The intramandibular joint in Girella: a mechanism for increased force production?

Intramandibular joints (IMJ) are novel articulations between bony elements of the lower jaw that have evolved independently in multiple fish lineages and are typically associated with biting herbivory. This novel joint is hypothesized to function by augmenting oral jaw expansion during mouth opening, which would increase contact between the tooth-bearing area of the jaws and algal substratum during feeding, resulting in more effective food removal from the substrate. Currently, it is not understood if increased flexibility in a double-jointed mandible also results in increased force generation during herbivorous biting and/or scraping. Therefore, we selected the herbivore Girella laevifrons for a mechanical study of the IMJ lower jaw lever system. For comparative purposes, we selected Graus nigra, a non-IMJ-bearing species, from a putative sister genus. Shortening of the lower jaw, during flexion at the IMJ, resulted in a more strongly force-amplifying closing lever system in the lower jaw, even in the absence of notable changes to the sizes of the muscles that power the lever system. To explain how the IMJ itself functions, we use a four-bar linkage that models the transmission of force and velocity to and through the lower jaw via the IMJ. When combined, the functionally interrelated lever and linkage models predict velocity to be amplified during jaw opening, whereas jaw closing is highly force modified by the presence of the IMJ. Moreover, the function of the IMJ late during jaw closure provides enough velocity to detach sturdy and resilient prey. Thus, this novel jaw system can alternate between amplifying the force or the velocity exerted onto the substrate where food items are attached. This unique mechanical configuration supports the argument that IMJs are functional innovations that have evolved to meet novel mechanical challenges and constraints placed on the feeding apparatus by attached and sturdy food sources.