The molly (Poecilia sphenops) has an unusual feeding mechanism among fishes in that its jaw is essentially “doublejointed”. They possess an extra joint within the lower jaw, or mandible, called an Intramandibular Joint (IMJ). Because of this joint, these fish are capable of opening their mouths to extreme angles. We wanted to understand how this mechanism functions during feeding and if the joint angles could be actively altered by the fish in different feeding situations. We asked specifically if there was a difference in the gape angle (the angle formed by the tip of upper jaw, the jaw joint, and the tip of the lower jaw) and intramandibular joint angle when offered food in the water column versus on the substrate, as well as with different food types in the water column. Feeding events were captured using high-speed video and the resulting footage analyzed using the software Image J. The average gape angle produced during midwater feedings was significantly larger than that produced during benthic feedings. Similarly, the average IMJ angle produced during midwater feedings was significantly larger than that produced during benthic feedings. This suggests that the joint angles are actively modulated in response to different feeding scenarios. Observations of benthic-feeding habits, including number of feeding attempts per hour, were also recorded to understand how often the jaws are experiencing bending at the IMJ. This speaks to the potential for subsequent longer-term deformation of the underlying cartilage and muscle based upon the stress and strain that the cartilage is undergoing daily. When observing benthic-feeding habits, it was found that most feeding, and therefore IMJ bending, occurred at midday and during the last two hours of the day and could exceed over 100 attempts per hour.
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