Adaptive phenotypic plasticity is being increasingly recognized as an important role in adaptive radiation. The plasticity leads to the production of phenotypes that are well suited for the ecological niches. The Midas cichlid fish, which are distributed in the Nicaraguan great lakes and several crater lakes, have been known for their intraspecific polymorphism, and phenotypic plasticity. The lower pharyngeal jaw of the Midas cichlid fish Amphilophus citrinellus was tested in different diets for its phenotypic plasticity. The study found that different diets induced changes on the lower pharyngeal jaw of the Midas cichlids fish, and that changes were due to the mechanical stimulations of the jaws, but not to nutritional composition. The result of jaw differentiation suggest that adaptive phenotypic plasticity of the Midas cichlid fish could be an important factor in its speciation. Introduction
Adaptive radiation generally refers to an event in which a lineage rapidly diversifies into the newly formed lineages evolving different adaptations (1). Several factors trigger adaptive radiation, for instances, the organisms evolve to exploit a new niches or resources, or specialization subdivide a single niche into many new niches by evolving to take advantage of different food (1). Some of known adaptive radiation are Darwin’s fitches(2), Honey creepers on the Hawaiian islands(3), and Cichlid fish in East African lakes(4). These examples show that specialization to certain diet is accompanied by morphological adaptations, and result in different shapes and sizes of species.
Phenotypic plasticity is being recognized as an important role in adaptive radiation by relatively rapid change on species morphology (5). For example, the length of the hindlimbs of Anolis sagrei lizard were found that the individuals lived on broad surfaces developed longer hindlimbs for rapid locomotion, to capture prey, and escape predators, but the ones lived in narrow surfaces grew shorter limbs for careful movement (5). The demonstration of plasticity in limb length could be an important implications for the evolutionary radiation of Anolis sagrei lizard in Caribbean and tropical mainland America (5).
The Midas cichlid fish species are distributed in Nicaragua, and inhabit the large Nicaraguan lakes along with several other volcanic crater lakes (6). The cichlid fish are known for adaptive radiation, but also for their phenotypic plasticity, and as an example of trophic and colour polymorphism (7). In the Midas cichlid fish, phenotypic plasticity is underlying in the basis of two different types of pharyngeal jaws that they have, which are mollariform jaws, and papilliform jaws (8). Mollariform jaws can crack larger and harder snail shells, on the other hand, papilliform jaws are more effective when dealing with soft food items (8). Moreover, phenotypic plasticity could link to species diversification through trophic polymorphism (8).
In the aspect of that the Midas cichlid fish shows phenotypic plasticity on its jaw, we assume food item that they consume are the main influence. With this hypothesis, there is a need in examination of adaptive phenotypic plasticity in trophic traits of Midas cichlid fish species differentiated in these traits. The lower pharyngeal jaw (LPJ), which is a component of cichlid fish trophic morphology was used to be investigated. The LPJs of the Midas cichlid fish Amphilopus citrinellus (A. citrinellus) were exposed to three types of diets differing in hardness, to verify whether a diet in hardness could cause changes in the LPJs of the fish, and whether the robust LPJs was the result in higher calcium content in the diet, or by mechanical response of the jaws crushing hard food items. Materials and Methods
A single Amphilophus citrinellus brood was divided from an inbred line into three similarly sized groups with thirty Amphilophus citrinellus individuals in each group (9). The fish stock...
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