Complex epidermal organs of Phascolion (Sipuncula): insights into the evolution of bimodal secretory cells in annelids |
| |
Authors: | Carsten HG Müller Jørgen Hylleberg Peter Michalik |
| |
Institution: | 1. Zoologisches Institut und Museum, Ernst‐Moritz‐Arndt‐Universit?t Greifswald, Greifswald, Germany;2. Department of Neuroscience, University of Arizona, Tucson, AZ, USA;3. Department of Marine Ecology, Institute of Biology, Aarhus University, Aarhus C, Denmark |
| |
Abstract: | The epidermal organs of an undescribed Phascolion species from the Balearic Islands were investigated using SEM, TEM, LM, CLSM and μCT methods. We found axial receptor cells confirming the previously assumed sensory function of epidermal organs. Our analyses also revealed six types of secretory cells. Some secretory cells types are capable of secreting filamentous and amorphous secretion in two different ways simultaneously (bimodal secretion). The high diversity of cell types, the complex pattern of acinar units, and the absence of a common gland pore make epidermal organs of Phascolion unique amongst sipunculans (Phascolion type). Our reconstruction of the evolution of the epidermal organs of Sipuncula revealed that Phascolion‐type epidermal organs may have derived from either Golfingia‐, Sipunculus‐ or Phascolosoma‐type epidermal organs. The oldest known sipunculans were Golfingia‐like and had epidermal organs, which might resemble the architecture of the Golfingia‐type epidermal organs in extant taxa. Thus, it can be hypothesized that bimodal secretory cells (e.g. basophilic secretory cells) were part of the sipunculan ground pattern. Moreover, bimodal secretory cells of Phascolion look strikingly similar to those found in various annelid glands and thus might even be part of the ground pattern of stem species of Sipuncula + Pleistoannelida. |
| |
Keywords: | ultrastructure evolutionary morphology phylogeny polychaetes multicellular glands secretory cells receptor cells cilia |
|
|