ISSN:
1432-1351
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Medicine
Notes:
Summary 1. Potamotrygon lacks ampullae of Lorenzini (defined by their long canals), otherwise general for elasmobranchs. There are present however microscopic ampullary organs with extremely short canals. A brief histologic description is provided, together with counts of their abundance in various parts of the body. They are chiefly concentrated ventrally in the head region. 2. The skin has a relatively high resistance compared to marine rays. This is measured in the physiologically significant way, by measuring the potential distribution in and around a living ray placed in a homogenous electric field. 3. The microscopical size of the ampullary organs and the high skin resistance are believed to be a specialization maintaining the electroreceptive function in the low conductivity, fresh water medium. 4. These rays are shown to be responsive to d.c. and low-frequency a.c. electric fields. They give specific movements seemingly related to feeding. They seem to be less sensitive than marine sharks and rays. The threshold stimulus is probably less than 120 μV/cm (corresponding to 0.03 μA/cm2 with water resistivity of 4 kOhm · cm). 5. Potamotrygon circularis appears to lack Savi's vesicles. However, an organ which may be equivalent is a tubular, subcutaneous, receptor in the Submandibular region. It does not open to the outside or connect to the skin or to the skeleton. Its spontaneous background nerve impulses and the increases in firing with mechanical stimuli are described.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00693615
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