The Cooks Cardinalfish (Ostorhinchus cookii) is also known as the blackbanded cardinalfish and the blackbanded cardinal. They are one of the few species of saltwater fish that are mouth brooders. As can be seen below, after the eggs are laid the male picks up the eggs and keeps them in his mouth until they hatch.
Cooks Cardinalfish has proportionally large eyes and have two dorsal fins. The fins are a translucent very light pink colour. The body is a silver colour with four dark lines running across the body. The lower two lines are broad and the upper two are thin. Just above the eye there is a dark grey stripe that extends part way down the body.
They are very similar in appearance to the Broadstripe Cardinalfish and this grey line is the distinguishing feature. Additionally on the Broadstripe Cardinalfish the spot on the tail merges with the center line whereas with the Cooks Cardinalfish it does not.
IN THE WILD
Cooks Cardinalfish is fairly common in Tanzanian waters. During the day they are usually seen in singularly or in small groups under overhangs or in small caves. At night they are normally always seen solitary and are almost never seen in pairs.
On one occasion however we spotted a male and female together. As documented in the images above and below, the female had just dropped a bundle of pink eggs. The male immediately tried to swallow them and initially battled to fit them in his mouth. He regurgitated them and swallowed them several times. After taking a few images I left him so as not to stress him.
Cooks Cardinalfish is found across the Indian ocean and in the west Pacific. They are found between depths of 2-60 meters.
Cooks Cardinalfish are nocturnal feeders , feeding mainly on zooplankton and small benthic organisms.
During breeding pairs are formed and after performing a mating dance, the eggs are laid by the female and fertilized by the male. The male then collects the fertilized eggs and carries the eggs in his mouth. Until they hatch the male cannot feed. Towards the end of the incubation period the males become quite skinny making the protruding mouth more noticeable. This can result in cannibalism with the male eating some of the eggs.
Cooks Cardinalfish are not commonly kept in aquariums but they are peaceful hardy fish. Some cardinalfish have been bred in captivity by breeders such as Orafarm where they are occasionally available.