The zebrafish is perhaps one of the most frequently used model organisms for genetic and developmental studies. This tropical freshwater fish can be found in the rivers of northern India, northern Pakistan, Bhutan, and Nepal. The zebrafish is known for its unique regenerative abilities and rapid embryonic development. The scientific name of zebrafish is Danio rerio and it belongs to the minnow family, Cyprinidae. The fish got its common name from the presence of five uniform and pigmented horizontal stripes on the side of its body, that resemble the stripes of a zebra. The characteristic stripes of zebrafish are blue in color and they extend from the gill cover to the end of the caudal fin. Let's find out some more interesting facts about zebrafish. » Zebrafish are vertebrates, which means that they have a backbone like humans. They possess a fusiform and laterally compressed body, adorned by five uniform, horizontal blue lines. » A zebrafish usually grows to a length of 6.4 cm. But in captivity, it rarely exceeds a length of 4 cm. » The male zebrafish is usually slender with a torpedo-shaped body. It can be distinguished from the female by the presence of golden stripes in between the blue stripes. The female zebrafish, on the other hand, has a larger belly and silver stripes between its blue stripes. » The adult female fish can also be identified by the presence of a small genital papilla, located in front of the anal fin. » Some adult zebrafish possess antennae or barbels near their mouth, which are mainly used to search for food in muddy water. These barbels can be found in many other fish that live at the bottom of nutrient-rich water bodies. » Zebrafish sexually mature at around 3 to 4 months. The female zebrafish can spawn every two to three days. Each female produces hundreds of eggs. The ovulation in females is believed to be induced by the release of steroid glucuronides by the males. » After ovulation, the female zebrafish release a hormone that stimulates the male fish to mate, which is then followed by oviposition and spawning. The females usually choose gravel substrates for depositing the eggs. Light can enhance spawning in zebrafish. This is the reason why they are commonly observed to spawn in the early morning. » The fertilized eggs of zebrafish become transparent and grow at an incredible speed, developing the precursors to all major organs within the first 36 hours of fertilization. After 1 to 3 days, hatching takes place and the young fish start looking for food about 36 hours after hatching. » The embryos of zebrafish are considered ideal for scientific research mainly due to their transparency. The transparent embryos can be closely observed and the stages of embryonic development, especially the formation of internal organs can be easily monitored. » Another interesting fact is that the embryos of zebrafish are much smaller and contain a small number of cells, as compared to the embryos of many other vertebrates. This helps scientists monitor the behavior and development of individual cells. » Zebrafish are omnivorous and can eat anything smaller than them. They mainly feed on insects and insect larvae, phytoplankton, and zooplankton. But, they can also feast on worms and small crustaceans, if their favorite foods are not available. » There are several varieties of zebrafish, such as long-finned, short-finned, albino, pink, striped, and speckled. Zebrafish have also been modified to create several genetic variants of the same species, mainly for scientific research. » The tiny zebrafish can regenerate not only their fins, but their heart as well. However, they do not regenerate heart muscles from stem cells. Instead, their mature heart muscles return to a stem cell-like state and generate new heart muscles. Zebrafish larvae can regenerate their brain as well. » Zebrafish can also regenerate damaged photoreceptor cells and retinal neurons after an injury. In a study conducted by Australian scientists in 2012, it was observed that zebrafish possess a special protein, known as 'fibroblast growth factor'. This specialized protein ensures the healing of their spinal cord after an injury without the formation of glial scar. » The zebrafish is considered a better model for scientific research than commonly studied invertebrates, like worms and insects. This preference is mainly due to the fact that the zebrafish is a vertebrate, and therefore more likely to have biological traits similar to humans. » Zebrafish can survive in different environments. They can be kept together in large numbers and can be easily bred. The embryonic development in zebrafish resembles the embryogenesis in higher vertebrates, including humans. This is the reason why zebrafish have become a very popular and convenient research model for genetic and developmental studies. Scientists consider them as an ideal tool for conducting research on certain human congenital and genetic diseases as well.