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Comparative Functional Anatomy of the Pharyngeal Jaw Apparatus in Two Morphs of Astatoreochromis Alluaudi (Pisces, Cichlidae)

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Comparative Functional Anatomy of the Pharyngeal Jaw Apparatus in Two Morphs of Astatoreochromis Alluaudi (Pisces, Cichlidae) COMPARATIVE FUNCTIONAL ANATOMY OF THE PHARYNGEAL JAW APPARATUS IN TWO MORPHS OF ASTATOREOCHROMIS ALLUAUDI (PISCES, CICHLIDAE) by JAN D. SMITS, GERRIT CH. ANKER, FRANS WITTE and KEES D.N. BAREL (Research Group in Ecological Morphology,Institute of Evolutionaryand Ecological Sciences, van der Klaauw Laboratorium, RijksuniversiteitLeiden, Postbus 9516, 2300 RA Leiden, The Netherlands) ABSTRACT Phenotypic differences of the pharyngealjaw apparatus between morphs of Astatoreochromis alluaudi are well known. But only a limited number of elements have been described in detail. In this study an anatomical description of elements markedly differing between the two extremes of the phenotypicrange - the snail-eatingand the non-snail-eatingmorph - is provided, and compared with the anatomy of a generalizedHaplochromis species, H. elegans. Many skeletal elements and muscles of the pharyngealjaw apparatus show hypertrophyin the snail-eating(hypertrophied) morph. However,the elementsof the pharyngealjaw apparatusof the non-snail-eating(non-hypertrophied) morph, which mainly feeds on insects, are generally more hypertrophiedthan the elements in the obligate insectivorousspecies H. elegans. Many of the differences between the two morphs can be interpreted functionally as adaptations to the increased crushing force of the hypertrophied morph. Differences between the morphs were compared with literaturedata of differencesbetween non-durophagousand durophagous trophic types in other labroid taxa and in the Centrarchidae. Comparabledifferences are found for the pharyngealjaw bones and the two largest pharyngealmuscles. The functionaldemands of crushing hard prey types appear to be very similar for the different groups. The small muscles show only slight similarity. Demands of other functions may co-determine their shape. KEYWORDS: Astatoreochromis alluaudi, Haplochromis elegans, phenotypic plasticity, mor- phology, labroids, Centrarchidae. INTRODUCTION During the past decade phenotypic plasticity in cichlid fishes has become a focus of interest (WITTE, 1984; HOOGERHOUD, 1986, 1989; VAN DER MEER & ANKER, 1986; MEYER, 1987; WITTE et al., 1990; KORNFIELD, 1991; VAN DER MEER, 1991; WIMBERGER, 1991, 1992). Most attention has centered on the plasticity of the trophic apparatus as it relates to different feeding regimes. But in almost all cases only a very limited number of 314 structures have been taken into account. Anatomical descriptions of the differences are rarely given. In the present paper a comparative description is presented of the differences in anatomy between two environmentally induced trophic morphs of the cichlid fish Astatoreochromis alluaudi. Morphological variation in the pharyngeal jaw apparatus of A. alluaudi was first reported by GREENWOOD (1959). He subdivided A. alluaudi, which occurs in several East African lakes, into two subspecies mainly on the basis of the shapes and sizes of the pharyngeal bones and teeth. This subspe- cific division was dropped after examining a tank-raised specimen with little hypertrophied pharyngeal jaws (GREENWOOD, 1965). This specimen origi- nated from Lake Victoria (East Africa), where A. alluaudi always exhibits hypertrophy of the pharyngeal jaw apparatus. Environmentally induced polymorphism was recognised. The degree of hypertrophy appeared to be geographically correlated (GREENWOOD, 1959). Individuals with marked hypertrophy (the hypertrophied morph) were found in Lake Victoria while individuals with slight hypertrophy of the pharyngeal jaw apparatus (the non-hypertrophied morph) were recorded from Lakes Edward (Uganda) and Rugwero (Burundi, Rwanda) (GREENWOOD, 1965; HOOGERHOUD, 1986). It was suggested that this variation resulted from differences among local food regimes. In Lake Victoria the dominant food items are the hard-shelled gastropods Melanoides tuberculata and Bellamya unicolor, and the bivalves Sphaerium stuhlmanni and Caelatura hauttecoeuri (HOOGERHOUD, 1987). A. alluaudi from Lakes Rugwero and Edward feed mainly on softbod- ied prey such as insects. In these lakes gastropods are virtually absent (GREENWOOD, 1965). The hypertrophied and non-hypertrophied morphs appear to be the two extremes of the range of possible phenotypes of the pharyngeal jaw apparatus. In laboratory experiments on specimens from the Lake Victoria population fish with different degrees of hypertrophy of the pharyngeal jaw apparatus, can be produced depending on food regime (SMITS, 1996). Thusfar, the anatomical descriptions of differences between the two ex- treme morphs of A. alluaudi focussed on the lower pharyngeal bone and on the apophysis on the skull (GREENWOOD, 1959, 1965; HOOGERHOUD, 1986; HUYSSEUNE et al., 1994). However, differences in other parts of the pharyngeal jaw apparatus are present as well. Using multi-variate analyses of muscle dry-weights and linear measurements of bony elements, SMITS et al. (1996a) surveyed the changes in a large number of characters of the pha- ryngeal jaw apparatus from one morph to the other, but gave no anatomical description. In the present paper a comparative description is presented of the anatomy of those elements which showed differences in the multivariate analysis (SMITS al., op. cit.), and of a number of additional elements that do not differ in size but differ in shape between the morphs. Dry-weight .
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