Tube morphology, ultrastructures and mineralogy in recent Spirorbinae (Annelida: Polychaeta: Serpulidae). III. Tribe Circeini

Abstract

This paper is a third part of the series started by Ippolitov and Rzhavsky (2014, 2015) and providing detailed descriptions of Recent spirorbin tubes, their mineralogy and ultrastructures. In the present part we describe species of the small tribe Circeini Knight-Jones, 1978, which includes the genera Circeis Saint-Joseph, 1894 and Paradexiospira Caullery et Mesnil, 1897. Mineralogically, tubes of both genera are purely calcitic. Ultrastructurally, most studied Circeis had three-layered tubes with spherulitic irregularly oriented prismatic (SIOP) structure of the main middle layer, while inner and outer surfaces were consolidated by dense spherulitic prismatic (SPHP) layers. All Paradexiospira species tend to have unilayered tubes with simple prismatic (SP) structure resulting in tube transparency. One unique species (Circeis vitreopsis Rzhavsky, 1992), shows a transition between these two types. A direct observation of ultrastructures at early stages of a tube development suggests that the presence of thick middle SIOP layer is a plesiomorphic state, while unilayered SP wall is apomorphic. Most plesiomorphic structures among Circeini look similar to structures of certain Spirorbini, probably indicating close phylogenetic relationships of these taxa. Development of apomorphic unilayered SP wall in Paradexiospira providing consolidation and strengthening of the tube is probably a result of adaptation to living on hard substrates with strong rate of sand abrasion in bottom waters.