From feeding to sensing: microanatomy and ultrastructure of the rudimentary polypide in the avicularia of Arctonula arctica (Bryozoa: Cheilostomatida)

Exaptations are among the most interesting evolutionary phenomena, involving a change of function in organs that were not originally adapted for that task.In the colonies of most cheilostome bryozoans, some basic feeding modules (autozooids) with protective operculum were transformed to avicularian polymorphs equipped with an enlarged defensive/cleaning mandible.The tentaculate, sensing, food-gathering and digesting apparatus of autozooids -the retractile polypide -was reduced to a tiny sensory organ retaining a limited mobility.Our first ultrastructural and immunocytochemical study on the adventitious sessile avicularia of Arctonula arctica focuses on muscular and nervous elements of the rudimentary polypide and also shows the presence of an associated unpaired gland.The overall structure suggests a mechano-and/or chemoreceptor function of the vestigial polypide.Based on the presence of the accessory gland, we also assume a chemical defense function of the avicularium.