The expansion apparatus in fish heads, a 3-D kinetic deduction

Among teleost fishes, suction feeding is the most frequent mode of obtaining food. Using a 3-D kinematic model of the suction apparatus, it is calculated for what quantitative combinations of the model's static parameters three hydrodynamic demands pertinent to suction feeding could be optimized under certain anatomical constraints. Based on these calculations and constraints each of the considered parameter value sets can be categorized as optimal, suboptimal or overruled. The morphospace thus subdivided is called the evaluated morphospace. It demonstrates, for instance, that optimal sets are not randomly distributed in the morphospace but exhibit a certain pattern implying an interdependency of the parameter values. Of 73 East-African, lacustrine cichlid species the parameter value sets were plotted in the evaluated morphospace where these species-specific sets appear to be located in a narrow zone around the border between optimal and suboptimal sets but quite far away from the overruled sets. Remarkably, of all optimal value sets theoretically feasible, a relatively small range is actually used by cichlids and possibly by teleosts in general. This and the border position of the value sets in the optimal domain suggest a constraint on optimizing the suction apparatus, but the nature of the constraint could not yet be detected. The importance of the results is based on the following considerations: (1) The 73 species investigated constitute a representative sample of a speciose and ecological diverse group, viz. the ca. 1500 cichlid species that make up the independently evolved flocks in the Great East-African Lakes. (2) Preliminary investigations indicate that the results hold also for noncichlids with a similar way of suction feeding. (3) The model may well explain the deviating value sets as observed among fishes with a different type of suction feeding.