Triplefins - New Zealand's answer to Darwin's finches

Triplefins will be familiar to virtually every diver who has plunged below the sea in New Zealand. They are the most diverse and abundant reef fishes throughout the country, and on any one given reef dive, there can be upwards of 15 species within arm’s reach.

Spotting them all, however, is another matter. This is because triplefins range from being highly animated with tropical-like colours, to cryptic and supremely camouflaged. None is more camouflaged than the topknots, who have evolved to match their surrounding algal habitat, even going so far as to have transparent membranes in their fins (in fact the specific name fenestratus is Latin for ‘window’).
All 27 species and 12 of the 14 genera are endemic – unique to New Zealand. This level of endemism is the highest of New Zealand’s fishes, some 10 times that of the rest put together. Their incredible geographic uniqueness is due to the fact that they are nearly all closely related to each other, radiating from a common ancestor. They have been shaped largely by the environment over a relatively short timeframe to arrive at the diversity we see today. We could go so far as to call them our equivalent of Darwin’s finches, being an ideal group to explore questions in biology, ecology and evolution.

Scientists have found in some species that each individual has a unique colour pattern, so each can be recognised by one another, and if we look close enough, by us. This all plays a part in the complex social lives these animals live. Triplefins establish and vigorously defend territories of a few square metres for their entire life. During winter months, spawning fervently takes place. With a polygynous reproductive strategy (one male, several females), the bolshie males spend much of the winter dressed in their spawning garb. Most species turn an almost jet black, whilst others intensify their already bright colours, such as the three Notoclinops species. With prepared nest sites that can include anything from the side of a rock to half a scallop shell, the males then presume to display to attract roving females to lay their eggs, and so continuing their lineage.

Triplefin’s sensory systems are surprisingly acute, and they possess an incredible navigational ability whereby they can home over half a kilometre. How this happens no one can say for certain, but it is likely true navigation – quite extraordinary for such a small animal living below the surface. It is probably equivalent to dropping your dog off blindfolded in the bush 20km from home and expecting him to turn back up on your doorstep. Another remarkable navigational ability is the active habitat selection of the larvae. Around the time when the planktonic larvae are ready to settle on to the seafloor (after around two months drifting near the surface), they actually listen to sound cues from different habitats, actively swimming towards areas with desirable sound characteristics (eg. a nice healthy reef). These provide avenues of research and understanding into navigation, hearing and habitat-related ecology.

Part of the reason triplefins are a good family to study is that they live hard and fast. That is, they generally only live two–three years, racing to maturity within their first six months or so, thereby jostling for territories and spawning habitat. The swift turnover of generations are partly why they have and can evolve so rapidly. An example of a lineage split is the two closely-related twisters that have diverged to occupy different zones in the intertidal. This dynamic zone is an extremely harsh place to live, with constant changes in salinity, temperature and levels of oxygen. The twister Bellapiscis medius has evolved to live in the highest areas of the intertidal, relative to the lower areas and upper subtidal habitat of B. lesleyae, thus making the most of an empty niche. Furthermore, insights into their physiology can shed light on how animals, even ourselves, deal with extreme levels of hypoxia.

There are many other examples of how populations have moved into available niches, under divergent natural selection, thereby generating new species. These include a species not many will have seen, the giant triplefin (Blennodon dorsale), found in the shallows and tide pools of the wild, pounding surf coasts. And also one familiar to most New Zealand divers, the schooling and planktonic oblique-swimming triplefin Forsterygion maryannae – they swim obliquely as all adult triplefins lack swimbladders to aid in buoyancy. Hopefully, their adaptive prowess will enable triplefins to keep up with the rapidly-changing environments in today’s oceans!

Triplefins are the most ubiquitous and arguably charismatic fish fauna in New Zealand. It is no wonder they benefit both recreational divers and scientists alike. They undoubtedly add to any diving experience, whether your interest is simply searching for all the little things that live beneath the waves, or enjoying watching the behaviours of marine life play out. They can also reveal a great deal about fish biology and ecology, and how groups of animals evolve within their environments. Certainly, there is much still to learn. None more evident than the discovery of new species, such as the polka dot triplefin (Ruanoho sp.), found as recently as 2009. This will continue to happen as we closely examine existing species' complexes, or when we explore lesser-known habitats.
Only one thing for it – get out there and keep discovering!