Abstract
BackgroundDespite significant methodological progress, Brachiopoda remains one of the lophotrochozoan phyla for which no recent ontogenetic data employing modern methodologies such as fluorescence labelling and confocal microscopy are available. This is particularly astonishing given the ongoing controversy concerning its phylogenetic position. In order to contribute new morphogenetic data for phylogenetic and evolutionary inferences, we describe herein the ontogeny and myoanatomy of larvae and adults of the rhynchonelliform brachiopods Argyrotheca cordata, A. cistellula, and Terebratalia transversa using fluorescence F-actin labelling combined with confocal laserscanning microscopy.ResultsFully grown larvae of A. cordata and T. transversa consist of three distinct body regions, namely an apical lobe, a mantle lobe with four bundles of setae, and a pedicle lobe. Myogenesis is very similar in these two species. The first anlagen of the musculature develop in the pedicle lobe, followed by setae muscles and the mantle lobe musculature. Late-stage larvae show a network of strong pedicle muscles, central mantle muscles, longitudinal muscles running from the mantle to the pedicle lobe, setae pouch muscles, setae muscles, a U-shaped muscle, serial mantle muscles, and apical longitudinal as well as apical transversal muscles. Fully developed A. cistellula larvae differ from the former species in that they have only two visible body lobes and lack setae. Nevertheless, we found corresponding muscle systems to all muscles present in the former two species, except for the musculature associated with the setae, in larvae of A. cistellula. With our survey of the adult myoanatomy of A. cordata and A. cistellula and the juvenile muscular architecture of T. transversa we confirm the presence of adductors, diductors, dorsal and ventral pedicle adjustors, mantle margin muscles, a distinct musculature of the intestine, and striated muscle fibres in the tentacles for all three species.ConclusionOur data indicate that larvae of rhynchonelliform brachiopods share a common muscular bodyplan and are thus derived from a common ancestral larval type. Comparison of the muscular phenotype of rhynchonelliform larvae to that of the other two lophophorate phyla, Phoronida and Ectoprocta, does not indicate homology of individual larval muscles. This may be due to an early evolutionary split of the ontogenetic pathways of Brachiopoda, Phoronida, and Ectoprocta that gave rise to the morphological diversity of these phyla.
Highlights
IntroductionBrachiopoda remains one of the lophotrochozoan phyla for which no recent ontogenetic data employing modern methodologies such as fluorescence labelling and confocal microscopy are available
Despite significant methodological progress, Brachiopoda remains one of the lophotrochozoan phyla for which no recent ontogenetic data employing modern methodologies such as fluorescence labelling and confocal microscopy are available
After cleavage and gastrulation (Fig. 1A), a three-lobed larva is established, which comprises an anterior apical lobe, a mantle lobe in the mid-body region, and a posterior pedicle lobe (Fig. 1B–F)
Summary
Brachiopoda remains one of the lophotrochozoan phyla for which no recent ontogenetic data employing modern methodologies such as fluorescence labelling and confocal microscopy are available. This is astonishing given the ongoing controversy concerning its phylogenetic position. Apart from some mainly gross morphological studies [1421], detailed data using modern techniques such as fluorescence labelling and confocal laserscanning microscopy are not yet available This is especially true with respect to the development of the musculature, despite the fact that myo-anatomical features may provide useful characters for reconstructing phylogenetic relationships [22,23]. We supplement our ontogenetic data with a detailed description of the adult muscle systems of all three species
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