Abstract
Hemichordates (Enteropneusta and Pterobranchia) belong to a small deuterostome invertebrate group that may offer insights on the origin and evolution of the chordate nervous system. Among them, the colonial pterobranch Rhabdopleuridae are recognized as living representatives of Graptolithina, a taxon with a rich fossil record. New information is provided here on the substrate selection and the life cycle of Rhabdopleura recondita Beli, Cameron and Piraino, 2018, and for the first time, we describe the nervous system organization of the larva and the adult zooid, as well as the morphological, neuroanatomical and behavioural changes occurring throughout metamorphosis. Immunohistochemical analyses disclosed a centralized nervous system in the sessile adult zooid, characterized by different neuronal subsets with three distinct neurotransmitters, i.e. serotonin, dopamine and RFamide. The peripheral nervous system comprises GABA-, serotonin-, and dopamine-immunoreactive cells. These observations support and integrate previous neuroanatomical findings on the pterobranch zooid of Cephalodiscus gracilis. Indeed, this is the first evidence of dopamine, RFamide and GABA neurotransmitters in hemichordates pterobranchs. In contrast, the lecithotrophic larva is characterized by a diffuse basiepidermal plexus of GABAergic cells, coupled with a small group of serotonin-immunoreactive cells localized in the characteristic ventral depression. It is envisaged the use of R. recondita as a novel and easily accessible hemichordate model organism to shed light on the evolution of hemichordates and more generally on the origin of deuterostome developmental mechanisms.
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Acknowledgements
Special thanks to Silvia Messinetti, University of Milan, for laboratory support and to Ittiturismo Anime Sante (Tricase), DWD Diving (Diso), Giorgio Aglieri, and Christian Vaglio that helped us in sampling procedures. The in vivo observations were made at the laboratory of the Avamposto MARE, Tricase Porto (Lecce), in the framework of the project Biodiversity MARE Tricase (http://www.biodiversitymaretricase.org/). The authors thank Prof. Cameron and Prof Maletz for the intensive and fruitful revision of the manuscript.
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Strano, F., Micaroni, V., Beli, E. et al. On the larva and the zooid of the pterobranch Rhabdopleura recondita Beli, Cameron and Piraino, 2018 (Hemichordata, Graptolithina). Mar Biodiv 49, 1657–1666 (2019). https://doi.org/10.1007/s12526-018-0933-2
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DOI: https://doi.org/10.1007/s12526-018-0933-2