Evolution of body‐wall musculature in the Platyhelminthes (Acoelomorpha, Catenulida, Rhabditophora)

Abstract

In an effort to understand the phylogeny of the Platyhelminthes, the patterns of body-wall musculature of flatworms were studied using fluorescence microscopy and Alexa-488-labeled phalloidin. Species of the Catenulida have a simple orthogonal gridwork of longitudinal and circular muscles. Members of the Rhabditophora have the same gridwork of musculature, but also have diagonal muscles over their entire body. Although a few species of Acoelomorpha possessed a simple orthogonal grid of musculature, most species typically have distinctly different patterns of dorsal and ventral body-wall musculature that include sets of longitudinal, circular, U-shaped, and several kinds of diagonal muscles. Several distinct patterns of musculature were identified, including 8 patterns in 11 families of acoels. These patterns have proven to be useful in clarifying the phylogeny of the Acoelomorpha, particularly with regard to the higher acoels. Patterns of musculature as well as other morphological characters are used here for revisions of acoel systematics, including the return of Eumecynostomum sanguineum (Mecynostomidae) to the genus Aphanostoma (Convolutidae), the revision of the family Childiidae, and the formation of a new family, Actinoposthiidae.

A cultivable acoel species from the Mediterranean, Aphanostoma pisae sp. nov. (Acoela, Acoelomorpha)

“…In A. pisae, the body wall musculature (Fig. 3) is generally similar to that of other isodiametrids (Tyler & Rieger, 1999;Ladurner & Rieger, 2000;Hooge, 2001). Unlike Isodiametra pulchra, there are no ventral diagonal fibers that terminate at the midline before crossing over in A. pisae (Tyler & Rieger, 1999;Ladurner & Rieger, 2000).…”

“…In this respect and also in the presence of thick wedge-shaped parenchymal muscles at the anterior part of the animal, A. pisae is most similar to Praeconvoluta tornuva (Hooge & Tyler, 1999). Regrettably, there are no other depictions of the musculature of species of Aphanostoma or Praeconvoluta available, but even with the data at hand it seems that the pattern of muscle fibers not only of the genital organs, but also of the remaining body is useful for comparison and classification of acoel species (Hooge, 2001;Hooge & Tyler, 2005).…”

“…Unlike Isodiametra pulchra, there are no ventral diagonal fibers that terminate at the midline before crossing over in A. pisae (Tyler & Rieger, 1999;Ladurner & Rieger, 2000). Hooge (2001) Hooge & Tyler, 2003; different from all aforementioned species, A. pisae's longitudinal ventral muscle fibers from mouth to posteriad stop short at the anterior part of the bursa, instead of reaching the posterior end. In this respect and also in the presence of thick wedge-shaped parenchymal muscles at the anterior part of the animal, A. pisae is most similar to Praeconvoluta tornuva (Hooge & Tyler, 1999).…”

“…Identification of acoels requires careful observation and in many cases sagittal sections of the genital organs of mature animals are required for identification (Westblad, 1948). Staining of the musculature (F-actin) can provide an additional level of detail for characterising these organs and the body wall musculature (Hooge, 2001;Hooge & Tyler, 2005).…”

New species of Acoela (Acoelomorpha) from the Gulf of California

“…Serial thick sections of 1.5µm were made according to Smith and Tyler (1984) and stained with toluidine blue. Body wall musculature was revealed through F-actin staining of whole mounts with fluorescently labeled phalloidin (Alexa 488; Molecular probes, Eugene, OR) according to Hooge (2001).…”