DUGi

La fecundació externa i interna en els vertebrats

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Fertilization in vertebrates has two main strategies: external and internal. The first is more characteristic of species belonging to fish and amphibians classes, whose male and female gametes are released outside the body, and meet and fuse with each other in the aquatic environment. Animals that use this reproductive strategy must produce large amounts of gametes and present synchronized behaviors to increase fertilization success, particularly when they face predation or adverse environmental conditions. On the other hand, internal fertilization is common in land animals, notwithstanding species from all vertebrate clades can use this strategy. This method involves the fusion of gametes within the female reproductive tract, providing gametes and the offspring with greater protection. This is achieved through a series of evolutionary adaptations such as hemipenes in reptiles or claspers in fish, which are reproductive organs for copulation. Each of the strategies has advantages and disadvantages. External fertilization is associated to a high genetic diversity and does not require a significant energetic investment from the parents, but has a high gamete mortality rate due to dispersal. In contrast, internal fertilization confers greater protection to gametes and embryo thanks to parental care, which increases the survival rate of the offspring; yet, adaptations such as the placenta and lactation represent a high wear and tear on the female. The transition from external to internal fertilization is linked to that of animals to terrestrial life and desiccation. Fossil evidences point out to the need of protecting gametes and embryos from adverse conditions such as predation and other environmental factors with the aim to increase the reproductive success. This suggests that the evolution of fertilization has entailed a convergence of similar characteristics in different groups of vertebrates to deal with the same adverse conditions.

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