Structure and evolution of the pharynx simplex in acoel flatworms (Acoela)

Area of Science:

• Marine Biology
• Evolutionary Biology
• Zoology

Background:

• The homology of pharynges in Acoela, a group largely lacking these structures, has been debated for decades.
• Key questions remain regarding whether the acoel pharynx is a primitive trait and homologous to that of platyhelminth turbellarians.

Purpose of the Study:

• To investigate the mouth and pharynx morphology in seven species of Acoela across multiple families.
• To compare acoel pharynges with those of Nemertodermatida and Catenulida to assess homology.

Main Methods:

• Utilized fluorescence staining of musculature.
• Employed conventional histological techniques.
• Conducted transmission electron microscopy (TEM) on acoel, nemertodermatid, and catenulid species.

Main Results:

• Observed significant variability in muscle systems associated with the mouth and pharynx among investigated acoel families.
• Found diverse pharyngeal histology and ultrastructural characteristics within Acoela.
• Noted a lack of similarity between acoel and catenulid pharynges, but a resemblance in mouth musculature between Paratomellidae and Nemertodermatida.

Conclusions:

• Acoel pharynges are not homologous to the pharynx simplex found in Catenulida and Macrostomida.
• Different muscular pharynx types within Acoela are not homologous across higher taxa, precluding a single acoel-type pharynx simplex definition.
• The presence of a muscular pharynx in acoels is unlikely to represent an ancestral characteristic.