Molecular and morphological phylogeny of Saccocirridae (Annelida) reveals two cosmopolitan clades with specific habitat preferences.

Saccocirrids are tiny, slender annelids inhabiting the interstices among coarse sand sediments in shallow waters. The 22 nominal species can be grouped into two morphological groups "papillocercus" and "krusadensis", based on the absence/presence of a pharyngeal bulbus muscle, absence/presence of ventral ciliary patterns, bilateral/unilateral gonad arrangement and chaetal differences. We present herein the first phylogenetic analyses of Saccocirridae based on four molecular markers and 34 morphological characters, employing maximum likelihood and Bayesian methods. All analyses confirmed the monophyly of each morphological group with high nodal support. As a consequence and based on several apomorphic characters, a new genus, Pharyngocirrus gen. nov., is erected for the "krusadensis" clade. Remarkably, the habitat preferences and trophic guilds are also shown to differ between the two genera, yet show strong consistency within each group. Geographic distribution analyses underscore the family preference for subtropical areas, but reject the previously proposed restriction of Pharyngocirrus gen. nov. to the Indo-Pacific. The finding of two morphologically diverging, cosmopolitan sister clades with different habitat preferences suggest an early ecological diversification of the family, conserved during the later evolution, speciation processes and dispersal of the family.