La fecundació externa i interna en els vertebrats

Abstract

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.