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date: 27 June 2017

Urochordate Nervous Systems


Urochordates are chordate siblings that comprise the following marine invertebrates: the sessile Ascidiaceae, or sea squirts; planktonic Larvacea; and the pelagic salps, doliolids, and pyrosomes (collectively the Thaliacea), each more beautiful than the next. Tadpole larvae of ascidians and adult larvaceans both have a body plan that is chordate, with a notochord and dorsal, tubular nervous system that forms from a neural plate. Salps have well developed eyes, but otherwise their central nervous system is little known, while the neurobiology of other Thaliacea is little reported. The ascidian tadpole larva is extensively reported, especially in the model species Ciona intestinalis, as is the caudal nerve cord in the larvacean Oikopleura dioica.

Chordate features that share proposed homology with vertebrate features include ciliary photoreceptors that hyperpolarize to light, descending decussating motor pathways that resemble Mauthner cell pathways, coronet cells in both the ascidian larva, and saccus vasculosus of fishes; a Reissner’s fiber in the neural canal of both urochordates and vertebrates; secondary mechanoreceptors that resemble hair cells; and ascidian bipolar cells that resemble dorsal root ganglion cells.