title: The evolution of annelids reveals two adaptive routes to the interstitial realm.
abstract: Many animals permanently inhabit the marine interstitium, the space between sand grains [1, 2]. Different evolutionary scenarios may explain the existence of interstitial animals [3, 4]. These scenarios include (1) that the interstitial realm is the ancestral habitat of bilaterians [5, 6], (2) that interstitial taxa evolved from larger ancestors by miniaturization, or (3) progenesis [3]. The first view mirrors the former hypothesis that interstitial annelids, called archiannelids, were at the base of the annelid radiation [7]. Based on morphological data, however, progenesis is generally favored for interstitial annelids today [3, 4, 8]. Herein, our phylogenomic approach revealed that interstitial archiannelids are robustly placed into two groups nested within the annelid tree. Evolution of the first group comprising among others Dinophilidae is best explained by progenesis. In contrast, the second group comprising Protodrilida and Polygordiidae appears to have evolved by stepwise miniaturization adapting from coarser to finer sediments. Thus, in addition to progenesis [3, 4], miniaturization, thought to be too slow for an adaptation to the interstitium [3], is an important second route allowing adaptation to interstitial environments. Both progenesis and miniaturization should be considered when investigating evolution of interstitial taxa [1, 3].
authors.affiliation: Centre of Molecular Biodiversity Research, Zoological Research Museum Alexander Koenig, Bonn 53113, Germany; Department of Biological Sciences, Auburn University, Auburn, AL 36849, USA. Electronic address: torsten.struck.zfmk@uni-bonn.de.
authors.forename: Torsten Hugo
authors.initials: TH
authors.lastname: Struck
authors.affiliation: Centre of Molecular Biodiversity Research, Zoological Research Museum Alexander Koenig, Bonn 53113, Germany; FB05 Biology/Chemistry, University of Osnabrück, Osnabrück 49069, Germany.
authors.forename: Anja
authors.initials: A
authors.lastname: Golombek
authors.affiliation: Institute of Biology, University of Leipzig, Leipzig 04103, Germany; Max Planck Institute for Evolutionary Anthropology, Leipzig 04103, Germany.
authors.forename: Anne
authors.initials: A
authors.lastname: Weigert
authors.affiliation: Department of Biological Sciences, Auburn University, Auburn, AL 36849, USA; Institute of Biology, University of Leipzig, Leipzig 04103, Germany.
authors.forename: Franziska Anni
authors.initials: FA
authors.lastname: Franke
authors.affiliation: University of Osnabrück, Wallenhorst 49134, Germany.
authors.forename: Wilfried
authors.initials: W
authors.lastname: Westheide
authors.affiliation: FB05 Biology/Chemistry, University of Osnabrück, Osnabrück 49069, Germany.
authors.forename: Günter
authors.initials: G
authors.lastname: Purschke
authors.affiliation: Institute of Biology, University of Leipzig, Leipzig 04103, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: Bleidorn
authors.affiliation: Department of Biological Sciences, Auburn University, Auburn, AL 36849, USA.
authors.forename: Kenneth Michael
authors.initials: KM
authors.lastname: Halanych
mesh_terms: D000220:Adaptation, Biological; D000818:Animals; D000845:Annelida / Q000502:physiology*; D005075:Biological Evolution; D017753:Ecosystem; D005247:Feeding Behavior; D010802:Phylogeny