Ecomorphology of Poeciliids in the Southwest

Authors: Amber Jones, Alice C. Gibb, Matthew W. O’Neill

Year: 2013 (xlv)

Abstract

The Intramandibular joint (IMJ), a second joint within the mandible, has evolved independently in many herbivorous salt water and fresh water fish. It is thought that the IMJ makes feeding off of algae and other encrusted food items more efficient. We believe that the Gila topminnow, Poeciliopsis occidentalis, has evolved an intramandibular joint to facilitate substrate-based feeding. In this study we compared the diet, feeding behavior, and the morphology of the lower jaw of Gila topminnow, with invasive mosquito fish, Gambusia affinis, and shortfin molly, Peociliea mexicana, to determine if the lower jaw of the topminnow is more similar in anatomy and function to the mosquito fish (non-bending jaw) or to the shortfin molly (bending jaw). Clearing, staining, and dissections were used to study the morphology of the lower jaws and diet information about all three species was culled from the literature. Based on our analyses, the Gila topminnow possesses an IMJ that is similar to that of the shortfin molly. The mosquito fish, on the other hand, does not have an IMJ. The diets of the mosquito fish and the Gila topminnow are similar, but the Gila topmminow feeds on attached algae when resources are limited, a behavior that is enhanced by the IMJ. This finding suggests that habitats that have large quantities of algae, but relatively few aquatic invertebrates, may favor the survival of Gila topminnow.