Refine
Year of publication
- 2012 (1) (remove)
Document Type
- Doctoral Thesis (1)
Language
- English (1) (remove)
Has Fulltext
- yes (1)
Is part of the Bibliography
- no (1)
Keywords
- Arrowworms (1) (remove)
Institute
Chaetognaths are a fascinating taxon with unique features and a great impact on marine food webs as primary predators of zooplankton. Their phylogenetic position has been subject to many speculations ever since their discovery and even contemporary phylogenomic methods have not yet been able to suggest a stable hypothesis on their phylogenetic position within the Bilateria. Neuroanatomical studies may contribute new aspects to this discussion. This study aims to provide new insights into the chaetognath nervous system using a fresh set of methods to determine characters for a phylogenetic discussion. The method of choice in this case was immunohistochemistry combined with confocal microscopy. Experiments were conducted with a host of antibodies. The most effective target antigenes were RFamides (a family of neuropeptides), synapsins (synaptic proteins), tyrosinated tubulin (a cytoskeletal element, especially in neurites) and BrdU (bromodeoxyuridin, a proliferation marker). Each of those markers was of great use in highlighting certain aspects of the nervous system. A fresh look at the development of juvenile chaetognaths shortly after hatching revealed that the ventral nerve center (VNC) is developing earlier than the brain and that the production of neurotransmitters has already started at hatching. Specifically, some neurons exhibit RFmide-like immunoreactivity (ir). Neurogenesis continues for about five days after hatching and the mode of division in the neuronal stemcells is asymmetrical. In adult chaetognaths, the brain is divided into a stomatogastric anterior and a sensory posterior neuropil domain. It contains a set of individually identifiable neurons that exhibit RFamide-like ir. The study highlights the interspecific variation of brain architecture between representatives of spadellids and sagittids. The VNC consists of two lateral bands of somata that flank a central neuropil. Within the VNC exists a serial arrangement of neurons with RFamide-like ir. A variety of other neurotransmitters and related substances are also present in both, the brain and the VNC. More interspecific differences and similarities were explored in another part of the study, comparing even more different chaetognath species and focusing on the VNC and its internal structure. The two species of Krohnitta have an unusual distribution of nuclei that is not clearly separated into two lateral bands like in other species. Many of the sagittid species exhibit a striation pattern of the neuropil that is mostly absent in other groups and some of their nerve nets show varying degrees of order as opposed to the rather disorganized nerve net in other groups. In addition, immunohistochemical methods were applied to several specimens of Gnathostomula sp. in order to test one of the many hypotheses about the chaetognaths phylogenetic position, a sister-group relationship to gnathostomulids. A comparison between the two taxa, taking into account also other gnathifera and platyhelminthes, makes a sistergroup relationship between chaetognaths and gnathostomulids very unlikely. In conclusion, chaetognaths remain in an enigmatic phylogenetic position and likely branched off close to the deuterostome/protostome split.