Open Access

This study reports on 25 species of hydroids occurring in the collections gathered during KANACONO and KANADEEP expeditions carried out in the SE of New Caledonia in 2016, and off the western coast of the island in 2017, respectively. Of these, 19 have not been dealt with in earlier reports on these collections. Two new genera and four new species are described, viz, Actinopluma mirifica Galea gen. et sp. nov., provisionally assigned to the family Kirchenpaueriidae Stechow, 1921, Schizoplumularia helicoidalis sp. nov., belonging to the Plumulariidae McCrady, 1859, and Corhiza patula Galea sp. nov. and Thamnopteros uniserius Galea gen. et sp. nov., both placed in the family Halopterididae Millard, 1962. The gonotheca and the medusoid gonophore of Plumularia contraria Ansín Agís et al., 2014 are described for the first time, allowing a genus transfer to Dentitheca Stechow, 1919, as D. contraria comb. nov. Plumularia conjuncta Billard, 1913, known earlier from a minute portion of colony, is redescribed based on a complete, though infertile, specimen. Similarly, complete specimens corresponding to the hydroid previously referred to as Antennella megatheca Ansín Agís et al., 2009 are documented, allowing a provisional reallocation to Corhiza Millard, 1962 and a description of its so far unknown gonothecae. Fertile material assignable to the poorly-known Monostaechas fisheri Nutting, 1905 allows the recognition of this hydroid as a valid species, distinct from M. quadridens (McCrady, 1859). Most taxa are illustrated to validate the reliability of their identifications. Finally, phylogenetic reconstructions of the families Aglaopheniidae, Plumulariidae, and Halopterididae, based on the 16S rRNA, allowed a first genetic characterization of some of the species dealt with in this work.

Thirty-six species of various thecate hydroids occur in two recent, deep-water collections from off New Caledonia. Of these, nine are new, namely Solenoscyphus subtilis Galea, sp. nov., Hincksella immersa Galea, sp. nov., Synthecium rectangulatum Galea, sp. nov., Diphasia alternata Galea, sp. nov., Dynamena opposita Galea, sp. nov., Hydrallmania clavaformis Galea, sp. nov., Symplectoscyphus acutustriatus Galea, sp. nov., Symplectoscyphus elongatulus Galea, sp. nov. and Zygophylax niger Galea, sp. nov. The male and female gonothecae of Caledoniana decussata Galea, 2015, the female gonothecae of Caledoniana microgona Galea, 2015, as well as the gonothecae of both sexes of Solenoscyphus striatus Galea, 2015 are described for the first time. The systematic position of the genera Solenoscyphus Galea, 2015 and Caledoniana Galea, 2015 is discussed on both morphological and molecular grounds, and both are confidently placed within the family Staurothecidae Maronna et al., 2016. In light of the molecular data, the genera Billardia Totton, 1930 and Dictyocladium Allman, 1888 are assigned to the families Syntheciidae Marktanner-Turneretscher, 1890 and Symplectoscyphidae Maronna et al., 2016, respectively. The previously undescribed gonothecae of Hincksella neocaledonica Galea, 2015, and the male gonothecae of Sertularella tronconica Galea, 2016, were found. Thyroscyphus scorpioides Vervoort, 1993, a peculiar hydroid with putative stem nematothecae, is redescribed and assigned to the new genus Tuberocaulus Galea, gen. nov. Noteworthy new records from the study area are: Tasmanaria edentula (Bale, 1924), Hincksella sibogae Billard, 1918, Dictyocladium reticulatum (Kirchenpauer, 1884), Salacia sinuosa (Bale, 1888) and Billardia hyalina Vervoort & Watson, 2003. Most species are illustrated to facilitate their identification, and the morphology of the new ones is compared to that of their related congeners.

Forty-three species of sertulariid hydroids (Cnidaria: Hydrozoa: Sertulariidae), collected from the tropical western Pacific (Taiwan, Philippines, New Caledonia, French Polynesia, Vanuatu, Fiji, Tonga, Solomon Islands) during various expeditions of the French Tropical Deep-Sea Benthos program, are discussed. Of these, nine are new to science: Gonaxia nova sp. nov., G. plumularioides sp. nov., Sertularella folliformis sp. nov., Se. plicata sp. nov., Se. pseudocatena sp. nov., Se. splendida sp. nov., Se. tronconica sp. nov., Se. tubulosa sp. nov., and Symplectoscyphus paucicatillus sp. nov. The subspecies Symplectoscyphus johnstoni (Gray, 1843) tropicus Vervoort, 1993 is raised to species but, in order to avoid the secondary homonymy with Sy. tropicus (Hartlaub, 1901), the replacement name, Sy. fasciculatus nom. nov., is introduced. The male and female gonothecae of Diphasia cristata Billard, 1920, the male gonothecae of Gonaxia elegans Vervoort, 1993, as well as the female gonothecae of Salacia macer Vervoort & Watson, 2003, are described for the first time. Additional notes on the morphology of several other species are provided. All taxa are illustrated, in most cases using figures drawn at the same scale, so as to highlight the differences between related species.