Amphibians, like most fish, lay their eggs externally. For this reason, as well as the rapid division and the simple cell cycle, the embryos are suitable for in vivo studies.
The small tropical freshwater fish D. rerio (zebrafish) is an important model organism, especially for biomedical studies and research on vertebrate development, due to its genetic and molecular similarity to humans. Manipulation of the genome is easy, resulting in numerous mutants in protein-coding genes and several transgenic lines of zebrafish have been produced to study human diseases that are commercially available. In addition, there is the advantage of a transparent eggshell, high fertility, fairly short generation time (about 3 months), rapid embryonic development and external fertilization.
The subtropical African killifish also brings advantages. Killifish live in ponds that dry out sessional. As an adaptation to this habitat, they have developed an annual life cycle with rapid maturation, have an active life span of several weeks to months and produce drought-resistant dormant eggs that survive the dry season in the pond sediment. In vertebrates, the occurrence of dormant eggs is unique and usually found in arthropods. However, due to its rapid life cycle and short generation time, the killifish is particularly suitable for life cycle analyses and experiments involving several generations. The species is emerging as a promising model organism for biomedical research, evolutionary biology, ecotoxicology, and developmental biology.