Are kissing gourami specialized for substrate-feeding? Prey capture kinematics of Helostoma temminckii and other anabantoid fishes.

Helostoma temminckii are known for a "kissing" behavior, which is often used in intraspecific interactions, and an unusual cranial morphology that is characterized by an intramandibular joint (IMJ). The IMJ is located within the lower jaw and aids in generating the eponymous kissing movement. In other teleost linages the IMJ is associated with the adoption of a substrate-grazing foraging habit. However, because of anatomical modifications of the gill-rakers, Helostoma has been considered a midwater filter-feeding species. We offered midwater, benthic, and attached food to Helostoma, Betta, and two "true" osphronemid gouramis, to ask: (1) how do food capture kinematics differ in different foraging contexts; and (2) are Helostoma feeding kinematics distinct when compared with closely related anabantoids that lack an IMJ? For all anabantoid species except Helostoma, benthic prey were captured using a greater contribution of effective suction relative to midwater prey, though Helostoma was rarely willing to feed in the midwater. Helostoma individuals produced relatively less suction than other species regardless of the food type. Helostoma produced a much larger gape and more premaxillary protrusion than other species, but also took longer to do so. We suggest that the jaw morphology of Helostoma facilitates an extremely large mouth-gape to enhance substrate-scraping. The large amplitude mouth-opening that characterizes substrate-feeding may represent a functional trade-off, whereby the enhanced ability to procure food from the substrate is accompanied by a concomitant reduction in the ability to produce suction.