Ovarian structure and oogenesis of two endemic pupfishes from the Yucatán Peninsula: morphofunctional traits related with an estuarine life history

Oogenesis has universal characteristics among fishes, in which structures of vital importance for embryos are assembled. However, these universal structures may have additional functions depending on the evolutionary ecology of each species. The aim of this research was to describe histologically the structure of the ovary and the oogenesis of the pupfishes Cyprinodon artifrons and Garmanella pulchra , with emphasis on the follicular structures linked to the resistance of embryos to desiccation. The embryos of pupfishes experience periodic deleterious environmental changes caused by estuarine tides. They can survive periods out of water protected by accessory egg layers assembled in the ovaries during oogenesis. This research found that both species have a single ovary with asynchronous follicle development, associated with a frequent breeding activity. Also, multiple generations of cortical alveoli were described, larger in G. pulchra , related with the formation of perivitelline space, a structure that can buffer the effects of desiccation when embryos remain out of water. In both species, the accumulation of oil droplets in the oocytes was simultaneous with the synthesis of cortical alveoli. Oil droplets may be related to energy reserves during extended incubation periods. The zona pellucida of both species presented an intermediate thickness with respect to other species, and hair-like filaments arranged on the zona pellucida of G. pulchra were clearly detected. These structures as a whole, could be related to the survival of the embryos of the pupfishes of Yucatan in the tidal fluctuations in the estuarine environments in which they inhabit.