Myo-anatomy of juvenile and adult loxosomatid Entoprocta and the use of muscular body plans for phylogenetic inferences.

The anatomy of the muscle bauplan in juvenile buds and adult specimens of the solitary loxosomatid entoprocts Loxosoma nielseni and L. annelidicola was studied by means of fluorescence staining of F-actin and confocal laser scanning microscopy. Although the general myo-anatomy of the body wall shows numerous similarities, both species express significant variations in the arrangement of their pedal muscles. In addition, L. annelidicola alone shows a distinct pair of rectum retractor muscles. Circular muscles are absent in the entire body wall of both species, as well as in previously investigated colonial taxa, which is therefore regarded as basal for Entoprocta. This is in striking contrast to the conditions found in other spiralian or lophotrochozoan taxa. The simple morphology of entoproct tentacle muscles, however, coincides with the phoronid-ectoproct condition and may be due to functional constraints of a simple filter-feeding system. This work shows that variations in the muscular anatomy provide useful characters for systematic analyses on species as well as phylum level and thus allow significant insight regarding metazoan body plan evolution. The phenomenon of phenotypic plasticity and its consequences for phylogenetic interpretations, however, must be carefully considered. Copyright 2004 Wiley-Liss, Inc.