GIBB, A.C.; BLANTON, J.M.; Northern Arizona University; Amherst College: Is there a free lunch? Elastic recoil and herbivory in Poecilia sphenops

Many species of teleost fish, including members of the Scaridae, Kyphosidae and Blennidae, have evolved a second lower jaw joint. This joint is located between the dentary and the angular-articular complex and termed the intramandibular joint. Many of the taxa that possess this mechanism use it for scraping periphyton (i.e., benthic autotrophs) from the substrate. Interestingly, this joint has evolved independently in all of these taxa and appears to maximize contact between the teeth of the mandible and the substrate. In Poecilia sphenops (Poeciliidae), the two bony elements of the mandible are bridged by a flexible, cartilaginous rod (Meckel�s cartilage). During jaw opening, the anterior bony element is pulled ventrally by the m. protractor hyoidei. The posterior jaw element is immobile, and the cartilage connecting the two bony elements flexes to produce greater than 90° of bending. During mouth closing, the lower jaw rapidly returns to its original conformation. This unusual morphology creates a unique jaw mechanism: energy is stored in the cartilage rod during jaw depression and elastic recoil provides energy for jaw elevation. We hypothesize that this mechanism reduces the net metabolic cost of feeding for P. sphenops, perhaps facilitating the exploitation of a relatively low-calorie food source. To our knowledge, no other vertebrate possesses a joint that functions in this manner.