Fishes diurnal

Longfinned Triplefin

Ruanoho decemdigitatus

The Longfinned Triplefin is the stealthy 'rock-hopper' of New Zealand's silty reefs. Masterfully camouflaged and fascinating to watch, this endemic fish brings a touch of wild marine life to every coastal backyard.

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Quick Identification

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Size

Up to 12 cm (4.7 inches) in total length

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Colors

Mottled brown, olive, and grey camouflage patterns; males possess distinctively long, darkened rays on the second dorsal fin during maturity

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Key Features

Three distinct dorsal fins characteristic of the family
Elongated rays on the second dorsal fin
Blunt, rounded head profile with large eyes
Broad, fan-like pectoral fins used for perching on rocks

When You’ll See Them

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Activity pattern diurnal

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Peak hours Daylight hours, especially during mid-tide when water flow is consistent.

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Season Year-round, with increased activity and coloration during the spring breeding season.

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Diet Carnivorous, preying on small benthic invertebrates like amphipods, tiny crabs, and marine worms found among the silt and cobbles.

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Habitat Shallow marine environments, specifically silty reefs, rocky shorelines, and semi-sheltered harbors and estuaries.

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Behavior

The Longfinned Triplefin is a specialized bottom-dweller that spends most of its life 'perched' on the seafloor rather than swimming in the open water. Lacking a swim bladder, these fish use their large, muscular pectoral fins to prop themselves up on rocks and silty substrates, moving in short, explosive hops to navigate their territory or evade predators.

They are primarily solitary and highly territorial, especially the males during breeding periods. While they appear stationary, they are highly alert, using their large eyes to monitor the water column for tiny prey and potential threats. In the presence of humans or larger fish, they quickly retreat into the shadows of rocky crevices or blend seamlessly into the silty background of the reef.

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Camera Tips

Capturing the Longfinned Triplefin requires an underwater action camera (like a GoPro) or a trail camera in a dedicated marine housing. Because these fish live on the bottom, your camera placement is critical. Use a low-profile weighted base—such as a dive weight or a sandbag—to position the lens just 2 to 5 inches off the seafloor. Aim the camera at a 'stage' of flat rock or a clearing in the silt near a known crevice where triplefins are seen hiding.

Patience is key, as these fish are easily spooked by movement. If you are placing a camera in a backyard rock pool or off a private dock, leave it submerged for at least 30 to 60 minutes to allow the fish to become comfortable with the object. A small amount of crushed mussel or fish bait tucked under a nearby rock (just out of frame) can act as a powerful lure, drawing the triplefin into the camera's field of view as it investigates the scent. Stirring up a tiny amount of sediment near the camera can also mimic the natural disturbance of prey, piquing their curiosity.

Since the Longfinned Triplefin lives in silty, semi-sheltered areas, water clarity can be a challenge. Try to film on days with low swell and during the rising tide when clearer ocean water is pushed into the shallows. Use a wide-angle setting to maximize the field of view, but ensure your camera is set to a high frame rate (60fps or higher); these fish move with lightning speed, and a standard frame rate may result in a blur when they dart between perches.

For the best results, utilize natural sunlight. If you are filming in deeper or shaded areas, a small, diffused underwater LED light can help bring out the subtle brown and green mottling of their skin. Check your housing for any bubbles on the lens before leaving it, as even a small bubble can ruin the clarity of these miniature subjects.

Similar Species

Species that look similar or are commonly confused with Longfinned Triplefin.

Common Triplefin

The Common Triplefin lacks the elongated rays on the second dorsal fin and typically inhabits more wave-exposed rocky shores.

Blue-eyed Triplefin

Easily distinguished by its striking blue eyes and reddish-brown body, whereas the Longfinned Triplefin is more mottled brown/grey.

Frequently Asked Questions

Longfinned Triplefins are diurnal, meaning they are most active during daylight hours. They are particularly busy during mid-tide cycles when the movement of water helps stir up the small invertebrates they feed on.

If your backyard borders a rocky New Zealand shoreline, you can attract Longfinned Triplefin by placing an underwater camera near silty crevices. Using a small amount of crushed shellfish as bait or gently disturbing the sand near the lens can draw them in.

They are opportunistic carnivores that eat small benthic invertebrates. Their diet mainly consists of amphipods, isopods, and various small marine worms found on the surface of rocks or within silty substrates.

Yes, they are very common in suburban coastal areas throughout New Zealand, particularly in semi-sheltered harbors, estuaries, and along rocky coastlines where there is plenty of cover.

Look for the elongated rays on the second dorsal fin, which are much longer than those of other species. They also prefer siltier, more sheltered habitats compared to the common triplefin which likes more wave-exposed rocks.

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