THE ZOOLOGIST 



No. 831.— September, 1910. 



NOTES FEOM THE MILLPOBT MAEINE BIOLOGICAL 



STATION. 



By Bichard Elmhirst, Superintendent. 



On the Yawning of Fishes. 



While watching the fish living in our Aquarium, I have 

 repeatedly observed them yawning, or at any rate perform an 

 action like yawning, consisting of a wide opening of the mouth, 

 slow expansion of the buccal cavity, erection of the gill-arches, 

 followed by a rapid expulsion of the indrawn water, most of 

 which is emitted from the mouth, although some certainly goes 

 through the gill- slits. This is often accompanied by a distinct 

 heaving of the pectoral region and erection of the pectoral fins, 

 and is quite different from the rapid movement of the operculum 

 and jaws which is used to remove a foreign object, such as a bit 

 of seaweed, from the gills. At first one would think that 

 yawning is only possible for an air-breathing vertebrate with 

 lungs. However, from numerous observations, I am led to 

 think that this action of fishes is a real yawn, and serves the 

 true physiological purpose of a yawn, i. e. flushing the brain 

 with blood during periods of sluggishness. The conditions con- 

 ducive to yawning are a slight increase in the temperature of the 

 water and, I suppose, the accompanying diminution of oxygen. 

 For instance, on Saturday morning we flush a lot of water 

 through our tanks, and at midday leave the storage tank full. 

 Now, we usually use a tankful in twenty-four hours, but to 

 avoid pumping on Sunday we make the tankful last from 

 Zool. 4th ser. vol. XIV., September, 1910, 2 c 



