--- title: Redmouth Cardinalfish (Apogon erythrosoma) scientific_name: Apogon erythrosoma common_name: Redmouth Cardinalfish taxon: Cardinals published: 2024-11-08T10:36:50.376Z updated: 2026-08-18T10:04:44.462Z tags: Redmouth Cardinalfish, Apogon erythrosoma, Cardinals, Red Sea, Marine Life, Diving url: https://redseacreatures.com/taxon/fishes/bony-fishes/cardinals/redmouth-cardinalfish --- # Redmouth Cardinalfish (*Apogon erythrosoma*) ## Quick facts - **Group:** Cardinals (*Apogonidae*) - **Size:** up to 3.8 cm - **Diet:** Carnivore - **Activity:** Nocturnal - **Rarity:** Uncommon - **Habitat:** Reef crevices and rubble - **Reproduction:** Mouthbrooder ## How to recognise it - under four centimeters, red to pinkish, with a reddish mouth - large eye taking up much of the head - deep in cover by day - up to 3.8 cm Under four centimeters, which puts it near the floor of what a mouthbrooding fish can be. The family's whole reproductive strategy runs on the male's mouth being big enough to hold a clutch and still let water through, and everything scales down together: a smaller fish carries fewer eggs, and a much smaller fish cannot carry a useful number at all. So there is a lower limit on a mouthbrooder that does not apply to a fish which glues its eggs to a rock, and this species sits close to it. What it gets in exchange is access. At this size the reef stops being a surface and becomes a volume: gaps in rubble, cavities in dead coral, spaces under plates that a six-centimeter fish cannot enter are all available, and inside them almost nothing can follow. It is red, which underwater is the same as being black, and it stays in cover through daylight, so most people who have swum over hundreds of them have never registered one. At night it comes out for small crustaceans, close to the bottom, and goes back in before dawn. ## Also known as - **English:** Diaphanous Cardinalfish - **Portuguese:** Cardeal-diáfano - **Czech:** parmovec malý --- *Source: [Red Sea Creatures](https://redseacreatures.com/taxon/fishes/bony-fishes/cardinals/redmouth-cardinalfish)* *Updated: August 18, 2026*