Common Triplefin

Common triplefin sexes are similar in appearance for most of their reproductive cycle, though there is some morphological diversity between populations. 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. This stripe is much fainter in South Island populations, and colouration tends to be darker in fish from more southern locations. When males are reproductively active, their bodies turn completely black and the anal fins gain a pale blue streak. The common triplefin is one of the smaller triplefins in family Tripterygiidae, reaching a maximum of 70 mm in length. Its maximum lifespan has been recorded at up to three years, shorter than most other triplefins, with individuals reaching mature sizes within a year of hatching.

The common triplefin is found across most of New Zealand's mainland coastline, but can also much less frequently be found on offshore islands and has been introduced in Australia. 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. 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. Habitats are most densely occupied by adult fish just prior to the breeding season, and populations gradually become more sparse as adults spread out again after its end. Adults occupy similar habitats to juveniles in terms of depth and substrate.

The common triplefin shares a similar diet with Yaldwyn's triplefin, the blue-eyed triplefin, the yellow-and-black triplefin, the blue dot triplefin, and the cryptic triplefin, with these species all having a diet consisting mostly of gammarid amphipods, ostracods, limpets, and errant polychaetes. The common triplefin was additionally found to eat a significant amount of caddisflies and sphaeromatid isopods, potentially due to their increased intertidal presence.

The feeding behaviour of common triplefins is predatory, with individuals stalking their prey. They will make a brief pause before launching towards it, generally swallowing the animal whole. 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.

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. 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's reproductive cycle typically begins in late winter and ends in early summer, although this can change from location to location. In Wellington alone, the breeding season has been recorded as running from August to November, September to December, and July to February, suggesting great variability between years. The males will firstly take on their reproductive colouration, and begin aggressively defending territories from other males as well as courting females. After choosing a male, females will lay large egg masses in the male's territory, most often under sloped surfaces of rocks and other structures. Multiple females may lay eggs in one male's territory over the course of the initial breeding period. The male will then guard the eggs, up to 2000 in total, for two to three weeks until they hatch.