Zebrafish

Several features make it unusually powerful for studying how animals, including humans, are built and how they go wrong. Its embryos are transparent and develop outside the body, so scientists can watch organs form in a living animal under a microscope. It is a vertebrate, so its heart, blood, brain, eyes, and immune system are far closer to a human's than a fly's or a worm's, and about 70 percent of human genes, and a similar share of disease-linked genes, have a zebrafish equivalent. It is small and cheap to keep in large numbers, it breeds fast and lays hundreds of eggs, and its genome can be edited easily, so researchers can create and test thousands of genetic variants quickly. This combination has made the zebrafish central to research on birth defects, cancer, heart disease, blindness, and drug discovery.

Yes, to a remarkable degree. If up to about 20 percent of an adult zebrafish's heart is surgically removed, the fish regenerates functional heart muscle over the following weeks, with existing heart-muscle cells losing some of their specialised structure, dividing, and rebuilding the missing tissue, and little or no permanent scar forming. Adult mammals, including humans, respond to the same kind of injury by forming a stiff scar and permanently losing pumping capacity, which is what a heart attack does. Understanding how the zebrafish avoids scarring and switches its heart cells back into a dividing state is a major goal of regenerative medicine, in the hope of eventually coaxing a human heart to do something similar.

They are among the easiest of all aquarium fish and a classic beginner species. They are small, at about 4 centimetres, extremely hardy, tolerant of a wide range of temperature and water quality, active, peaceful, and inexpensive, and they do well in cooler water than most tropical fish, so a heater is often unnecessary in a warm home. They are shoaling fish and should be kept in groups of at least six in a tank of 40 litres or more, where they will constantly patrol the upper water in a tight school. Their main drawback is that they are fast and busy, which can unsettle slower or more timid tank mates.

A GloFish is a zebrafish, or in some cases another species, that has been genetically engineered to contain a fluorescent-protein gene taken from a jellyfish, sea anemone, or coral, so that the fish glows bright green, red, orange, blue, or purple under normal or blue light. The colour is heritable, passed to the offspring. The original project aimed to make fish that would fluoresce in polluted water as a pollution sensor, but the modified zebrafish were instead licensed as ornamental pets and became the first genetically modified animal sold to the public, first in the United States in 2003. They are the same species as the wild zebrafish, are not more delicate, and should not be released, as with any aquarium fish.