1. Figure 2: Illustration of the different Mechanical Advantages. 1: articulo-angular; 2: dentary; 3: premaxillary; 4: maxillary; 5: palatine. Comparison of the adductor mandibulae and jaw lever mechanisms between three damselfishes (Pomacentridae) Laboratoire de Morphologie Evolutive et Fonctionnelle, Institut de Chimie (B6c) Université de Liège, 4000 Liège, Belgique. Corresponding author: dolivier@student.ulg.ac.be Damien OLIVIER, Bruno FREDERICH and Eric PARMENTIER Feeding ability is determined by functional morphology of the trophic apparatus because it constrains the nature of consumed food. One way to understand the structural bases of the generation and transmission of force and motion during feeding is to realize comparative studies between closely related species having different food habits. In this study we chose to make comparisons of the adductor mandibulae, which is muscle that closes the mouth in all teleost, between three species of damselfishes. We chose this family because of their great ecological and food diversity. The goal was to determine whether the food diversity in this family was related to any mechanic and/or anatomical differences in the adductor mandibulae of the different species. Dissections were made on three species of damselfishes (five adults/species) ; two herbivorous that graze algae: Plectroglyphidodon lacrymatus and Abudefduf sordidus ; and one zooplanktivorous: Dascyllus trimaculatus. Each bundle of the adductor mandibulae muscle (A 1, A 2 and A 3, see Figure 1) was weighed and their respective Mechanical Advantage (MA) (Figure 2) values were calculated in each species. The MA is the ratio between an in-lever and an out- lever. The in-lever is the distance between the angular-quadrate joint and where a bundle inserts on the jaws. The out-lever is the distance between the angular-articulate joint and the tip of the lower jaw. High MAs indicate high force transmission and low MAs indicate high velocity transmission. In other fish families, it has already been shown the MAs values were different according to bundles of the adductor mandibulae. Generally, A 1 and A 2 are considered as force transmitter and A 3 as a speed transmitter. Adductor A 3 was the heaviest and A 2 the lightest bundle in each species (Figure 3). A 3 had the smallest MA value and A 1 had the highest MA value in each species (Figure 4). Despite these similarities, we found out many differences according to the trophic diversity as summed up in table 1. Plectroglyphidodon lacrymatus Dascyllus trimaculatus Abudefduf sordidus Results & Discussion Materials & Methods Introduction A 3 was proportionally more developed and had lower MA value in the zooplanktivorous D. trimaculatus than in both herbivorous species: faster mouth closing is required to capture efficiently evasive preys. Conversely, herbivorous need forceful mouth closing to graze benthic algae: their A 1 and A 2 are more developed. However, conversely to A 2, A 1 had higher MA value in the zooplanktivorous species, future studies are necessary to investigate and understand this result. Figure 1: Illustration of the bundles of the adductor mandibulae (A1-A3) in a lateral view of the cranium in the three species. Figure 4: A1, A2,and A3 levers. Figure 3: Relative importance of A1, A2 and A3 bundles. Table 1: Comparison between herbivorous and planktivorous species HerbivorousZooplanktivorous Bundles MAs A1A2A3A1A2A3 A1A2A3A1A2A3 Conclusion System built to increase force transmission System built to increase speed transmission + ++ - ++ - + ++ - High MA  Force transmitter Low MA  Speed transmitter