Common Threefin

Interesting Info

• The Common Threefin is a little triplefin fish, accidentally introduced it to Port Phillip Bay in Victoria, Australia, likely via ballast water in the 1990s.
• It’s endemic to New Zealand’s coastal waters, where it’s one of the most widespread triplefins, found on rocky reefs, tide pools, and even jetties.
• Although small, it covers a lot of ground—from intertidal pools down to about 30 m (100 ft), with most found within 5–10 m.
• Males and females generally look alike—dappled light brown with a pale belly—but males turn solid black (often with red dorsal fins) during breeding.
• They munch on tiny reef snacks like little crabs, worms, snails, and other mini critters that crawl over rocks and algae.
• Males guard sticky eggs that females lay on hard surfaces, fanning them with fins to keep them fresh until hatching.
• Their lifespan is short—only about 3 years in the wild. They grow fast, reaching adult size within a year.
• Because they eat algae and tiny creatures and get eaten themselves, they play a big role in passing energy up the reef food chain.
• Lots of researchers study them, they’re easy to spot and handle, so scientists use them to learn about fish behaviour and reef ecology.
• Their name lapillum means “small pebble,” a nod to their habitat among cobbles and rocky rubble.