Tessa Mills, manager at the New Zealand Marine Studies Centre and Aquarium at Portobello, knows the answer: "a lanternfish".
And she should know, after recently finding a beautiful silver-blue specimen washed up at Allans Beach, on the Otago Peninsula. She had spotted its slender, compressed body glinting on the sand.
English-born, and in her 50s, Mrs Mills has, since childhood, been a regular beachcomber, starting at Southport , in the English North West, where she grew up, then on Otago Peninsula since moving to Dunedin about 14 years ago.
The fish was in remarkably good condition, and it was also surprising to find anything washed up on the beach, usually swept clean by wind and water.
"It is quite unusual to find anything at all on Allans Beach, except for the odd sea lion or penguin."
As soon as she saw the fish's "big eye" and photophores (light emitting organs), Mrs Mills realised this was something "quite unusual".
It was the first time she had found a lanternfish, which often operates in deep waters.
Andrew Stewart, Te Papa fishes collection manager, confirmed it was a Hector's Lanternfish.
The fish can occur in vast schools "numbering in their millions" in southern hemisphere waters, but their scales were very fragile, "so seeing one with most of them is unusual".
University of Otago zoologist Associate Prof Liz Slooten said some species of lanternfish could operate at depths of down to 1500 metres, but the Hector's Lanternfish was often found in shallower waters - about 100m or less.
The specimen is being preserved at the marine centre.
