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This paper gives a systematic account of 67 species, referable to 22 families and 40 genera, identified in a small collection of hydroids from the central Atlantic coast of Florida between Melbourne and Palm Beach. The fauna mostly comprises an assemblage of tropical western Atlantic species ranging northwards along the southeastern coast of the United States. One new species, Lafoea intorta, is described. Applying Reversal of Precedence provisions in zoological nomenclature, the widely-used generic name Halopteris Allman, 1877 is designated as valid and as a nomen protectum, while its virtually unused senior synonym Halicornaria Hincks, 1865 (not Halicornaria Allman, 1874) is reduced to a nomen oblitum. The genus Pasya Stechow, 1922 is resurrected for the hydroid generally known as Dynamena quadridentata (Ellis & Solander, 1786). Laomedea tottoni Leloup, 1935 is shown to be a junior objective synonym of Clytia fragilis Congdon, 1907, which in turn is a junior subjective synonym of Clytia linearis (Thornely, 1900). Obelia oxydentata Stechow, 1914 is recognized as distinct from O. bidentata Clark, 1875. Hincksella brevitheca Galea, 2009, first described from Cuba, is reported for only the second time; records of the species are added here from Grand Cayman Island and the Caribbean coast of Panama as well as from the Atlantic coast of Florida. Also reported for the second time is Antennella incerta Galea, 2010, previ-ously known only from Guadeloupe in the Caribbean Sea. The true Halopteris diaphana (Heller, 1868), known from the Mediterranean Sea and from Brazil, is reported for the first time from the western North Atlantic. Earlier records of the species in the region are based on misidentifications of H. alternata (Nutting, 1900). Male gonothecae of Halecium calderi Galea, 2010 are reported and illustrated for the first time.

A systematic account is given of 17 families, 25 genera, and 32 species of anthoathecate hydroids and limnopolyps reported from Hawaii. Applying Reversal of Precedence provisions in zoological nomenclature, the familiar hydrozoan genus names Hydractinia Van Beneden, 1844a, Bimeria Wright, 1859a, and Porpita Lamarck, 1801 are designated as valid and as nomena protecta, while seldom-used older names threatening them (the synonyms Echinochorium Hassall, 1841 and Manicella Allman, 1859a, and the homonym Porpita Soldani, 1789 respectively) are relegated to nomena oblita. Also designated a nomen oblitum is the name Pyxidium Leuckart, 1856, threatening its junior but widely used synonym Ectopleura L. Agassiz, 1862. The species name Bimeria vestita Wright, 1859a is rendered valid and a nomen protectum, while its virtually unused senior synonym Manicella fusca Allman, 1859 becomes a nomen oblitum. Hydrodendrium Nutting, 1905 is reinstated as a valid genus, distinct from Hydractinia and replacing its junior objective synonym Nuttingia Stechow, 1909. The spelling of Hydrodendridae Nutting, 1905 is emended to Hydrodendriidae, but that family name is retained as a synonym of Hydractiniidae. Usage of the familiar generic name Sphaerocoryne Pictet, 1893 is upheld by recognizing it and its former senior subjective synonym Corynetes Haeckel, 1879 as valid. The correct spelling of the family name originally founded as Olindiadae Haeckel, 1879 is taken to be Olindiidae, and spelling of the species name Solanderia misakinensis (Inaba, 1892), first established as Dendrocryne (sic) misakii, is stabilized. One new species, Stylactaria munita, is described from shallow waters at Hawaii Kai, Oahu. Lectotypes are designated for Corydendrium corrugatum Nutting, 1905 and Corydendrium minor Nutting, 1905 (=Turritopsis minor), both originally described from Hawaii. Type material of Balea mirabilis Nutting, 1905 (=Balella mirabilis), originally described from waters between the islands of Molokai and Maui, could not be located. Six anthoathecate species [Corydendrium parasiticum (Linnaeus, 1767), Bimeria vestita, Amphinema sp., Eudendrium carneum Clarke, 1882, Ectopleura viridis (Pictet, 1893), and Sphaerocoryne bedoti Pictet, 1893] are recorded from Hawaii for the first time.

Massive accumulations of pelagic species of Sargassum have generated recent social, economic and ecological problems along Caribbean shores. In the Mexican Caribbean, these events have prompted the study of diverse biological and ecological aspects of these macroalgae. However, studies on their associated biota, including Hydrozoa, remain scarce. This research provides important species observations in an area where data is lacking. The occurrence and percent cover of hydroids on Sargassum thalli collected on the beach at Puerto Morelos, Quintana Roo, Mexico from April 2018 to March 2019 was studied. Three pelagic species and morphotypes of Sargassum from this area were analyzed: Sargassum fluitans III, S. natans I and S. natans VIII, as well as a benthic species, S. polyceratium var. ovatum. A total of 14 taxa of hydroids, belonging to the superorders “Anthoathecata” and Leptothecata, were identified. In our study, more hydroid taxa were observed on axes of the different species of Sargassum than on leaves or aerocysts. In general, the greatest species richness of hydroids was observed from February to April. Results show that live hydrozoans attached to pelagic Sargassum are transported into the area. This should be considered in future management measures that address the recurring coastal abundance of Sargassum and its associated biota in the Caribbean region.

The hydroid fauna of the Galápagos Islands, Ecuador, is known largely from reports of collections made between 1932 and 1938 during several Allan Hancock Pacific Expeditions. Of some 100 nominal species (excluding Stylasteridae) reported from the archipelago overall, including species obtained during other expeditions, 81 are recognized as valid. An additional 15 species are added here, 14 of them based on collections undertaken between 1992 and 2000. Leptothecates account for 66 of the 96 species, while anthoathecates comprise the remaining 30. One previously undescribed leptothecate, Halopteris violae, sp. nov., is characterized and compared with H. tenella (Verrill, 1874) and H. minuta (Trebilcock, 1928), species it closely resembles. Gonothecae of Sertularella costata Leloup, 1940 are described for the first time. Plumularia galapagensis, nom. nov., is proposed as a replacement name for the invalid junior primary homonym Plumularia tenuissima Fraser, 1938b (not Plumularia tenuissima Totton, 1930). Under provisions of the International Code of Zoological Nomenclature, the widely used name Cladocoryne floccosa Rotch, 1871 is designated a nomen protectum and assigned precedence over its largely unknown senior synonym Hydra corynaria Bosc, 1797, which is reduced to the status of nomen oblitum. Balella irregularis (Fraser, 1938) from the Galápagos is considered conspecific with Balella mirabilis (Nutting, 1905) from Hawaii and Japan. Six new combinations are introduced: Bimeria laxa Fraser, 1938a is assigned to the genus Garveia Wright, 1859, as G. laxa (Fraser, 1938a); Tubularia integra Fraser, 1938a is assigned to Ectopleura L. Agassiz, 1862, as E. integra (Fraser, 1938a); Campanulina ramosa Fraser, 1938a is assigned to Opercularella Hincks, 1868, as O. ramosa (Fraser, 1938a); Bonneviella minor Fraser, 1938a is assigned to Scandia Fraser, 1912, as S. minor (Fraser, 1938a); Campanularia gracilicaulis Fraser, 1938a is assigned to Clytia Lamouroux, 1812, as C. gracilicaulis (Fraser, 1938a); Sertularia anceps Fraser, 1938a is assigned to Dynamena Lamouroux, 1812, as D. anceps (Fraser, 1938a). Hydroids of the Galápagos are moderately well known faunistically, at least in comparison with those of other areas in the Eastern Pacific Tropical Region.

Sixty species of hydroids, assigned to 24 families and 39 genera, are recognized and discussed in a collection of material from the southwest coast of Florida. One new species (Clytia joycei) is described from turtlegrass (Thalassia testudinum) and reported as well from the Caribbean coast of Panama. Under provisions of the First Reviser Principle in the International Code of Zoological Nomenclature, Antennopsis nigra Nutting, 1900 is assigned precedence over its simultaneous synonym A. longicorna Nutting, 1900. Also included as a synonym of A. nigra is A. sinuosa Fraser, 1947b. The species is now assigned to Nemertesia Lamouroux, 1816, as N. nigra. Following Reversal of Precedence provisions in the code, the well-known name Halecium nanum Alder, 1859 is designated as valid and conserved as a nomen protectum, while Hydra articulata Bosc, 1797 is relegated to a nomen oblitum. The genus Monotheca Nutting, 1900 is upheld as valid on the basis of both morphological and molecular evidence. Sertularia pourtalesi Nutting, 1904, a seldom-reported species, is assigned to Dynamena Lamouroux, 1812 and recognized as distinct from D. disticha (Bosc, 1802). Calyptospadix cerulea Clarke, 1882 and Bimeria franciscana Torrey, 1902 (also known as Garveia franciscana) are taken to be conspecific, with Calyptospadix Clarke, 1882 treated as a valid genus. Confusion over the identity of Lovenella gracilis Clarke, 1882 in literature on North American hydroids is addressed, and topotypic specimens from the Chesapeake Bay region are illustrated to document characters that distinguish the species. Campanularia colombiana (Wedler, 1976) is reported for the first time outside the Caribbean coast of Colombia. The fauna studied here consists largely of species known to occur in the tropical and subtropical western Atlantic. [Zoobank URL: urn:lsid:zoobank.org:act:4C926BE2-D75D-449A-9EAD-14CADACFFADD]