Abstract
BackgroundEctoprocta is a large lophotrochozoan clade of colonial suspension feeders comprising over 5.000 extant species. Their phylogenetic position within the Lophotrochzoa remains controversially discussed, but also the internal relationships of the major ectoproct subclades -Phylactolaemata, Stenolaemata, and Gymnolaemata - remains elusive. To gain more insight into the basic configuration of ectoproct muscle systems for phylogenetic considerations, we analysed the adult myoanatomy and the serotonergic nervous system as well as myogenesis in budding stages of the ctenostome Hislopia malayensis.ResultsIn adults, the serotonergic nervous system is restricted to the lophophoral base with a high concentration in the cerebral ganglion and serotonergic perikarya between each pair of tentacles. Prominent smooth apertural muscles extend from the basal cystid wall to each lateral side of the vestibular wall. The musculature of the tentacle sheath consists of regular strands of smooth longitudinal muscles. Each tentacle is supplied with two bands of longitudinal muscles that show irregular striation. At the lophophoral base several muscles are present: (i) Short muscle fibres that proximally diverge from a single point from where they split distally into two separate strands. (ii) Proximally of the first group are smooth, longitudinal fibres that extend to the proximal-most side of the lophophoral base. (iii) Smooth muscle fibres, the buccal dilatators, traverse obliquely towards the pharynx, and (iv) a circular ring of smooth muscle fibres situated distally of the buccal dilatators. Retractor muscles are mainly smooth with short distal striated parts. The foregut consists mainly of striated ring musculature with only few longitudinal muscle fibres in the esophagus, while the remaining parts of the digestive tract solely exhibit smooth musculature. During budding, apertural and retractor muscles are first to appear, while the parietal muscles appear at a later stage.ConclusionsThe apertural muscles show high similarity within Ectoprocta and always consist of two sets of muscles. Gymnolaemates and Phylactolaemates show clear differences within their digestive tract musculature, the former showing smooth and longitudinal muscles to a much greater extent than the latter. The complex musculature at the lophophoral base appears promising for inferring phylogenetic relationships, but sufficient comparative data are currently lacking.
Highlights
The lophotrochozoan phylum Ectoprocta consists of benthic, colonial filter feeders that live on various substrates
Each tentacle is supplied with two bands of longitudinal muscles that show irregular striation (Figure 4b, c, e)
A second set of muscles is situated proximally to the first group and consists of smooth, longitudinal fibres that extend to the proximal-most side of the lophophoral base (Figure 4a, e)
Summary
The lophotrochozoan phylum Ectoprocta consists of benthic, colonial filter feeders that live on various substrates It currently contains over 5.000 extant and approximately 20.000 described extinct species. Within the Gymnolaemata two distinct groups, the Ctenostomata and Cheilostomata, are recognized [1] The relationships between these higher taxa are currently not well understood: The Phylactolaemata are considered monophyletic, while the Cheilostomata and Stenolaemata have been considered both monophyletic [2] or polyphyletic [3,4]. Ectoprocta is a large lophotrochozoan clade of colonial suspension feeders comprising over 5.000 extant species Their phylogenetic position within the Lophotrochzoa remains controversially discussed, and the internal relationships of the major ectoproct subclades -Phylactolaemata, Stenolaemata, and Gymnolaemata remains elusive. To gain more insight into the basic configuration of ectoproct muscle systems for phylogenetic considerations, we analysed the adult myoanatomy and the serotonergic nervous system as well as myogenesis in budding stages of the ctenostome Hislopia malayensis
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