Brain size, brain architecture, and eye size vary extensively in vertebrates. However, the extent to which the evolution of these components is intricately connected remains unclear. Trinidadian killifish, Anablepsoides hartii, are found in sites that differ in the presence and absence of large predatory fish. Decreased rates of predation are associated with evolutionary shifts in brain size; males from sites without predators have evolved a relatively larger brain and eye size than males from sites with predators. Here, we evaluated the extent to which the evolution of brain size, brain structure, and eye size covary in male killifish. We utilized wild-caught and common garden reared specimens to determine if specific components of the brain have evolved in response to differences in predation and to determine if there is covariation between the evolution of brain size, brain structure, and eye size. We observed consistent shifts in brain architecture in second generation common garden reared, but not wild caught preserved fish. Male killifish from sites that lack predators exhibited a significantly larger telencephalon, optic tectum, cerebellum, and dorsal medulla when compared with fish from sites with predators. We also found positive connections between the evolution of brain structure and eye size but not between overall brain size and eye size. These results provide evidence for evolutionary covariation between the components of the brain and eye size. Such results suggest that selection, directly or indirectly, acts upon specific regions of the brain, rather than overall brain size, to enhance visual capabilities.

Data includes wild-caught and common garden reared specimen. Variables are Code - fish ID, River - river of origin, population of origin- HP (high predation), RO (Rivulus-only), figh length (mm), ln transformed length, fish weight (g), ln transformed weight, brain weight (g) and ln transformed brain weight, eye size (mm) and ln transformed eye size. Telencephalon, dorsal medulla, cerebellum, optic tetcum width (mm) and ln transformed widths. Telencephalon, dorsal medulla, cerebellum, optic tetcum volume (mm^3) and ln transformed volumes (for wild caught only). Telencephalon, dorsal medulla, cerebellum, optic tetcum, and overall brain size residuals. Residuals were output from regressions between ln-brain (or brain structure) and ln-length. For common garden specimen, age at maturation is included (days) as well as food treatment (H= high food, L = low food). See ReadMe file.