Intramandibular Joint in Helostoma temmincki: Relationship with Quadratomandibular Joint and Lateral Expansion

The intramandibular joint (IMJ) in Helostoma temminckii, also known as kissing gourami, is located in the lower jaw at the articulation of the dentary and angular-articular bones. These fish are known for a ‘kissing’ behavior, facilitated by the IMJ, which allows the jaws to protrude and rotate making the mouth appear to look like pursed lips. The IMJ has independently evolved in the teleost lineage multiple times, and the mechanics are slightly different in each lineage. In kissing gourami, there are no muscular connections to the region of the lower jaw anterior to the IMJ, thus the IMJ is something of an enigma in terms of how it is mobilized during feeding, and moreover returned to a resting state after feeding. High-speed video was used to examine lateral expansion particularly in the regions of the intramandibular joint (IMJ) and the quadratomandibular (QMJ) joint. Individuals were filmed from a lateral and ventral view simultaneously during food capture. From the data, it appears that the expansion at the IMJ contributes to lateral expansion of the head during food capture. Lateral expansion at the location of the IMJ appears to occur in tandem with rotation about that joint; however, lateral expansion in this species is also rather slight due to their extremely rigid heads.