title: The cephalopod arm crown: appendage formation and differentiation in the Hawaiian bobtail squidEuprymna scolopes.
abstract: BACKGROUND
  Cephalopods are a highly derived class of molluscs that adapted their body plan to a more active and predatory lifestyle. One intriguing adaptation is the modification of the ventral foot to form a bilaterally symmetric arm crown, which constitutes a true morphological novelty in evolution. In addition, this structure shows many diversifications within the class of cephalopods and therefore offers an interesting opportunity to study the molecular underpinnings of the emergence of phenotypic novelties and their diversification. Here we use the sepiolidEuprymna scolopesas a model to study the formation and differentiation of the decabrachian arm crown, which consists of four pairs of sessile arms and one pair of retractile tentacles. We provide a detailed description of arm crown formation in order to understand the basic morphology and the developmental dynamics of this structure.
  
  
  RESULTS
  We show that the morphological formation of the cephalopod appendages occurs during distinct phases, including outgrowth, elongation, and tissue differentiation. Early outgrowth is characterized by uniform cell proliferation, while the elongation of the appendages initiates tissue differentiation. The latter progresses in a gradient from proximal to distal, whereas cell proliferation becomes restricted to the distal-most end of the arm. Differences in the formation of arms and tentacles exist, with the tentacles showing an expedite growth rate and higher complexity at younger stages.
  
  
  CONCLUSIONS
  The early outgrowth and differentiation of theE. scolopesarm crown shows similarities to the related, yet derived cephalopodOctopus vulgaris. Parallels in the growth and differentiation of appendages seem to exist throughout the animal kingdom, raising the question of whether these similarities reflect a recruitment of similar molecular patterning pathways.
authors.affiliation: Department of Theoretical Biology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria ; Department of Biology, University of Hawaii at Manoa, 2538 McCarthy Mall, Edmondson Hall 413, Honolulu, HI 96822 USA.
authors.forename: Marie-Therese
authors.identifier: 0000-0001-6210-6044
authors.initials: MT
authors.lastname: Nödl
authors.affiliation: Marine Biology Section - Department of Biology, University of Copenhagen, Universitetsparken 4, 2100 Copenhagen, Denmark.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Kerbl
authors.affiliation: Department of Integrative Zoology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria.
authors.forename: Manfred G
authors.initials: MG
authors.lastname: Walzl
authors.affiliation: Department of Theoretical Biology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria.
authors.forename: Gerd B
authors.initials: GB
authors.lastname: Müller
authors.affiliation: Department of Biology, University of Hawaii at Manoa, 2538 McCarthy Mall, Edmondson Hall 413, Honolulu, HI 96822 USA.
authors.forename: Heinz Gert
authors.initials: HG
authors.lastname: de Couet
keywords: Appendage; Arm crown; Bobtail squid; Cephalopod; Development; Euprymna scolopes; Evolution; Lophotrochozoa; Tentacle