Body and head shape in fish responds to environmental factors such as water flow rate, food sources, and niche availability. However, the way in which fish respond to these environmental factors varies. In Central Chile, multiple river and lake systems along the coast provide an ideal study site to investigate these types of shape changes. We use geometric morphometrics to characterize shape differences in Galaxias maculatus (Jenyns) between river and lake populations. Lake fish converge on a shape with a more fusiform body, narrower head, and larger eyes, while river fish have a more robust body, rounder head, and smaller eyes. These shape changes are consistent with a shift to zooplanktivorous foraging in lakes, as evidenced in other systems. Unlike some fish species that develop polymorphisms in body shape after colonization (e.g., benthic and limnetic forms), G. maculatus in lakes exhibit a monomorphic limnetic form.

Collections were done under the auspices of Dirección de Investigación, Universidad de Concepción, following Institutional Animal Care and Use Committee (IACUC) protocols. Specimens were collected with seines and backpack electrofishing equipment and euthanized with an overdose of BZ-20 (20% ethylp-aminobenzoate). The body and head of each freshly-killed fish were photographed from the left lateral view for morphometric analysis. Using the program tpsDig, we digitized 10 landmarks and semi-landmarks on each body photograph, and 11 landmarks and semi-landmarks on each head photograph. 

The program tpsRelW was used to generate shape variables. A generalized Procrustes analysis was used to remove non-shape variation. We used a multivariate linear mixed model to determine the effects of our variables (river or lake environment and centroid size) on Galaxias maculatus shape variation. Proc MIXED in SAS was used to run all analyses (SAS version 9.4, SAS Institute Inc., Cary NC, USA).

ReadMe file titled README_DataMercer2020.txt contains the necessary information for re-use.