The novel ‘tongue‐bite apparatus’ in the knifefish family Notopteridae (Teleostei: Osteoglossomorpha): are kinematic patterns conserved within a clade?

Osteoglossomorph fishes are unique in possessing a specialized feeding mechanism, the tongue‐bite apparatus (TBA) involving the hyoid apparatus. The TBA is associated with two unique behaviour patterns ‐ raking and open‐mouth chewing ‐ used to disable and macerate prey. The kinematics of these two behaviours was compared in two species of knifefishes (family Notopteridae): Xenomystus nigri (Gunther, 1868) and Chitala ornata (Gray, 1831) using high‐speed video (250 frames s“ 1 ). Both univariate and multivariate analyses indicated that there were significant interspecific differences in both raking and open‐mouth chewing. Raking can be divided into three stages; the preparatory phase, power stroke, and recovery phase. During the power stroke posterior motion of the pectoral girdle and neurocranial elevation both appear to play a major role in prey reduction. In Xenomystus the power stroke involves significantly greater levels of neurocranial elevation (35 o ) and pectoral girdle motion (38% of head length; 9.5 o ) than that found in Chitala (neurocranial elevation 11 o ; pectoral girdle motion 11% of head length and 5 o ). Indeed, Xenomystus represents the most extreme raking behaviour of any osteoglossomorph thus far studied. Temporal displacement variables demonstrated that the power stroke in Xenomystus is significantly faster than in Chitala. Although some of the interspecific differences might be size related, these data suggest that a greater degree of difference exists in these highly specialized behaviours than previous work has demonstrated. These findings support the notion that biomechanical duplication (an additional ligament found in osteoglossomorphs) could be linked to increased functional versatility.