Comparative 3D‐anatomy of Appendicularian Endostyles (Tunicata, Chordata) ‐ A Tale of Reduction

Appendicularia comprises about 70 holoplanktonic species traditionally classified in three families: Oikopleuridae, Fritillariidae, Kowalevskiidae. Despite their eminent phylogenetic position and their important role in ocean ecosystems, most research focuses on the model organism Oikopleura dioica while the diversity of appendicularians remains underexplored. Here, we present a comparative morphological analysis of appendicularian endostyles, a pharyngeal gland homologous to the vertebrate thyroid. Based on light- and transmission electron-microscopical investigations in 12 species representing seven (of 15) genera from all three family-level taxa, we describe the 3D-anatomy of endostyles, histologically recognizable cell-types, and discuss our findings in a cladistic framework. We identified seven different cell types arranged in species specific patterns, including the formerly unrecognized 'bright cells'. Two ciliary bands - the peripharyngeal band and the retropharyngeal band - are associated with the endostyles. Outgroup comparison indicates that repeated apomorphic reductions of cell types, rows of cells, the retropharyngeal band, and the complete endostyle occurred within Appendicularia. We propose a phylogenetic hypothesis that suggests that "Oikopleuridae" is a paraphyletic grouping and supports an evolutionary scenario with multiple reductions functionally related to the evolution of the external filter house of appendicularians. While we document the diversity of endostyle anatomy, more detailed cladistic analysis, including other organ systems, is needed to resolve the phylogenetic relationships and to understand the evolution of appendicularian taxa.