What lies beneath? A new species of endemic triplefin

What lies beneath? A new species of endemic triplefin

A recent discovery has highlighted, yet again, just how poorly we know what lives around our coasts – some within sight of our major cities – especially among rocky reefs. Te Papa’s fish expert Andrew Stewart introduces us to the polkadot triplefin.

In 2009 and 2011, NIWA’s RV Tangaroa undertook a series of surveys around the coast of New Zealand to sample the shelf to about 200 m depth to gain an understanding of the fauna there.

NIWA’s sampling included small, robust rock dredges hauled over rough, rocky sea bed, and a number of rare fish species were caught, including new records for NZ EEZ, and some new to science – including the polkadot triplefin, Ruanoho sp. (see photo).

A pale fish lying on a grey surface
Polkadot triplefin, Ruanoho sp. Te Papa (P046307)

Teeny weeny yellow polka dots

This is a pretty little fish with distinctive bright yellow spots over the head and gills, and narrow, bright yellow lines over its dorsal, caudal and anal fins. Mature males develop a dusky charcoal hue, typical of others in the family. Maximum size is 75 mm total length. Currently, it is known from just six specimens caught at 108–216 m depth, off North Cape to Rakiura/Stewart Island and is being described by Andrew Stewart (Te Papa) and Prof. Kendall Clements (Auckland University).

Its discovery has come as a surprise, because it belongs to a genus with species only found in very shallow water – less than 35 m deep.

We are currently awaiting the genetic analysis being done by a former student of Kendall’s who is in Denmark. He is recovering from having had Covid-19 so it might take a few months. In the meantime, we continue to ponder the possible conditions that cause this fish to evolve from its sister species R decemdigitatus, the longfinned triplefin and R. whero, the spectacled triplefin.

Cycles of Pliocene-Pleistocene ice ages (from about 5 million years ago to about 12 thousand years ago), with associated large rises and falls of sea levels, coupled with our long geographic isolation, and emergence of New Zealand from the sea, have rapidly driven the evolution and radiation of triplefins.

As a result, New Zealand has the most diverse triplefin fauna in the world: 12 genera (10 endemic), and 27 species (all endemic) – about one-sixth of the global total. These include the deepest living species – 110 m to 500 m depth, and this new discovery increases the ‘deep-water’ species to five.

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