Aschwin H. Engelen

1 Introduction The species is one of the key units to characterize biodiversity, and the delineation of species boundaries is fundamental to evolutionary biology and conservation (Cracraft 1989; Eric et al. 2015; Prada et al. 2008). Accurate species delimitation is not only central for understanding biodiversity patterns but also essential for identifying threatened species and formulating effective conservation strategies (Agapow et al. 2004; Bickford et al. 2007; Dillon and Fjeldså 2005).

This is an early access version, the complete PDF, HTML, and XML versions will be available soon.

1 INTRODUCTION Bacteria are of vital importance to marine multicellular organisms and often play a crucial role throughout their host's life (Bordenstein & Theis, 2015; McFall-Ngai et al., 2013). Seaweeds—important primary producers in coastal ecosystems worldwide—likewise depend on their associated microbiota for optimal functioning, including nutrient exchange, defence mechanisms and reproduction (Egan et al., 2013; Weinberger et al., 2007).