Of all the creatures that inhabit our waterways, the tadpole shrimp might be among the most intriguing.

Shaped like a horseshoe crab, and with two long, protruding tails, it glides through shallow pools and ponds using the paddle-like limbs beneath its body.

Scientists considered it as something of a living fossil, as its lineage hasn't changed much from the Triassic period that ended some 199 million years ago.

It's also rarely seen – and a new stocktake suggests we soon might never see it again.

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The Department of Conservation's (DoC) latest threat classification shifted the tiny species from at risk/naturally uncommon to threatened/nationally endangered.

DoC's freshwater manager, Natasha Grainger, said the species was declining on both mainland islands, as its habitat of small ponds continued to be lost.

"Concern is mounting for its continued persistence in very small, widely dispersed locations."

The species was among 675 species assessed in the new report, of which 47 were classified as "nationally critical", 10 were "nationally endangered", 14 were "nationally vulnerable", nine were "declining" and 81 were "naturally uncommon".

The conservation status of 17 had changed since the last stocktake five years ago.

The stonefly Zelandobius wardi and the mayfly Nesameletus vulcanus were now also considered threatened/nationally endangered, while the mayfly Zephlebia pirongia, the isopods Notamphisopus dunedinensis and N. benhami, and the amphipod Chiltonia minua were now at risk/naturally uncommon.

Better information had allowed some to be assessed for the first time – but in other cases, such as with an unnamed crayfish Paranephrops sp, and a mussel, Echyridella onekaka, there simply wasn't enough data to classify them.

In all, one quarter of the listed species – or 178 – couldn't be assessed for this reason, so they were categorised as "data deficient".

A further 88 taxa were assessed with the qualifier "data poor" - reflecting a lack of comprehensive data to support the assessments.

"This lack of knowledge reflects the limited number of researchers working on freshwater invertebrates," Grainger said.

"This is a problem because the presence or absence of many freshwater invertebrate species provide strong indicators of the health of aquatic ecosystems.

"Our ability to effectively plan for the conservation of freshwater species and their habitats is hindered because of these reasons."

The problem wasn't just limited to freshwater invertebrates.

The most recent DoC reports showed 54 per cent of native lichens, 37 per cent of chimaeras, sharks and rays, 23 per cent of butterflies and moths, 21 per cent of marine mammals, 14 per cent of marine invertebrates and 18 per cent of mosses were tagged as "data deficient".

The assessment followed that of New Zealand's 76 known native freshwater fish species, of which 22 were threatened.

Almost all of those belonged to the to the Galaxiidae family, which included Canterbury mudfish, now on the brink of extinction.

A further 17 were classified as "at risk".