Johnston Island damsel
Plectroglyphidodon johnstonianus
The Johnston Island damsel exhibits a vibrant blue body with yellowish accents along the fins and a distinctive black spot at the base of the dorsal fin.
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About the Johnston Island damsel
A bold little reef damselfish with a thick black bar near the tail, yellow body, and bright blue eyes. It hangs tight to branching corals on wave-washed reef crests and has plenty of attitude for its size, so it can boss tankmates and may peck at SPS.
Also known as
Quick Facts
Size
14 cm
Temperament
Semi-aggressive
Difficulty
Beginner
Min Tank Size
50 gallons
Lifespan
5-7 years
Origin
Indo-Pacific
Diet
Omnivore - benthic algae and small crustaceans; accepts marine pellets and frozen foods; may nip coral polyps
Water Parameters
24.7-29.2°C
8.1-8.4
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This species needs 24.7-29.2°C in a 50 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- Start with at least 50 gallons; this damselfish can reach 14 cm and likes room to claim a territory.
- Stack branching rockwork or coral skeletons; in the wild it hugs branching corals on wave-washed reef crests and will spend its time tight to that structure.
- Provide lively water movement to suit its reef-crest habits.
- Run 24.7-29.2 C and pH 8.1-8.4, and keep these stable for a marine setup.
- Offer a varied omnivore menu: algae-based marine pellets plus frozen crustacean foods, matching its habit of grazing benthic algae and picking small crustaceans.
- Reef caution: it may nip coral polyps and peck at SPS, so plan coral placement or choose livestock accordingly.
- Semi-aggressive with plenty of attitude; choose sturdy tankmates and introduce this fish after more peaceful species to reduce bullying.
- Expect about 5-7 years in captivity when its needs are met.
Compatibility
Good Tankmates
- Robust clownfish like ocellaris or percula - similar size and temperament, unlikely to be bullied
- Centropyge dwarf angelfish (e.g., coral beauty, flame angel) - semi-aggressive and sturdy enough for a damsel
- Hardy Halichoeres wrasses (melanurus, yellow/chrsyus) - fast, alert swimmers that can handle some attitude
- Hawkfish (flame or longnose) - perching, semi-aggressive fish that are not easily intimidated
- Algae blennies (Salarias/Ecsenius spp.) - tough, mid-sized benthic grazers that usually hold their ground
- Royal gramma and other robust basslets - cave-oriented and typically capable of coexisting with a feisty damsel
Avoid
- Other damselfish and chromis - similar body shape and niche invite territorial fighting, especially in modest tanks
- Very slow or long-finned fish that cannot handle nipping or high flow (seahorses, pipefish, lionfish)
- Timid or delicate nano fish that get stressed by harassment (firefish/dartfish, mandarins, assessors, tiny gobies)
- Tiny ornamental shrimp and very small crabs - viewed as bite-sized crustaceans and may be picked at
The Johnston Island damsel (Plectroglyphidodon johnstonianus) is a bold, medium-sized Indo-Pacific damselfish that reaches about 14 cm. It shows a yellow body, a thick black bar near the tail, and bright blue eyes. Hardy and suitable for beginners, it still carries the typical damsel attitude. Expect a lifespan of roughly 5-7 years with stable marine conditions.
Quick ID: yellow body, bright blue eyes, and a prominent black bar just ahead of the tail.
Origin and natural habitat
This species comes from wave-washed reef crests in the Indo-Pacific. It stays close to branching corals, ducking in and out of cover while grazing benthic algae and picking at small crustaceans. The habitat is high-energy with strong water movement and excellent oxygenation.
Aquarium setup and water conditions
Plan on a minimum 50 US gallons. Provide branching rockwork or sturdy coral-like structure so the fish can claim a patch and retreat quickly. Include brisk flow areas to mirror its natural surge zone, but also create calmer leeward spots so it can rest. Keep reef-stable water chemistry and avoid rapid swings.
- Temperature: 24.7-29.2 C
- pH: 8.1-8.4
Use a layout with multiple sight breaks. This helps contain territorial behavior by letting fish get out of each others line of sight.
Diet and feeding
An omnivore that naturally grazes benthic algae and picks at small crustaceans. In the aquarium it accepts quality marine pellets and a variety of frozen foods. Include algae- or seaweed-based items regularly to match its grazing habits. Keeping the diet varied can reduce interest in coral polyps, though some individuals still peck at them.
Behavior and tankmates
Semi-aggressive and territorial for its size, this fish will pick a patch of rock or branching structure and defend it. It can boss timid or very small tankmates and may harass similar-shaped damsels. Choose companions that can handle a bit of attitude and provide enough space that each fish can hold its own area.
- Keep one per tank in modest aquaria; multiple individuals require far more space.
- Add it later in the stocking order so it is less likely to claim the entire tank.
- Pair with fish of similar size and confidence rather than delicate, shy species.
Not guaranteed coral-safe. This species may nip coral polyps, especially on SPS near its chosen territory.
Common problems and how to avoid them
- Territorial bullying: provide at least 50 gallons, complex rockwork, and clear sight breaks; avoid crowding similar damsels.
- Coral pecking: expect some risk with SPS; keep prized colonies out of its territory or choose a different fish for SPS-dominant reefs.
- Stress from unstable water: keep temperature and pH within the listed ranges and avoid rapid changes.
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