---
title: Blackbelly Cardinalfish (Rhabdamia nigrimentum)
scientific_name: Rhabdamia nigrimentum
common_name: Blackbelly Cardinalfish
taxon: Cardinals
published: 2024-11-08T21:15:00.151Z
updated: 2026-08-18T22:11:36.037Z
tags: Blackbelly Cardinalfish, Rhabdamia nigrimentum, Cardinals, Red Sea, Marine Life, Diving
url: https://redseacreatures.com/taxon/fishes/bony-fishes/cardinals/blackbelly-cardinalfish
---

# Blackbelly Cardinalfish (*Rhabdamia nigrimentum*)

## Quick facts

- **Group:** Cardinals (*Apogonidae*)
- **Size:** 5.5 cm
- **Depth:** 6-12 m
- **Diet:** Planktivore
- **Activity:** Nocturnal
- **Rarity:** Uncommon
- **Habitat:** Coral heads on sheltered reef, in schools
- **Reproduction:** Mouthbrooder

## How to recognise it

- small, translucent, with a dark patch on the chin and belly
- dark spot at the tail base
- in dense schools over and around coral heads
- up to 5.5 cm

A school is not a crowd of fish hiding behind each other. It is a single object that behaves in ways individual fish cannot.

Three things happen when hundreds of small fish pack together. A predator has to pick one target out of a mass of identical moving shapes, and the picking is measurably slower and less accurate than against a single fish. The group has hundreds of pairs of eyes, so the odds that something is spotted early are far better than for any individual. And when the attack comes, the school splits and reforms around the attacker, which repeatedly costs the predator the target it had chosen.

The cost is that a school is easy to find. What it buys is that finding it and eating from it are different problems.

These cardinalfish do it over coral heads on sheltered reef, in dense clouds by day that break apart at dusk when the fish spread out to feed. The individual is five centimeters of translucent fish with a dark chin, which is not much; several hundred of them together is something else.

## Also known as

- **Czech:** parmovec pestrý

## Scientific synonyms

- *Bentuviaichthys nigrimentum* Smith, 1961

## Range

Eritrea, Red Sea

---

*Source: [Red Sea Creatures](https://redseacreatures.com/taxon/fishes/bony-fishes/cardinals/blackbelly-cardinalfish)*
*Updated: August 18, 2026*