Fish Active day and night

Convict Blenny

Pholidichthys leucotaenia

iNaturalist Wikipedia

The ultimate architect of the reef, the Convict Blenny builds vast underground cities and exhibits some of the most unique parental care in the ocean.

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Quick Identification

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Size

Adults typically reach 25–34 cm (10–13.4 inches) in length.

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Colors

Juveniles are black with a bright white horizontal stripe. Adults transition to a dark body with intricate, maze-like yellow or white reticulated patterns.

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Key Features

Elongated, eel-like body shape
Juveniles mimic the appearance of venomous catfish
Adults feature a complex 'marbled' or reticulated skin pattern
Continuous dorsal and anal fins joined to the caudal fin
Strongly protruding eyes used for monitoring burrow entrances

When You’ll See Them

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Activity pattern Active day and night

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Peak hours 8 AM - 4 PM (Juvenile swarming)

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Season Year-round

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Diet Juveniles are planktivores that swarm in the water column; adults are rarely seen feeding but likely subsist on food brought back to the burrow by the juveniles.

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Habitat Shallow coral reefs, silty lagoons, and sandy seabeds with enough rubble to support burrow structures.

public Geographic range

Where Does the Convict Blenny Live?

The Convict Blenny is a signature species of the tropical West-Central Pacific Ocean. Its range is centered within the Coral Triangle, with high population densities found throughout the Philippines, Indonesia, and the Solomon Islands. They are also frequently documented along the northern reaches of the Great Barrier Reef in Australia and the coastal waters of Papua New Guinea. This species is strictly marine and is not found in freshwater or introduced populations outside its native tropical range.

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6 Countries

2.5M km² Range

Least Concern Conservation

PH Philippines ID Indonesia Solomon Islands AU Australia Papua New Guinea Palau

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iNaturalist / Verified observation data

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Behavior

The Convict Blenny is one of the marine world's most industrious excavators. Despite the 'blenny' name, these fish belong to their own unique family and are famous for their engineering prowess. They spend the vast majority of their adult lives underground, constructing and maintaining massive, complex tunnel systems in the sand and rubble beneath coral reefs. Using their mouths to carry sand and pebbles, they can move a surprising amount of substrate in a single day, constantly remodeling their subterranean homes.

Their social behavior is truly remarkable and rare among fish. While the adults remain in the security of the burrow, the juveniles emerge in massive, coordinated swarms to feed on plankton during the day. At night, the young return to the burrow, where they hang from the ceiling by mucous threads. Fascinatingly, the adults are believed to be fed by their offspring, either through the young regurgitating food or by the adults consuming the nutrient-rich mucus produced by the juveniles.

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Camera Tips

To capture high-quality footage of a Convict Blenny, you will need an underwater camera setup or a waterproof action camera mounted on a stable weighted base. Look for tell-tale signs of their 'engineering' work: small craters in the sand or piles of coral rubble near a hole. Position your camera about 12 to 18 inches away from a burrow entrance, keeping the lens at a low, horizontal angle to catch the adults as they poke their heads out to spit out sand. Using a camera with a wide-angle lens is ideal for capturing the scale of their excavation sites.

If you are looking for the striking juvenile swarms, aim your camera upward from the seabed toward the light. Juveniles hover in a vertical orientation, looking like a curtain of stripes. For the best visual results, record during the late morning when the sun is directly overhead; this provides maximum natural light to penetrate the water and highlight the white stripes of the young fish. Setting your camera to 60 frames per second (fps) will help capture the fluid, eel-like swimming motion of the adults without motion blur.

For nighttime observation, use a camera with high ISO capabilities or a diffused red LED light. White lights often cause these shy fish to retreat deep into their tunnels, but they are less sensitive to red spectrum light. At night, you may be able to film the fascinating 'hanging' behavior of the juveniles or catch a rare glimpse of an adult performing heavy maintenance on the burrow entrance. Ensure your camera housing is thoroughly rinsed with fresh water after every deployment to prevent salt crystals from damaging the seals.

Similar Species

Species that look similar or are commonly confused with Convict Blenny.

Striped Eel Catfish

Catfish have prominent whiskers (barbels) around their mouths and venomous spines, whereas Convict Blennies lack both.

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Frequently Asked Questions

Juvenile Convict Blennies are most active during the daylight hours, typically between 8 AM and 4 PM, when they emerge from their burrows in large swarms to feed on plankton. Adults remain active day and night within their burrows, constantly performing maintenance and excavation.

You cannot easily attract them with bait, as they are shy and specialized feeders. Instead, look for active burrow entrances marked by piles of fresh sand and coral rubble. Placing a camera near these 'construction sites' is the best way to ensure you capture them on film.

Juveniles feed on tiny zooplankton in the water column. Adults are rarely seen feeding in the open; it is widely believed they are fed by the juveniles that return to the burrow after a day of foraging, a very unusual behavior in the fish world.

No, they are strictly marine fish and are found only on tropical coral reefs or in coastal lagoons. They are not found in freshwater or suburban backyard ponds unless you live on a tropical beachfront with a healthy reef system directly offshore.

Juvenile Convict Blennies look almost identical to Striped Catfish (Plotosus lineatus), but you can tell them apart by their swimming style; blennies have a more eel-like, undulating movement and lack the barbels (whiskers) found on the mouths of catfish.

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