA sequence obtained from a Balinese specimen does not cluster with other Tridentata sequences, the latter forming a well-supported monophyletic group ( Fig. 28 ) also including a sequence of T. trigonostoma ( Busk, 1852 ) from Bali, which is identical to a conspecific sequence from Thailand . Two other sertulariids, namely Idiellana pristis ( Lamouroux, 1816 ) and Diphasia mutulata ( Busk, 1852 ) , were also sequenced. The first clusters with a sequence of the same species from Thailand (with an intra-specific genetic distance of 0.4 ± 0.2%), whereas the second, here sequenced for the first time, is sister to all other Diphasia sequences included in the analyses, even though the support values for the genus are low ( Fig. 28 ).