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Marine Animal Encyclopedia

Blue Chromis Chromis cyanea


Species ID: P.CC

Description: A small, oval-bodied fish with a long and deeply forked tail. This species is a brilliant blue colour with black along the back and on the outer edges of the tail, although it can pale or darken at will. Sexes appear similar and juveniles resemble adults

Maximum Size: 15 cm (6 in)

Longevity: Unknown, but up to 15 years based on the longevity of close relatives

Status: Not currently on the IUCN endangered species list

Blue Chromis & People: Not a food fish, but often collected for export in the aquarium trade


Geographical Range: Very common throughout the Caribbean, somewhat less common in the northern Gulf of Mexico and Florida

Coral Reef Zone: Generally found in the fore reef zone, where currents from the open sea supply plenty of planktonic food; can also be found in the back reef zone

Favourite Habitat: Blue chromis prefer steep reef slopes or patch reefs in areas with many holes and crevices where they can hide at night

Depth Range: 3–60 m (10 –200 ft)

A Day in the Life:

Dawn: Fish emerge from their shelter and rise above the reef to feed on plankton

Day: Chromis feed in schools above the reef; spawning also occurs during the day

Dusk: Fish seek out shelter and bed-down for the night

Night: Fish remain hidden in the reef until sunrise

Who Eats Who

The blue chromis is an omnivore that feeds on small planktonic food, including jellyfish. Blue chromis are eaten regularly by a variety of large reef predators including trumpetfish, grouper, and snapper.

Scuba Diver & Snorkeler Best Practices

Don’t touch the living coral : Corals are made up of many living polyps that secrete a thin mucous for their protection. Touching these tiny animals is like giving the kiss of death; contact may remove their protective coating leaving the polyps vulnerable to infections that can spread through the entire colony. Look, but don’t touch, and remember corals can also injury you if you touch them. Snorkelers are often positively buoyant so it is particularly important to avoid the temptation to hold the reef to maintain position while duck-diving.


These fish are somewhat wary and may dart into hiding holes unless divers approach slowly. If chromis retreat into the reef, divers and snorkelers may need to wait for several minutes before they come out again.

Feeding Behaviour

Blue chromis are plankton feeders. They spend their days above the reef in schools, actively picking microscopic plankton from the currents. Occasionally, a school of chromis will attack larger planktonic prey such as a jellyfish.

Observe, record & share:

O P.CC-101 – Feeding school: Schools numbering up to a hundred or so individuals pick plankton from currents above the reef

O P.CC-102 – Attacking jellyfish: Schools sometimes take advantage of larger prey floating through the ocean’s currents, such as jellyfish and other oceanic invertebrates

Attack & Defence Behaviour

Only blue chromis males are territorial, defending the areas they use as hiding and nesting sites. They will repel intruders of other species by chasing and biting, but have developed a special aggressive language for use with other blue chromis males. If two males meet at a territorial border, they spread their fins and swim short circuits side-by-side at the boundary. If the opponent refuses to back down, the defending male spreads his fins and shakes his entire body while beating his tail, or performs a headstand display in front of the intruder. If the intruding male still refuses to budge, the fish get physical, pressing their mouths together and wrestling, as well as biting and shoving each other. Such aggressive behaviour is especially common during the spawning season, when ompetition for nesting sites is fierce. Blue chromis are quick enough to avoid most predators by escaping to reef shelters spots and can darken their body considerably to camouflage themselves against the reef.

Observe, record & share:

O P.CC-201 – Camouflage colour: Chromis blend into a reef background by darkening their entire body

O P.CC-202 – Parallel swimming: Two males swim short circuits side-by-side at the border between territories

O P.CC-203 – Shaking and tail beating: Males spread their fins and shake their entire body, often followed by strong beats of the tail intended to ward off intruders

O P.CC-204 – Headstand: Males assume a head-down position in front of intruders

OP.CC-205 – Mouth-fighting: Territorial males press their open mouths together and push against each other until the stronger fish wins


Reproductive Behaviour

These fish are somewhat wary and may dart into hiding holes unless divers approach slowly. If chromis retreat into the reef, divers and snorkelers may need to wait for several minutes before they come out again. Blue chromis reproduce sexually by laying demersal eggs, which rest on the reef until hatching, and do not undergo sex change during reproductive development. Courting males clear a nest area within their territory and adopt a pale blue to white belly with a wider than normal black band along their back. When females approach, courting males swim in a vertical zig-zag pattern above the nest, called “dipping”, and may also swim a rapid vertical loop known as a “signal jump” to indicate the location of the nest. If the female shows interest, the male leads the way to the nest and gently quiver to encourage spawning. While sitting upon the reef, the female darkens and begins to lay her eggs, while the male periodically quivers as he fertilizes them. After spawning, the male keeps the nest clean by removing debris and fanning the eggs with his fins and both parents aggressively defend the nest from predators. Blue chromis reproduce during the day, all year round, but spawning activity peaks during the full moons of spring.

Observe, record & share:

O P.CC-301 – Nest and egg care: Males fan the eggs with the tail fin and pick away debris. Both fish defend the nest aggressively until the eggs hatch

O P.CC-302 – Courtship colours: Courting males have dark backs and white bellies

O P.CC-303 – Dipping: Males swim in a vertical zig-zag pattern to court females

O P.CC-304 – Signal jumps: Males swim a rapid vertical loop to indicate the location ofthe nest

O P.CC-305 – Quivering: Males gently quiver to encourage spawning and also while fertilizing the eggs

O P.CC-306 – Spawning: Females darken and lay eggs which the male fertilizes


Highlight Behaviours

Aggressive behaviour: Blue chromis, like many other  animals, possess a formal language of aggressive signalsused with members of their own species. These are used to resolve confrontations before physical violence, which carries a risk of injury for both fish. Signals are displayed when two males meet at a territorial boundary and often involve swimming side-by-side, or spreading the fins and shaking while beating the tail in order to send out shockwaves proportional to the fish’s strength. In the process, one fish establishes dominance and the weakerfish backs down and swims away. Only when fish are evenly matched do confrontations escalate to a physical battle involving mouth-fighting or rapid circling with open mouths in an attempt to bite the opponent.

Did You Know?

• Young black snapper (Apsilus dentatus) closely resemble adult blue chromis, and like the chromis, they feed on plankton. The juvenile snapper are believed to mimic the chromis in order to benefit from the protection their schools offer against predation.

• Blue chromis are much more cautious than their cousins, brown chromis. By staying much closer to their shelters while they feed, blue chromis do a better job of avoiding predation than brown chromis, who feed high above the reef and far from their shelters. In some diet studies, brown chromis outnumbered blue chromis in the stomachs of predators by nearly fifty to one!

What to do?

Share your observations today!: Discover your species of interest, observe its behaviour, and share your pictures and videos with friends and coral reef enthusiasts around the world! Upload media to the web, tagged with species common name (ex.: trumpetfish) and species ID code (ex.: A.AM) or species behaviour code (ex.: A.AM-101)

Blue Chromiszoom image
  • Order Pomacentridae
  • Length 15 cm (6 in)
  • Weight Not recorded
  • Depth 3–60 m (10 –200 ft)
  • Habitat Back reef, fore reef zones
  • Distribution Common throughout the Caribbean, less common in the northern Gulf of Mexico and Florida
Blue Chromis habitat mapzoom image