## 1 Introduction  
Body size influences many traits in animals [1, 2, 3, 4, 5]. Allometry is defined as the variation in morphometric variables or other features of organisms associated with size. Thus, ontogenetic, static, and evolutionary allometries are recognized based on whether this relationship is over the development of individuals, across individuals at a similar developmental stage, or across species, respectively [2, 6]. Ontogenetic allometry could contribute to evolutionary diversification, as has been described in studies of morphological diversification associated with ontogenetic allometric trajectories at intra- and interspecific levels (e.g., [7, 8, 9, 10, 11, 12]). According to the latter idea, changes in ontogenetic trajectories could have an adaptive basis, being useful to the study of how different selective pressures imposed by ecological factors could affect development [10, 12, 13]. The fish genus _Astyanax_ Baird [PERSON], 1854, distributed in North and Central America, shows ideal features for the study of ontogenetic allometric trajectories associated with trophic adaptation in an ecological divergence context. These fishes are known to adapt to a very wide range of ecological conditions, the cave-adapted morphs being the most conspicuous example [14, 15, 16, 17]. Interestingly, previous works have found evidence of the parallel evolution of sympatric morphs associated with trophic morphology and body shape divergence in different Mesoamerican lakes inhabited by lineages of _Astyanax_, which evolved independently [18; 19; 20; 21; 22]. This has been shown in two distant geographical systems, the San Juan River basin in Central America (which includes the Lakes Managua and Nicaragua), and the Lake Catemaco system in Mexico [20; 23; 24]. The lacustrine morphs, consisting of an elongate-body and a deep-body morph (Figure 1), show major morphological differences, which prompted their original consideration as different genera (i.e., _Astyanax_ and _Bramocharax_, [25; 26]). Some of the morphological differences distinguishing these morphs correspond to the body depth and the head length, as well as to their trophic morphology (i.e., number of maxillary teeth and of cuspids on the anterior tooth of premaxilla, see [21; 22]), which are associated with alternative trophic habits. Additionally, a geographical cline (i.e., a differentiation gradient between north and south systems) in morphological differentiation between morphs was suggested between the Lake Catemaco and San Juan River systems [22], and, recently, we explored the genetic and ecological differentiation between sympatric morphs of _Astyanax_ in these systems, finding evidence of a geographical cline also in the genetic differentiation [23; 24]. The morphs from the San Juan River system show greater genetic differentiation in contrast to the lack of thereof between morphs in Lake Catemaco, suggesting that both systems represent different stages of divergence between sympatric _Astyanax_ morphs pairs [23; 24]. Previous studies have shown that size is an important factor determining body shape variation in _Astyanax_ morphs from the Lake Catemaco and San Juan River systems [19; 21]. Particularly, there are differences in body size between morphs in Lake Catemaco [19], where the elongate-body morph reaches a larger size than the deep-body morph. Despite the possible relevance of size in trophic morphology variation and its potential role on the diversification in the _Astyanax_ genus [21; 22] and in Characteriforms [27], it remains to be explored how trophic morphology varies throughout ontogeny and if it can be considered as a mechanism promoting morphological diversification. Here, we focused on determining the ontogenetic allometric trajectories of trophic morphology between sympatric morphs of _Astyanax_ in two different geographic systems, by analyzing the shape of a relevant trophic structure, the premaxillary bone. We have a two-fold objective.