On the larva and the zooid of the pterobranch Rhabdopleura recondita Beli, Cameron and Piraino, 2018 (Hemichordata, Graptolithina)

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 shed light on the evolution of hemichordates and more generally on the origin of deuterostome developmental mechanisms.

Serotonin system in tunicates: insight from morphological and molecular approaches

“…5-HT roles as neurotransmitter, in morphogenesis and melanogenesis, are likely to be ancient ones since they have been described also in platyhelminthes (Lambrus et al, 2015). Conversely, the presence of 5-HT positive cells in the intestine in tunicates (Pennati et al, 2001;Tiozzo et al, 2009;Stach, 2016, 2018), cephalochordates (Candiani et al, 2001) and vertebrates (Gershon, 2004) could be a chordate specific novelty, since it has never been described in other groups (Strano et al, 2019;Mercurio et al, 2019b). Within the tunicate clade, several events of loss and gain of function might have occurred in relation to the evolution of different life cycles.…”

“…Based on the detailed morphological description provided by Burighel and Milanesi, the serotonergic cells described in the digestive system of ascidians may be identified as endocrine cells (Burighel and Milanesi, 1975), and considered homolog to the vertebrate enterochromaffin cells (Gershon, 2004). 5-HT-positive cells are also present in the enteric system of cephalochordates (Candiani et al, 2001) but not in hemichordates or echinoderms (Strano et al, 2019;Mercurio et al, 2019b), suggesting that enteric 5-HT may be a novel feature of chordate evolution.…”

Project “Biodiversity MARE Tricase”: A Species Inventory of the Coastal Area of Southeastern Salento (Ionian Sea, Italy)

“…The MARE Outpost was established in Tricase (southeastern Salento, Apulia, Italy) to monitor the local coastal and marine biodiversity and to investigate the phenology and behaviour of marine organisms [24,25]. The remarkable biological uniqueness of this stretch of coastline has been recognized since the 1970s [26,27] and was confirmed by recent findings [28].…”

“…For 63 of the taxa found, phenology traits were also reported. These included the presence of reproductive structures (31), the occurrence of seasonal organisms (25), and instances of reproductive behaviour (6) (Tables 1 and S1).…”

Development and Phenotypic Plasticity of Tubes and Tubaria of the Living Graptolite Rhabdopleura recondita (Pterobranchia, Hemichordata)

“…A colony starts from a larva that secretes a dome (or prosiculum), where it metamorphoses into the foundational sicular zooid. The zooid emerges from the prosiculum and then secretes, using glandular cells on the ventral cephalic shield, the first tube of the colony [2,3]. Stebbing [2] provided information on the early steps of tubarium development in R. compacta.…”