6 World’s oldest whitebait swam in volcanic lake

Osteichthyes, Galaxiiformes: Galaxias effusus

‘Galaxiid’ is the name often given to a distinctive Southern Hemisphere group of small to medium-sized freshwater fish (100 mm to 580 mm long as adults) that have no scales and lack an adipose fin. More than 20 species are currently recognised in the modern New Zealand fauna, but the exact number of species is a little unclear. The juveniles of five species that make up ‘whitebait’ were once abundant in many New Zealand streams, rivers, lakes and swamps, but are now under threat from habitat modification and overfishing. Local names for galaxiids include galaxias, mudfish, inanga or inaka (for Kāi Tahu) and kōkopu, amongst many other te reo Māori names used variously by iwi for adult and juvenile fish. The family name Galaxiidae refers to the small, silvery-gold star-like patterning seen on many galaxiid fish.

Long considered to be of Gondwanan origin, galaxiids are found today in Tasmania, Australia, New Caledonia, South America, including the Falkland Islands, as well as New Zealand. In Aotearoa today, species occur from Northland to Stewart Island and as far afield as the Chatham Islands, Auckland and Campbell Islands. Most species are diadromous, spending the early part of their life cycle at sea before returning to freshwater environments to reproduce. A few species are land-locked and non-diadromous (from choice or chance) and their entire life cycle takes place in lakes.

Globally, the only known galaxiid fossils occur in lake deposits in Otago. The species Galaxias effusus found at Foulden Maar, at 23 million years old, is the oldest found so far. The best-preserved and most complete fish to be found at Foulden was effectively ‘filleted’ by a field knife splitting the laminated diatomite in which it occurred, with one side preserved on one layer and the counterpart on the other. The fish is more stout-bodied than most living Galaxias and differs from all modern species in its relatively low vertebral count, and expansive dorsal, anal and caudal fins.

More than 100 galaxiid fossils have now been collected from the two small Foulden mining pits; most are articulated, laterally compressed skeletons. They range from 40 mm larvae (whitebait) to 140mm long adults. Randomly dispersed throughout the exposed diatomite sequence, the little fish show no preferred orientation and no mass mortality layers have been observed. The exceptional preservation of soft tissue with eyes, skin and skin patterning is one of the reasons why the term Konservat-Lagerstätte is applied to Foulden Maar.

The stratified nature of the maar lake meant that only the upper few metres had sufficient oxygen for fish to thrive. The deeper bottom waters were anoxic so the fish may have succumbed to low oxygen levels if they swam down too far (many specimens died with gaping mouths). The lack of current activity and absence of bioturbation or scavenging at the bottom of the deep lake meant that the little fish remains lay undisturbed until covered by each successive seasonal rain of diatoms.

At Foulden Maar, the remarkable level of preservation allows us to identify the food of these 23 million year old fish. Living galaxiids in New Zealand are generalised invertebrate predators, eating beetles, moths and caterpillars, as well as aquatic insects including caddisflies, mayflies, midges and their larvae. Some of the Foulden fish have gut contents that preserve their final meal and indirect evidence for their food is abundant in the form of coprolites.

—Written by Daphne E Lee