---
title: Bluetail Boxfish (Ostracion cyanurus)
scientific_name: Ostracion cyanurus
common_name: Bluetail Boxfish
taxon: Boxfishes
published: 2023-12-28T19:47:42.764Z
updated: 2026-08-20T14:28:43.483Z
tags: Bluetail Boxfish, Ostracion cyanurus, Boxfishes, Red Sea, Marine Life, Diving
url: https://redseacreatures.com/taxon/fishes/bony-fishes/boxfishes/bluetail-boxfish
---

# Bluetail Boxfish (*Ostracion cyanurus*)

## Quick facts

- **Group:** Boxfishes (*Ostraciidae*)
- **Size:** up to 15 cm
- **Depth:** 3-50 m
- **Diet:** Omnivore
- **Activity:** Diurnal
- **Rarity:** Common
- **Habitat:** Coral and rubble, close to holes and overhangs
- **Reproduction:** Broadcast spawner, Gonochoric
- **Hazard:** Poisonous

## How to recognise it

- males brilliant blue on the sides with a yellow-brown back
- females and juveniles dull olive with dark brown spots and a blue tail only
- rigid box with holes cut for the eyes, mouth, fins and tail
- sculls with tiny fins and hovers rather than swims
- up to 15 cm

A small armored box, fifteen centimeters, that comes in two versions. Males are brilliant blue on the sides with a yellow-brown back; females and juveniles are dull olive spotted with dark brown and carry only a blue tail. They are the same species, and photographs of the two circulate as if they were not.

Its range is narrow without being confined to this sea: Red Sea, Gulf of Aden and on into the Persian Gulf, and nothing beyond that. Species with that particular footprint are common here and they say something about the region - this is a warm, salty, semi-enclosed corner of the Indian Ocean, and a fish adapted to it can spread along the Arabian coast and still get no further.

The body is a shell of fused plates with holes for the eyes, mouth, fins and tail. Nothing bends, so the fish sculls with fins that look far too small and hovers rather than swims, and it never travels far from a hole.

The defense is chemical and worth respecting in a closed space: stressed boxfish release ostracitoxin from the skin, which is lethal to other fish in a confined volume and is a well-known way to lose an aquarium. In open water it does nothing to anyone, and the fish's habit of hanging motionless a meter away makes it one of the easiest subjects on the reef.

## Also known as

- **English:** bluetail trunkfish, Bluetail Boxfish
- **Arabic:** Sandook Al-Bahar
- **Hebrew:** קופסינון סגול
- **French:** Poisson-coffre à points bleus
- **Italian:** Pesce scatola coda blu
- **German:** Arabischer Kofferfisch
- **Spanish:** Cofre Azul
- **Portuguese:** Peixe-cofre-de-cauda-azul
- **Dutch:** Blauwe Koffervis
- **Czech:** havýš modrý
- **Russian:** Аравийский кузовок
- **CA:** Peix cofre àrab
- **DA:** Blåhalet kuffertfisk

## Range

Red Sea

---

*Source: [Red Sea Creatures](https://redseacreatures.com/taxon/fishes/bony-fishes/boxfishes/bluetail-boxfish)*
*Updated: August 20, 2026*