Trophic apparatus in cyprinodontiform fishes: functional specializations for picking and scraping behaviors

Cyprinodontiforms are a diverse and speciose order that includes topminnows, pupfishes, swordtails, mosquitofishes, guppies, and mollies. Sister group to the Beloniformes and Atheriniformes, Cyprinodontiformes contains approximately twice the number of species of these other two orders combined. Recent studies suggest that this group is well suited to capturing prey by "picking" small items from the water surface, water column, and the substrate. Because picking places unusual performance demands on the feeding apparatus, this mode of prey capture may rely upon novel morphological modifications not found in more widespread ram- or suction-based feeding mechanisms. To assess this evolutionary hypothesis, we describe the trophic anatomy of 16 cyprinodontiform species, selected to broadly represent the order as well as capture intrageneric variation. The group appears to have undergone gradual morphological changes to become increasingly specialized for picking and scraping behaviors. We also identify a suite of functional characters related to the acquisition of a novel and previously undescribed mechanism of premaxillary protrusion and retraction, including: modification of the "premaxillomandibular" ligament (which connects each side of the premaxilla to the ipsilateral mandible, or lower jaw), a novel architecture of the ligaments and bony elements that unite the premaxillae, maxillae and palatine bones, and novel insertions of the adductor muscles onto the jaws. These morphological changes to both the upper and lower jaws suggest an evolutionary trend within this group toward increased reliance on picking individual prey from the water column/substrate or for scraping encrusting material from the substrate. We propose that the suite of morphological characters described here enable a functional innovation, "picking," which leads to novel trophic habits.