Danio rerio

It belongs to the family Danionidae within the ray-finned fishes.

Its body is slender and flattened from side to side, with small barbels around the mouth.

The characteristic five body stripes continue onto the tail fin, and the anal fin is also striped.

At the cellular level, dark melanophores, yellow or orange xanthophores and reflective iridophores together produce the adult pattern.

These cells interact during development: experimentally removing iridophores can disrupt the arrangement of the other pigment cells.

The adult stripes are not simply the embryo’s markings enlarged, because a new pattern develops during the transition toward adulthood.

Zebrafish reproduce through eggs that are fertilized outside the female’s body.

During courtship, males swim close to females and try to guide them toward a spawning site, where females release eggs onto the substrate.

Some males defend a spawning area, whereas others pursue females without holding such a territory.

After fertilization, yolk-poor cytoplasm gathers at one pole of the yolky egg and forms the disc in which early cell divisions occur.

Cleavage partitions that disc into cells without dividing the whole yolk into separate cells.

Later, the embryo lengthens, repeated body segments form and early organ structures become visible.

Hatching occurs at variable times, commonly during the third day under the standard laboratory conditions used in the classic staging series.

That series uses 28.5 °C as its reference temperature, and changing temperature changes the pace of development.

The jaws, gills and paired pectoral fins continue taking shape around this period.

A field survey in Bangladesh found zebrafish especially abundant in shallow ponds, lakes and ditches in open settings, often with vegetation along their margins.

The findings associate these fish with floodplain waters rather than making them specialists of the main river channel.

A separate Bangladesh study found that zooplankton and insects were the main foods.

In that study, the principal breeding season began just before the monsoon, and growth was fastest early in life during the monsoon season.

Those field observations differ from laboratory conditions, under which spawning can occur throughout the year.

Because the embryo is accessible and optically clear, researchers can watch tissues develop and follow cells in a living vertebrate.

Pigment-cell experiments show how zebrafish can connect an easily visible feature, such as a stripe, to the interactions that build it.

Behaviour also depends on circumstances: in an aquarium experiment, territorial males had greater reproductive success at low density, but not at high density.

Kimmel and colleagues stress that embryos can differ in developmental stage even within one clutch.

Morphology is therefore more informative than assuming that all embryos of a given age are equivalent.

An embryo remaining inside its chorion need not be developmentally behind one that has already hatched.

Patterson and Parichy used iridophore ablation to test effects on the developing arrangements of melanophores and xanthophores, rather than inferring interactions from colour alone.

Spence, Jordan and Smith assigned parentage genetically to offspring sampled after hatching.

They explicitly distinguish this measure from fertilization success, because survival between fertilization and hatching can also affect which offspring are counted.

The two Bangladesh field-study abstracts report habitat associations, food and seasonal growth, not one universal schedule for every population or laboratory line.