[8] Males will aggressively defend their territory during the breeding season as well as guard eggs after they are laid by females, who contribute no parental care.

Both sexes usually have dappled light brown colouration and white undersides outside of the breeding season, but the sides of females and younger males may have a longitudinal black stripe.

[7] When males are reproductively active, their bodies turn completely black and the anal fins gain a pale blue streak.

[6] It is most abundant in subtidal depths up to 5 m, and unlike most other New Zealand triplefins who prefer higher amounts of cover, it occupies a wide range of levels of shelter.

[6][11] The fish can occasionally occupy intertidal rock pools during low tides, but this is thought to be temporary and has not been associated with reproductive activity like its usual habitat.

This preference is potentially a limiting factor for the number of common triplefin within a given population, with these specific algae species providing food, cover, or a combination of both for the developing juveniles.

This lack of specificity may help them by allowing them to occupy whichever microhabitat is unused by other triplefins in their surroundings, letting multiple species coexist together without competing.

Here, despite the potential gene flow between populations of triplefins, the ecological boundary of habitat selection created reproductive isolation between groups.

[4] The common triplefin is often used for laboratory work on fish physiology and behaviour, as it can remain calm but still active enough to move around and engage with tasks while in experimental conditions.

[15] They are also easy to observe in a natural setting due to their lack of skittishness, and so are a popular study species for multiple types of research.

The common triplefin was additionally found to eat a significant amount of caddisflies and sphaeromatid isopods, potentially due to their increased intertidal presence.

[7]  However, feeding does seem to be largely opportunistic, with individuals usually moving after whichever food is closest to them rather than making the effort to hunt any one specific prey type.

In Wellington alone, the breeding season has been recorded as running from August to November,[9] September to December,[18] and July to February,[19] suggesting great variability between years.

By having their nest out in the open, there is potential for them to be visited by higher numbers of females, even if they have to invest more into the defence of their territory from both other males and predators looking to eat their eggs.

These qualities make them preferrable to females, who do not contribute parental care and so after laying have no control over the behaviour of the male safeguarding their eggs.

As ocean temperature continues to increase with global warming, it is possible that the common triplefin may shift its preferred habitat to warmer regions of New Zealand as waters disproportionately heat.