Morphological Variation and Trophic Partitioning Among Central Mexican Lake Silversides Daniel L. Powell and Kyle R. Piller Southeastern Louisiana University, Department of Biological Sciences, Hammond, Louisiana and Abstract Central Mexican Lakes Silverside (CML) species (Teleostomi:Chirostoma) show limited genetic variation for individuals with significant levels of morphological variation. Trophic partitioning is believed to influence the elevated diversity in the region but is not well understood. This study examined the association between diet and morphology, focusing on internal digestive anatomy and head shape. Dissections were performed to determine relative intestine length as related to trophic position. No clear relationship was uncovered. Head shape was analyzed using geometric morphometric analysis to quantify replicated patterns of species divergence between lakes. Independent divergence between lakes along similar axes in morphospace was largely supported. Results showed some degree of grouping according to trophic position. Materials & Methods Head Shape Museum lots from TU, UMMZ, and SLU collections. Pin difficult to see landmarks and photograph fish heads. Digitize landmarks using tpsDIG v. 2 (Figure 1). Perform new Procrustes fit, align by principle axes, and perform canonical variate analysis (CVA) using MorphoJ v. 105a. Relative Intestine Length Specimens from TU and SLU collections. Dissection and removal of alimentary tract. Measure standard length and length of alimentary tract (without stretching). Calculate intestine length relative to standard length. Introduction CML silversides show significant morphological variation but only limited genetic variation. Factors influencing diversification poorly understood, however trophic partitioning has been proposed a being potentially important driver of diversification. Objectives Determine relationship between relative intestine length and trophic position. Use geometric morphometrics to quantify differences in head shape among member of the CML clade. Results Head Shape Species sharing trophic position occupy the same morphospace to varying degrees – particularly evident for piscivorous species from Lake Chapala (Figure 2). Lake Chapala and Lake Patzcuaro Chirostoma groups overlap in morphospace (Figure 5). Relative Intestine Length Little variation in RIL among CML species as well (Figure 6). Little variation in RIL across all members of Tribe: Menidiini examined. No clear relationship between intestinal morphology and trophic position uncovered (Figure 6). Discussion RIL does not predict trophic position for CML Chirostoma. Stable isotope data are needed for a clearer understanding of trophic partitioning. CVA supports independent divergence of Lake Chapala and Lake Patzcuaro Chirostoma along the same axes through morphospace. Literature Cited Bloom, D. D., K. R. Piller, J. Lyons, N. Mercado-Silva, and M. Medina-Nava Systematics and biogeography of the silverside Tribe Menidiini (Teleostomi: Atherinopsidae) based on the mitochondrial ND2 Gene. Copeia 2009: Cooper, W. J., K. Parsons, A. McIntyre, B. Kern, A. McGee-Moore, et al Bentho-pelagic divergence of cichlid feeding architecture was prodigious and consistent during multiple adaptive radiations within African rift-lakes. PLoS ONE 5(3): e9551. doi: /journal.pone German, D. P. and M. H. Horn2006. Gut length and mass in herbivorous and carnivorous prickleback fishes (Teleostei: Stichaeidae); ontogenetic, dietar and phylogenetic effeects. Marine Biology 148: Wagner, C. E., P. B. McIntyre, K. S. Buels, D. M. Gilbert, and E. Michael Diet predicts intestine length in Lake Tanganyika’s cichlid fishes. Functional Ecology 23: Acknowledgements: The Piller Lab: Mollie Cashner, Elizabeth Marchio, David Camak, Luke Bower, and Marisa Stachowski. Specimens and other support: Chris Boeckman, Henry Bart (TU), Doug Nelson and William Fink (UMMZ), and Southeastern Louisiana University. This project was supported in part by a grant to KRP (NSF DEB ). Lake Chapala Figure 2. CVA for head shape of Chirostoma species from Lake Chapala. Lake Patzcuaro Figure 3. CVA for head shape of Chirostoma species from Lake Patzcuaro. Central Mexican Lakes Figure 5. CVA for head shape of Chirostoma across three Central Mexican lakes. Figure 1. Anatomical landmarks examined. C. labaracae Figure 4. Chirostoma at market in Patzcuaro. Figure 6. Relative Intestine Length (RIL) and trophic position for Chirostoma according to species. Planktivore