The study of prenatal interactions of embryonic viviparous Cyprinodontidae (Pisces) can illuminate the evolution of this reproductive mode. Yet the small size, relative scarcity, and conservation status severely limit this potential. Magnetic resonance imaging (MRI) can solve or palliate these limitations. We developed a protocol to visualize the intraovarian location and arrangement of embryos of several species of the Goodeidae. This allowed us, in some cases, to gather images of the same specimens more than once during gestations, to be subsequently analyzed using ITK‐SNAP. We show that it is possible to visualize several characteristics of the embryos as they develop within the maternal ovarium lumen, including their number, developmental stage as inferred from the presence and completeness of the mouth, gills, and eyes, and their arrangement within the lumen. The latter can allow inferences about the mother–embryo interactions, as we found, for instance, differences in the relative positions of embryos in species with and without functional trophotaeniae—the embryonic component of the goodeid placenta. In sum, we show that (1) it is possible to visualize the embryonic and maternal structures without dissection and (2) without sacrificing the fish, thus allowing to include in these studies even endangered species; (3) once calibrated to particular species, the technique enables repeated visualization of the same developing brood, opening the opportunity to study prenatal interactions nonlethally; and (4) MRI allowed the detection of abnormalities within the female body, potentially providing a tool for husbandry of endangered or rare species.
Olivera‐Tlahuel et al. (Thu,) studied this question.