TRIPLEFIN FISHES (family Tripterygiidae) are the most abundant of New Zealand’s shallow-water subtidal reef fish. It is thus surprising that they are so poorly known by most New Zealanders, who refer to them simply as cockabullies or blennies. Blennies are in fact a distinct group (family Blenniidae). Both triplefins and blennies are small fish that lack swim bladders as adults, but triplefins are characterised by having three dorsal fins and scales on the sides of the body, while blennies have one continuous dorsal fin and lack scales. Only one species of blenny—the crested blenny (Parablennius laticlavius)—is common in New Zealand coastal waters.

Approximately 30 genera and 130 species of triplefin have been identified worldwide, in tropical, subtropical, temperate, subantarctic and—in the case of one species recorded from the Antarctic Peninsula—polar seas. But triplefins reach their greatest diversity and size in the New Zealand region, and are the most characteristic and ubiquitous feature of the New Zealand coastal fish fauna. It is no exaggeration to say that triplefins are as iconic among New Zealand fishes as weta are among New Zealand insects and kiwi among New Zealand birds.

The New Zealand triplefin fauna includes 14 genera and 26 species, and thus constitutes a significant proportion of triplefin diversity worldwide. What is even more surprising is that 12 of these genera and all 26 species are unique to New Zealand, except for three species accidentally introduced to Australia. Just two species—the ocellate triplefin (Apopterygion oculus) and the Kermadec triplefin (Enneapterygius kermadecensis)—have close evolutionary affinities to fishes elsewhere. The latter species is endemic to the Kermadec Islands and belongs to a genus of tropical species.

This level of endemism is approximately ten times that of the New Zealand marine-fish fauna as a whole. How it has arisen is an interesting biological question. Are the seas around New Zealand some kind of living museum for triplefins, a refuge where relic species hang on like aquatic tuatara? Preliminary DNA evidence suggests not. New Zealand species seem generally to be closely related to one another, suggesting that most have radiated from an ancestral species over the last 12–25 million years.