The pouched lamprey (Geotria australis), also known as the piharau[1] in the North Island, korokoro,[2] kanakana in the South Island, or wide-mouthed lamprey, is a species in the genus Geotria, which is the only genus in the family Geotriidae.[3] The second species in the genus is the Argentinian lamprey (Geotria macrostoma), which was revalidated as a separate species in 2020. The pouched lamprey is native to the southern hemisphere. It spends the early part of its life in fresh water, migrating to the sea as an adult, and returning to fresh water to spawn and die.
G. australis, like other lampreys, has a thin eel-like body, and grows up to 60cm (20inches) long.[4] It has two low dorsal fins on the back half. Like other lampreys, it has no jaws, only a sucker. The skin is a striking silver in adult lampreys caught fresh from the sea but soon changes to brown after they have been in fresh water for some time, due to deposition of biliverdin.[5] Adult's eyes are relatively small and located on the side of the head. When fully mature, males develop a baggy pouch under their eyes, which may be used to massage and oxygenate their eggs.[6] There have also been suggestions that the pouch in northern hemisphere species has been used by males during breeding times for gathering stones to make a nest.[7]
The freshwater ammocoete or larval stage of the life cycle are a dull brown in colour for most of their lives. Ammocoetes remain in fresh water for about four years until undergoing a six-month metamorphosis,[8] changing to silver with blue-green stripes. The central nervous system of the pouched lamprey develops notably during metamorphosis to the large-eyed macropthalmia stage, with particularly large increases in the volume of visual areas of the brain.[9] At this point they migrate downstream to the sea.
Adults spend some of their lives in the open sea, living as parasites on other fish. They attach themselves to the gills or side of the fish and rasp at the tissues below. Adults return to fresh water to breed, spending up to eighteen months sexually maturing before spawning. Adults have been recorded living up to 105 days after spawning and wrapping themselves around egg masses to provide parental care.[10]
The pouched lamprey is widespread in the Southern Hemisphere, occurring in New Zealand, Chile, Argentina, the Falkland Islands, South Georgia and the southwest and southeast corners of Australia. It is the only lamprey species found in New Zealand.[11]
Lampreys are preyed on by albatrosses, shags, large fish and marine mammals. It has been hypothesised that the apparent decline in lamprey numbers could be caused by the degradation of water quality in lowland waterways.
Pouched lampreys are a traditional Māori delicacy in New Zealand. Traditional methods for catching lampreys included disturbing the lampreys as they ascended waterfalls and capturing them, or by using Maori: utu piharau, which involved placing a weir across larger rivers which led to a hīnaki (woven trap). Some Maori: utu piharau across the Whanganui River were more than across.[11] Pouched lampreys were widely seen in New Zealand in the mid-19th century, and were adopted as a food by European settlers, due to the history of lampreys as delicacies in Europe.[11]