Saccoglossus kowalevskii
Creators: name: Wanninger, Sabrina Kaul-Strehlow Makoto Urata Takuya Minokawa Thomas Stach Andreas
Publisher: name: Zenodo
Year: 2015
Subjects: Biodiversity
Taxonomy
Animalia
Hemichordata
Enteropneusta
Harrimaniidae
Saccoglossus
Saccoglossus kowalevskii

description: Saccoglossus kowalevskii: dorsal kink stage   
Embryos at this stage are of elongate shape measuring approximately 700 μm in maximum length (Fig. 6a). The anterior proboscis region is of ovoid shape with a tapering anterior tip and is subdivided from the posterior body regions by a deep circular constriction (Fig. 6a, d). The proboscis region is covered by short cilia, whereas at the anterior tip, a ciliary tuft composed of long cilia is present (Fig. 6a, c, d). A second, yet shallow circular groove is discernable within the prospective collar region (Fig. 6d). The opisthotroch is composed of compound cilia of multiciliary cells and demarcates the posterior perianal field (Fig. 6a, d).  
Serotonin-LIR reveals the presence of several bipolar neurons that are located circumferentially around the midlevel of the proboscis region (Fig. 6a). These bipolar neurons project a proximal neurite into a basiepidermal plexus of the proboscis region. A distal neurite connects to the apical cell surface where a single cilium is present. α- Tubulin-LIR reveals an apical ciliary tuft at the anterior tip; yet, no serotonin-LIR somata are detectable within the apical region close to the apical tuft (Fig. 6a, c). Throughout the prospective collar and trunk region, basiepidermal serotonin-LIR neurites form a loose network that thins out posteriorly into individual neurites within the perianal field (Fig. 6b). No traces of a serotonin-LIR opisthotroch neurite bundle are present within the trunk region, although tubulin staining reveals the existence of an opisthotroch (Fig. 6a).  
 S. kowalevskii: 1-gill-slit hatchling   
The 1-gill-slit hatchling measures between 800 μm and 1 mm in length when fully expanded (Fig. 6e, h). The animals exhibit a conical proboscis with an apical tuft composed of long cilia at the anterior tip (Fig. 6f, h). The opisthotroch is present within the trunk region, just posterior to the position of the first pair of dorsolateral gill pores (Fig. 6h). The posterior trunk region including the perianal field is elongated ventrally to a total length of 150 μm (Fig. 6e, h).  
 In transmission electron micrographs, numerous basiepidermal neurites of different diameters, exhibiting oblique to oval profiles, are present in 1-gill-slit hatchlings of S. kowalevkii (Fig. 7a–c). The high number of neurites composes a continuous and circumferential neurite layer of 2- to 4-μm thickness (Fig. 7b, c).   
 S. kowalevskii: 3-gill-slit juvenile   
The juveniles at this stage of development resemble the adult acorn worms in many aspects (Fig. 6k, l). Obvious differences are a lower number of gill slits and the presence of the postanal tail, a feature only present in juvenile acorn worms of the family Harrimaniidae (Cameron 2005; Stach and Kaul 2012). Scanning electron micrographs of the anterior tip of  
the proboscis show that the former ciliary tuft is degraded (Fig. 6k, l). The opisthotroch is partially altered and has transformed into a ventral creeping sole.  
 Within the proboscis and collar region, a serotonin-LIR basiepidermal plexus is present. The concentration of neurites is denser at the dorsoposterior base of the proboscis, constituting the proboscis plexus (Fig. 6k, o). Serotonin-LIR bipolar neurons are distributed throughout the epidermis of the proboscis with the exception of the most anterior region. Each soma has a bulbous central part containing the nucleus and a slender distal neurite reaching the apical cell surface, where a single cilium is present (Fig. 6n). Basally from the bulbous part, each soma sends a proximal neurite into the basiepidermal plexus. The dorsal proboscis stem sends two serotonin-LIR neurite bundles posteriorly into the subepidermal collar cord, thereby passing the proboscis neck region. At the posterior end of the collar region, the serotonin-LIR becomes superficial again and continues into a dorsal neurite bundle (Fig. 6m, o). Unlike B. misakiensis, S. kowalevskii lacks mesocoelic pores as well as a prebranchial nerve ring at this stage of development. A vast number of serotonin-LIR bipolar neurons are present within the epidermis of the anterior half of the collar region (Fig. 6k, o). The architecture of the serotonin-LIR nervous system within the trunk region differs considerably from that described for the preceding hatchling stage. Serotonin-LIR is present in a longitudinal ventral as well as dorsal neurite bundle along the trunk region (Fig. 6k, m). The ventral bundle is more extensive and extends to the posterior end of the postanal tail. The dorsal serotonin-LIR neurite bundle is less voluminous and is composed of few neurites only. It runs along the dorsal midline to the anus, with an interruption of the serotonin-LIR signal in the middle region of the trunk (Fig. 6k, m). Several serotonin-LIR bipolar neurons are present within the trunk region (Fig. 6m). The majority of the somata are located dorsolaterally, sending a proximal neurite into the ventral nerve cord (Fig. 6m). The number of serotonin-LIR somata in the trunk region is particularly low compared to the collar and proboscis region. Between the gill pores, a higher density of serotonin-LIR neurons is present. Notably, serotonin-LIR does not show a plexus-like pattern in the trunk region as in earlier stages.   
Ultrastructural investigations of transmission electron micrographs of a 2-gill-slit juvenile show that only scattered neurites are present below the one-layered epidermis (Fig. 7d–f). The majority of neurites is concentrated within the dorsal and ventral nerve cord (read also Kaul and Stach 2010).
description: Published as part of Wanninger, Sabrina Kaul-Strehlow Makoto Urata Takuya Minokawa Thomas Stach Andreas, 2015, Neurogenesis in directly and indirectly developing enteropneusts: of nets and cords, pp. 405-422 in Organisms Diversity & Evolution 15 (2) on pages 412-416, DOI: 10.1007/s13127-015-0201-2, http://zenodo.org/record/12764142