Moving beyond medusae: integrating siphonophores into marine food web ecology

Our perception of community composition and species interactions in the deep ocean has evolved over time, as various sampling approaches have been developed that reveal different habitat components. We are now better-suited to examine the ecological functions of fragile, gelatinous animals that were traditionally understudied due to difficulties associated with their collection. Contemporary research highlights the potential importance of gelatinous taxa in midwater ecosystems. However, this shifting paradigm has predominantly focused on scyphozoan medusae (phylum Cnidaria), while the ecological functions of other abundant taxa remain poorly-understood. We address a critical knowledge gap regarding the trophic ecology of siphonophores, a clade of colonial cnidarians (Class Hydrozoa). Siphonophores are highly abundant in zooplankton communities, have broad geographic distributions, are present across the entire water column, and have potential to serve various functions in pelagic food webs. To examine the ecological roles of siphonophores, we first quantified midwater community composition in Monterey Bay, California, using multiple sampling approaches (trawl and ROV video surveys). We examined potential collection biases against gelatinous taxa to inform our understanding siphonophore abundance in the midwater food web. We then conducted an extensive literature review of siphonophore predator-prey interactions and analyzed siphonophore tissues for stable isotope analyses to evaluate their trophic niches. Our results indicate that the view of community composition is strongly dependent on sampling methodology, where diversity and abundance of gelatinous taxa were considerably lower in trawl compared to ROV data. Diet data indicate that siphonophores occupy numerous ecological niches, span multiple trophic levels, consume a diverse assemblage of pelagic taxa, and some species employ a high degree of specialization. Our study highlights the need for complementary sampling and analytical approaches in midwater research and provides a framework for integrating siphonophores into food web ecology.