Body and head shape among fishes both vary between environments influenced by water velocity and across ontogeny. Although the shape changes associated with variation in average water velocity and ontogeny are well documented, few studies have tested for the interaction between these two variables (i.e., does ontogenetic shape variation differ between velocity environments). We use geometric morphometrics to characterize shape differences in Trichomycterus areolatus, a freshwater catfish found in high and low-velocity environments in Chile. We identify a significant interaction between velocity environment and body size (i.e., ontogeny). Ontogenetic patterns of shape change are consistent with other studies, but velocity environment differentially affects the ontogenetic trajectory of shape development in T. areolatus. Shape change over ontogeny appears more constrained in high-velocity environments compared to low-velocity environments.

All collections were done under the auspices of Dirección de Investigación, Universidad de Concepción (Collections were made before IACUC protocols were required for field studies in Chile, but we followed the Guidelines for The Use of Fishes in Research provided by The American Fisheries Society, the American Institute of Fisheries Research Biologists, and the American Society of Ichthyologists and Herpetologists). Trichomycterus areolatus were collected from the Andalién River and two of its tributaries (Nonguén and Queule Rivers) in summer 2003 and spring 2004 using electrofishing backpack equipment. Each specimen was euthanized with an overdose of BZ-20 (20% ethyl p-aminobenzoate), assigned an ID number and preserved in ethyl alcohol. The right laterval view of the body and dorsal view of the head of each specimen were photographed at Brigham Young University. 

Landmark-based geometric morphometrics were used to quantify body and head shape in Trichomycterus areolatus. Landmarks and semi-landmarks were digitized using tpsDig (http://www.sbmorphometrics.org/soft-dataacq.html). tpsRelW (http://www.sbmorphometrics.org/index.html) was used to generate shape variables (i.e. relative warps) from the landmark data. Relative warps were used for all subsequent analyses.

These methodologies are clearly outlined in the manuscript.

ReadMe file titled Searle2021_ReadMe.txt contains the necessary information for re-use.