Shark pit organs: enhancement of mechanosensitivity by potassium ions

The mandibular pit organs of pelagic sharks, which respond sensitively to monovalent cations, often show neural discharges synchronized with respiratory gill movement. The mechanosensitivity of the organs is remarkably enhanced by application of potassium ions on the same end organ, respiratory movement remaining constant. In view of their michanosensitivity to an incr-ease of potassium ions in the cell environment, as well as their chemosensitivity,the pit organs of sarks, rather than the canal organs which have no chemosensitivity, may be designated as a better model of the inner ear of higher animals.     

Shark pit organs: response to chemicals

Nerve fibers from pit organs and canal neuromasts are distinguished by the nature of their electrophysiological response to mechanical and chemical stimulation. Pit organs respond to touch but have a relatively high threshold compared with canal neuromasts. They respond readily to sodium and potassium chloride solutions, the rate of discharge increasing with the concentration of the solution. Order of effectiveness with 1 molar solutions of monovalent cations is as follows: potassium, rubidium > sodium, ammonium > cesium, lithium. Anions are ineffective. Divalent cations such as calcium and magnesium are inhibitory. Responses to acid, sugar, and quinine are either very slight or inhibitory.     

Geographical variation in the skeletal morphology of red jungle fowl

1. The skulls and postcranial skeletons of the red jungle fowl (Gallus gallus) were compared osteometrically between the populations from North and South Vietnam, North and Central Laos and Southeast Bangladesh. The populations include the three subspecies of G. g. spadiceus, G. g. gallus and G. g. murghi and were sampled to reveal the geographical morphological variations among populations in G. gallus.

2. The morphometric characteristics of subspecies murghi could be clearly distinguished from those of the other subspecies using a canonical discriminant analysis. However, the size and shape of the skull of the gallus population from South Vietnam were not statistically different from that of the subspecies spadiceus from North Laos. The canonical discriminant scores also clearly indicated that there were morphological similarities in the skulls of the populations from North Laos and South Vietnam.

3. From the results, therefore, it is concluded that red jungle fowls do not exhibit high levels of osteometric variation between geographical localities at least within the Indochinese Peninsula.

4. This contrasts with previous studies which have described these subspecies as having various external morphological differences and have argued that zoogeographical barriers exist between the north and south areas of the Indochinese Peninsula.