Inter- and intraspecific plasticity in distribution patterns of immunoreactive compounds in actinotroch larvae of Phoronida (Lophotrochozoa)

Abstract

Recent molecular data place Phoronida within the protostome superclade Lophotrochozoa, where they have been suggested to form a monophyletic assemblage with Brachiopoda and/or Nemertea. Herein, the anatomy of the nervous system and the structure of the apical organ are described for two larval stages of Phoronis muelleri in order to contribute to the discussion concerning the evolution of lophotrochozoan nervous systems. Specimens were investigated by confocal laser scanning microscopy using antibodies against the serotonin‐like immunoreactive (serotonin‐lir), the FMRF‐amide‐like immunoreactive (FMRFamide‐lir) and the small cardioactive peptide B‐like immunoreactive (small cardioactive peptide B‐lir) compounds of the nervous system. Consistent with larvae of other phoronid species, we found a complex apical organ that consists of numerous serotonin‐lir flask‐shaped cells, additional bi‐ or multipolar serotonin‐lir cells and several FMRFamide‐lir perikarya. A detailed comparison between our results and those of a previous study on the same species shows significant differences in the innervation of the preoral lobe, the tentacles and the telotroch. Our work is the first to prove the presence of small cardioactive peptide B in phoronids. In larvae of P. muelleri, it is expressed in neurites along the margin of the preoral hood, in the mesosome, in the tentacles, in the trunk as well as in the apical organ. A positive signal for this peptide is also known from molluscs, annelids and arthropods, indicating that it was also part of the protostomian groundplan. In contrast to a recent study on another phoronid species, Phoronopsis harmeri, we did not find a ventral neurite bundle in the larval stages investigated herein, thus leaving the question open whether this structure was part of the phoronid groundplan or evolved de novo in P. harmeri.

Phoronida

“…Beyond the apical organ, signifi cant larval nerves include the main hood nerve and two dorsolateral nerves that run down into the collar (mesosome) region of the larval body and merge with the minor and major tentacle nerve rings. From these nerve rings, the larval tentacles (and sometimes the juvenile tentacle rudiments; Santagata and Zimmer 2002 ) are innervated on frontal and abfrontal sides (Hay-Schmidt 1989Lacalli 1990 ;Santagata 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013 ;Temereva and Tsitrin 2014 ). Basiepithelial fi bers also project from these nerve rings through the larval trunk epithelium and merge with a telotrochal nerve ring at the posterior end of the larva (Hay-Schmidt 1990 ; Santagata and Zimmer 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013 ).…”

“…Cellular domains within the apical organ include groups of cells that express serotonin (Hay-Schmidt 1990 ;Santagata 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013), catecholamines (Hay-Schmidt 1990Santagata 2002), FMRFamide (Hay-Schmidt 1990Sonnleitner et al 2013 ), and a cardioactive B-like peptide (Sonnleitner et al 2013 ). Serotonin-like immunoreactive cells include numerous fl ask-shaped cells that overlie other bipolar or multipolar neuronal cells (HaySchmidt 1990 ;Santagata 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013 ). Neuronal cells and fi bers that have catecholaminelike immunoreactivity are situated at the periphery of the apical organ (Hay-Schmidt 1990 ;Santagata 2002 ).…”

“…Neuronal cells and fi bers that have catecholaminelike immunoreactivity are situated at the periphery of the apical organ (Hay-Schmidt 1990 ;Santagata 2002 ). Only a few to perhaps ten bipolar neuronal cell bodies on dorsolateral sides of the apical organ have FMRFamide-like immunoreactivity, depending upon the species and larval stage (Hay-Schmidt 1990 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013 ). The FMRFamide-like immunoreactive cells do not appear to be sensory, as they lack cilia and their fi ber projections go to the apical neuropil and the tentacular neurite bundles, leading into the major tentacle nerve rings.…”

“…From these nerve rings, the larval tentacles (and sometimes the juvenile tentacle rudiments; Santagata and Zimmer 2002 ) are innervated on frontal and abfrontal sides (Hay-Schmidt 1989Lacalli 1990 ;Santagata 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013 ;Temereva and Tsitrin 2014 ). Basiepithelial fi bers also project from these nerve rings through the larval trunk epithelium and merge with a telotrochal nerve ring at the posterior end of the larva (Hay-Schmidt 1990 ; Santagata and Zimmer 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013 ). Although a possible vestigial ventral nerve cord has been reported for Phoronopsis harmeri (Temereva 2012 ), the cytoarchitecture of these neural fi bers has very little in common with the ventral nerve cords of molluscan or annelid larvae in that phoronid larvae do not have centralized ventral nerve cords with repeated neuronal cell bodies along them Denes et al 2007 ;Meyer and Seaver 2009 ).…”

“…The greatest concentration of neuronal cells in the larval nervous system is found in the apical organ, a U-shaped structure containing at least four different neuronal cell types, all of which send axonal fi bers into a central neuropil (Hay-Schmidt 1989Lacalli 1990 ;Santagata 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013 ;Temereva and Tsitrin 2014 ). Cellular domains within the apical organ include groups of cells that express serotonin (Hay-Schmidt 1990 ;Santagata 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013), catecholamines (Hay-Schmidt 1990Santagata 2002), FMRFamide (Hay-Schmidt 1990Sonnleitner et al 2013 ), and a cardioactive B-like peptide (Sonnleitner et al 2013 ). Serotonin-like immunoreactive cells include numerous fl ask-shaped cells that overlie other bipolar or multipolar neuronal cells (HaySchmidt 1990 ;Santagata 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013 ).…”