Haematococcus pluvialis is a freshwater species of Chlorophyta from the family Haematococcaceae.

This species is well known for its high content of the strong antioxidant astaxanthin, which is important in aquaculture, and cosmetics.

Examples of such conditions include bright light, high salinity, and low availability of nutrients.

Their resting cysts are often responsible for the blood-red colour seen in the bottom of dried out rock pools and bird baths.

This colour is caused by astaxanthin which is believed to protect the resting cysts from the detrimental effect of UV-radiation, when exposed to direct sunlight.