ORIGINAL PAPER Allometric shape change of the lower pharyngeal jaw correlates with a dietary shift to piscivory in a cichlid fish Christoph J. Hellig & Michaela Kerschbaumer & Kristina M. Sefc & Stephan Koblmüller Received: 26 March 2010 / Revised: 18 May 2010 / Accepted: 19 May 2010 / Published online: 8 June 2010 # Springer-Verlag 2010 Abstract The morphological versatility of the pharyngeal jaw of cichlid fishes is assumed to represent a key factor facilitating their unparalleled trophic diversification and explosive radiation. It is generally believed that the functional design of an organism relates to its ecology, and thus, specializations to different diets are typically associated with distinct morphological designs, especially manifested in the cichlids’ pharyngeal jaw apparatus. Thereby, the lower pharyngeal jaw (LPJ) incorporates some of the most predictive features for distinct diet-related morphotypes. Thus, considering that piscivorous cichlids experience an ontogenetic dietary shift from typically various kinds of invertebrates to fish, concomitant morpho- logical changes in the LPJ are expected. Using Lepidio- lamprologus elongatus, a top predator in the shallow rocky habitat of Lake Tanganyika, as model, and applying geometric and traditional morphometric techniques, we demonstrate an allometric change in ontogenetic LPJ shape development coinciding with the completion of the dietary shift toward piscivory. The piscivorous LPJ morphotype is initiated in juvenile fish by increasing elongation and narrowing of the LPJ and—when the fish reach a size of 80–90 mm standard length—further refined by the elonga- tion of the posterior muscular processes, which serve as insertion for the fourth musculus levator externus. The enlarged muscular processes of the fully mature piscivorous morphotype provide for the construction of a powerful lever system, which allows the large individuals to process large prey fish and rely on exclusive piscivory. Keywords Lepidiolamprologus elongatus . Feeding apparatus . Trophic specialization . Lake Tanganyika . Ontogeny . Allometric growth Introduction The pharyngeal jaw apparatus (PJA) of cichlid fishes (Perciformes: Cichlidae), a derived morphological feature shared with the Embiotocidae, Pomacentridae, and Labridae (Yamaoka 1978; Liem 1986; Liem and Sanderson 1986; Stiassny and Jensen 1987; Drucker and Jensen 1991), is a functionally integrated and highly specialized system and considered to be a major adaptive complex (Liem 1973). Decoupled from the oral jaws, the pharyngeal jaws (PJs) are used for efficient crushing and processing of food items and thus freed the oral jaws from their dual task of food collection and preparation. This division of functions permitted the development of numerous specializations of food collection and procession mechanisms (Liem 1973). Minor modifications in their structure afford the utilization of novel food resources within a few generations, so that unexploited ecological niches can be rapidly occupied (Stiassny 1991; Sturmbauer 1998; Albertson et al. 1999). Presumably, ready differentiation in trophic specializations allowed for effective resource partitioning and contributed to the evolution of complex cichlid communities by ecological segregation (e.g., McKaye and Marsh 1983; Barluenga et al. 2006). Considered a key innovation, the PJA is therefore believed to represent a key factor for the emergence of a diversity unparalleled among vertebrates (Liem 1973; Greenwood 1973; Galis and Metz 1998; Hulsey Electronic supplementary material The online version of this article (doi:10.1007/s00114-010-0682-y) contains supplementary material, which is available to authorized users. C. J. Hellig : M. Kerschbaumer : K. M. Sefc : S. Koblmüller (*) Department of Zoology, Karl-Franzens-University Graz, Universitätsplatz 2, 8010 Graz, Austria e-mail: stephan.koblmueller@uni-graz.at Naturwissenschaften (2010) 97:663–672 DOI 10.1007/s00114-010-0682-y