The Cardinalfish is a small, reddish fish that lives primarily on rocky bottoms and in caves. It belongs to the Apogonidae family, and its scientific name translates to "beardless" because it lacks the whiskers found on red mullet, with which it shares a similar red colour. The body is short and high, with an intense orange-to-red colouration. Its most characteristic features include the massive eyes, which betray its nocturnal habits, and the large oblique mouth. It often bears two or three dark spots at the base of its tail.

This is a cryptic species that lives on rocky coasts and reefs, as well as in marine caves. During the day, it remains hidden in dark cavities and rock crevices, often in small groups, and sometimes swims upside down with its belly toward the cave ceiling. At night, it ventures into open water to hunt. It is found at depths from 2 to 200 metres, although it moves to shallower waters during the summer for reproduction.

The Cardinalfish is a small fish. Its usual length ranges from 8 to 12 centimetres, while the maximum length it can reach is 15 centimetres.

The species is a nocturnal predator. It feeds mainly on small invertebrates such as shrimp and copepods, as well as on the eggs and larvae of other fish it finds in the plankton at night.

The Cardinalfish has an extremely interesting reproductive method known as mouthbrooding. Breeding takes place in the summer and usually from June to September. After fertilisation, the male takes the eggs, which are joined in a ball, into his mouth. He holds and protects them there until they hatch, which takes approximately 5 to 7 days. During this period, the male does not feed at all, and he aerates the eggs by opening and closing his mouth.

It has no commercial value and is not sold in fish markets due to its small size. It is only caught accidentally as bycatch in nets and traps. However, it is sometimes used by recreational fishers as live bait for larger predators such as groupers, although it is difficult to keep alive.

The International Union for Conservation of Nature classifies the Cardinalfish as a species of Least Concern because it is abundant throughout the Mediterranean. Scientific studies have shown that it represents an important link in the food chain of marine caves by transferring energy from the external environment into the cave through its droppings. It has also been observed that males incubating eggs are more vulnerable to predators because their ability to feed and breathe is limited.