Kathrin Lunzer


Muscle development in chaetognaths

MSc Student
Advisor: Tim Wollesen

Unit for Integrative Zoology, Department of Evolutionary Biology
University of Vienna


Chaetognaths or “arrow worms” are small, marine predators that are among the most abundant planktonic organisms, but several epibenthic species also are also known. So far, the phylum comprises 133 species from all geographical and vertical ranges of the ocean (meiobenthic to planktonic). There is evidence that the taxon has been present since the early Cambrian (ca. 520–540 Mya) with chaetognath representatives from the fossil record being remarkably similar in body organization to present day forms. Their phylogenetic position remained elusive until 2019, when an extensive phylogenetic study by Marlétaz and colleagues found chaetognaths to be part of a previously identified monophyletic group, the gnathiferans.

There has been a longstanding interest in chaetognath body plan organization, the somatic musculature of the adults is known quite well. In contrast, obliquely striated musculature, as is typical for many other invertebrates, is absent in arrow worms, this taxon has “experimented” with various types of cross striated muscles and evolved a unique type of secondary muscles with two different kinds of sarcomeres that alternate within the muscles.

To gain insights in chaetognath evolution and to improve our understanding of the ancestral condition in the gnathiferan clade, we study muscle development during different embryological stages of the benthic chaetognath Spadella cephaloptera. For this purpose, F–actin staining combined with confocal laser scanning microscopy will be employed to address the following questions: When and where do the first muscle fibers appear during ontogeny? Does myogenesis progress in an anterior to posterior manner? S. cephaloptera has been proven to be a suitable research organism, since it can be kept quite easily under laboratory conditions, reproducing all year round. Since chaetognaths and all other remaining gnathiferan clades are direct developers, I would expect the musculature to develop gradually into the adult condition.