SWANSON, B.O.: Intra-specific competition in a polymorphic cichlid, Herichthys minckleyi: Does variation in jaw morphology lead to resource partitioning?

At least two distinct prey processing morphotypes are observed in the Cuatro CiÉnegas cichlid (Herichthys minckleyi). One morphotype has gracile pharyngeal jaws with papilliform pharyngeal teeth. The other morphotype has robust pharyngeal jaws and molariform pharyngeal teeth. In this experiment, we test the hypothesis that these morphotypes reduce intraspecific competition. Competitive interactions were examined within and between the morphotypes using field enclosures. High density and low-density cage treatments containing a single morphotype were used to quantify within-morph competition. Cages containing both morphotypes were used to quantify between-morph competition. Results indicate that the morphotypes show similar levels of within-morph competition. The mixed-morph treatments indicate that the competitive effect of papilliforms on molariforms is higher than the effect of molariforms on papilliforms. Under these experimental conditions, the polymorphism reduced intra-specific competition for papilliforms, but not for molariforms. Feeding behaviors were digitally recorded in the field to examine the behavioral mechanisms underlying resource partitioning. Feeding events were placed into behavioral categories based on kinematic variables. These categories included suction feeding on soft or hard substrates, scraping material off of hard substrates, and diving or scooping in soft substrates. Molariform individuals performed diving behaviors more than expected and papilliform individuals performed scraping and scooping behaviors more than expected. Together, these experiments suggest that morphological and behavioral differences between the morphs allow them to exploit different food resources.