The capsular organ of Chelyosoma productum (Ascidiacea: Corellidae): a new tunicate hydrodynamic sense organ.
Chelyosoma has about 200 sense organs in the atrial wall of the branchial sac that show structural features suggestive of a role in hydrodynamic sensing. They consist of a fluid-filled capsule with an acellular diaphragm spanning an opening in the top. The floor of the capsule is a sensory macula, with 5-6 primary sensory neurons whose cilia project into the capsular cavity and whose axons go to the brain. Animals were tested for vibrational sensitivity using a loudspeaker probe. They responded by muscular contractions of the siphons and/or arrests of the branchial cilia, both of which caused changes in water flow velocity through the siphons, which were monitored non-invasively using a thermistor flow meter. The animals showed peak sensitivity at 240-260 Hz. Subsequent calibration of the loudspeaker probe indicated that the animals could detect 104 decibels re 1 microPa from a source 80 cm away. The majority of axons leaving the capsular organs go to the brain via the visceral nerve. Cutting this nerve abolished or greatly reduced responses. Electrophysiological recordings from the distal nerve stump showed bursts of electrical events following vibrational stimulation. No such records could be obtained from other major nerves and cutting them did not affect responsivity. Corella inflata, a close relative of Chelyosoma, lacks capsular organs and failed to show any responses when the source was more than 3.5 cm away. We conclude that Chelyosoma's vibration-sensing ability is due to its capsular organs and is adaptive in terms of detecting the movements of objects in the vicinity. The findings are discussed in relation to the evolution of hydrodynamic mechanoreceptors in tunicates, amphioxus and craniates.
Mackie
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Biology Department, University of Victoria, Victoria, B.C., Canada. mackie@uvic.ca
Singla
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CL
D000818:Animals; D008465:Mechanoreceptors / Q000502:physiology; D008854:Microscopy, Electron; D009475:Neurons, Afferent / Q000648:ultrastructure; D012679:Sense Organs / Q000648:ultrastructure*; D014561:Urochordata; D014732:Vibration