Ultrastructure, number, distribution and innervation of electroreceptors and mechanoreceptors in the bill skin of the platypus, Ornithorhynchus anatinus.

The platypus is presently the only mammal demonstrated to use electroreception to obtain food. The electroreceptive system of the platypus is far more complex than that of its close relative the echidna. This paper presents an anatomical study of the basis of electroreception in the platypus. The innervation of the bill by the trigeminal nerve is described, as are three sensory structures, associated with food gathering, within the bill skin. There are 40,000 mucous gland electroreceptors found in the bill skin of the platypus. The papillary portion of each of these sensory mucous glands is modified to accommodate electrosensory nerve terminals. In contrast to fish electroreceptors, the electrosensory terminals of the platypus are not associated with a sensory cell. These mucous gland electroreceptors are arranged in a series of parasagittal stripes on the bill. This array suggests a basis for the ability of the platypus to quickly and accurately locate the origin of an electrical stimulus. A push-rod mechanoreceptor, similar in morphology to Eimer's organ of the mole, and bill-tip organs in birds, was also found in the bill skin. The slightly differing morphology of these mechanoreceptors when compared to their avian and talpid counterparts suggests that this is another example of convergent evolution, with the common need to provide a solution to increasing tactile sensitivity on bare rhinarial skin. These push-rods are found to be most dense around the labial margins of the bill, with a marked decrease in density towards the middle and caudal portions of the bill. The distribution of the push-rods is similar to the distribution of the third sensory structure found on the bill, the sensory serous gland. Although less numerous than the mechanoreceptors (46,500 mechanoreceptors compared with 13,500 sensory serous glands), these sensory serous glands have a similar distribution and similar changes in density. These concurrent distributions argue for some functional correlation of these two sensory structures. The papillary region of the serous gland is modified in a manner similar to that of the mucous gland electroreceptor to accommodate sensory input. The sensory terminals of the serous glands are very similar to those of the mucous gland electroreceptors, and so it is presumed that these sensory serous glands are a type of electroreceptor that might be involved in detection of electrical signals at close quarters where the mechanoreceptors are also engaged.