Global distribution patterns of siphonophores across horizontal and vertical oceanic gradients.

Background: Siphonophores are diverse, globally distributed hydrozoans that play a central role in marine trophic webs worldwide. However, they still constitute an understudied fraction of the open ocean gelatinous taxa, mainly due to challenges related to siphonophore sampling and identification, which have led to a general knowledge gap about their diversity, distribution and abundance. Methods: Here, we provide a global overview of the oceanic vertical distribution of siphonophores using DNA metabarcoding data from 77 bulk mesozooplankton samples collected at four different depth ranges (0-200, 200-500, 500-1000, 1000-3000 m depth) along the Atlantic, Pacific, and Indian Oceans during the MALASPINA-2010 circumnavigation expedition. Results: We detected a total of 44 siphonophore species (which represents about one quarter of the described siphonophore species) from which 26 corresponded to Calycophores, 14 to Physonectae and 2 to Cystonectae. Our results suggest wider horizontal and vertical distributions of siphonophore species than previously described, including novel records of some species in certain oceanic basins. Also, we provide insights into the intraspecific variation of widely distributed species. Finally, we show a vertical structuring of siphonophores along the water column; Calycophores (siphonophores without pneumatophores) dominated the epipelagic (from the surface to 200 m depth) and upper mesopelagic layers (from 200 to 500 m depth), while the proportion Physonectids (siphonophores with pneumatophore) notably increased below 500 meters and were dominant at bathypelagic depths (>1000 m depth). Conclusions: Our results support that the siphonophore community composition is vertically structured. Also, we provide insights into the potential existence of genetic variations within certain species that dominate some ocean basins or depth ranges. To our knowledge, this is the first time that DNA metabarcoding data is retrieved to study siphonophore distribution patterns, and the study provides evidence of the potential of molecular techniques to study the distribution of gelatinous organisms often destroyed in net sampling.