Comparative anatomy and evolution of the gastrotrich muscular system
Gastrotrichs figure prominently in metazoan phylogeny because they share a suite of complex morphological characteristics with several other members of the Bilateria. But their microscopic size, cryptic interstitial habitat, and lack of fossil record have exacerbated the usual barriers to phylogenetic analysis. To arrive at a better understanding of gastrotrich systematics and evolution, cladistic analyses and detailed studies of the muscular system were performed. A fluorescent F-actin stain was applied to whole mounts of 26 species of Gastrotricha to characterize the musculature. Muscle patterns were mapped, their functions inferred, and the direction of evolution hypothesized for several families. The musculature of all gastrotrichs is arranged as a series of circular, helicoidal, and longitudinal bands around the digestive tract. Circular muscles are generally present in splanchnic and somatic positions. Helicoidal muscles in 50--60&deg; angles are present on the pharynx and intestine of most species. Longitudinal muscles are arranged radially around the digestive tract in dorsal, lateral, ventral and ventrolateral positions. Extraordinary muscle orientations are present in several species. In macrodasyidan gastrotrichs, the musculature of Dactylopodola baltica (Dactylopodolidae) is considered to be closest to the ground pattern of the phylum and consists of the following: splanchnic circular muscles on the pharynx and intestine, longitudinal muscles in dorsal, lateral, ventral and ventrolateral positions, pharyngeal and intestinal helicoidal muscles, and somatic circular muscles. Within the Chaetonotida, species of Neodasys and Xenotrichula have the most plesiomorphic muscle topologies. Muscle patterns are similar to macrodasyidans though several muscle orientations have become reduced (splanchnic and somatic circular muscles), are the result of evolutionary modification to existing muscles (incomplete splanchnic and somatic circular muscles, dorsoventral muscles) or evolved independently (the branched Ruckenhautmuskel). This study relied on a phylogenetic perspective to delineate the origin of specific muscle patterns in gastrotrichs and allow for the separation of phyletic heritage from adaptation. Several species from both orders possess muscle patterns that can be regarded as apomorphic and may therefore serve as taxonomic characters. Closer scrutiny of these species may reveal the underlying selective processes that led to the origin and maintenance of novel muscle orientations in gastrotrichs.
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