Blackwater diving illuminates biodiversity and ecology of siphonophores in the Gulf Stream

Maja Trochanowska¹Richard Collins²Peter Schuchert³Maciej K Mańko^1*

• ¹Department of Marine Biology and Biotechnology, University of Gdansk, Gdansk, Poland
• ²Florida Museum of Natural History, Gainesville, United States
• ³Museum d'histoire naturelle de la Ville de Geneve, Geneva, Switzerland

Siphonophores are gelatinous, colonial cnidarians, distributed throughout the world's oceans. They inhabit various water depths, and are important predators in the open waters, consuming multiple prey types. Siphonophore colonies are fragile and easily damaged during collection and preservation. They are composed of several morphologically different elements (zooids), what further hampers proper identification of collected specimens. As a result, siphonophores remain understudied, with significant knowledge gaps concerning distribution, trophic ecology, and life cycles. The objective of this work is to identify siphonophores occurring in the Gulf Stream off the Florida coast, reconstruct their molecular diversity using 16S rRNA barcoding and shed light on their ecology. The siphonophore data were obtained through underwater photographs taken by scuba divers during hundreds of night drift dives. Additionally, samples of whole colonies were occasionally taken for molecular work. Overall, twenty-two siphonophore species were identified, and their morphology is described here based on in situ observations. Eight taxa have never been recorded in the study area, and one, Lilyopsis problematica sp. nov., is formally described here. Morphological identifications were corroborated with molecular methods: phylogenetic analyses, and calculation of genetic distances, that has uncovered hidden diversity even within well-established taxa. The photography employed here, documented ecological associations between siphonophores and other marine taxa (preying and being preyed upon), some of which have important ecological implications, such as the observation of larval Physalia physalis feeding on fish. Blackwater diving has proved to be a promising method for obtaining unique data on the siphonophore biodiversity, morphology, and ecology in the upper layer of the epipelagic.