Natasha Drury, a summer research scholarship student working with DOC on threatened freshwater invertebrates, Tom Drinan Freshwater Technical Advisor , and Jess MacKenzie share some thoughts about one of New Zealand’s strange invertebrates – the tadpole shrimp.
A few years ago, a group from Eastern Institute of Technology (EIT) doing some fieldwork close by to the Tutaekuri River unexpectedly stumbled across some tiny prehistoric-looking critters in a wetland.
These creatures looked like an extinct trilobite and a horseshoe crab traversed the bounds of time to have a teeny tiny baby. It’s not what you’d generally expect to find in a temporary pond – unless you’re in the know.
They’d found tadpole shrimp, or Lepidurus apus viridis, a freshwater invertebrate found only in New Zealand, Tasmania, and parts of coastal mainland Australia. People also refer to them as shield shrimps due to the large carapace (kind of like a shell) that wraps around their body.
Although this little population found in Hawke’s Bay by EIT is still the only one we currently know about in that region, it’s likely there are more out there. They’re not common in New Zealand, but there are certainly more populations around the country – they’re just hard to find.
Let us tell you why it’s amazing when you do.
These little critters are primitive crustaceans of ancient origin – they haven’t changed much since the Triassic period, when they lived alongside dinosaurs. They’re considered to be ‘living fossils’ and are one of the oldest surviving species on the planet. Interestingly, the oldest notostracan fossil record dates back to the Upper Carboniferous period (327 to 299 million years ago!). Tadpole shrimp are typically up to 5–6 cm in length. They swim around (usually on their back) using the paddle-like limbs underneath their carapace, and have two long ‘tails’ called caudal filaments the stick out from their carapace. They can be a variety of colours, from pale green to dark brown (as well as yellow-red), and may be mottled or have a solid colour.
Tadpole shrimp are omnivorous, meaning they feed on both plant and animal matter (like plant detritus and small aquatic invertebrates). Their main mode of feeding is by filtering particles from the sediments stirred up as the animals move around. This may explain why they spend most of their time near soft sediments at the bottom of their watery home.
Most records of tadpole shrimp from both islands are from temporary ponds, tarns (small mountain and high country lakes) and wetlands. Populations have been recorded in Canterbury, Otago and Marlborough in the South Island, and in Hawke’s Bay, Manawatu-Wanganui, Wellington and Taranaki in the North Island.
Tadpole shrimp have adapted perfectly for this type of habitat – although some populations are known to live in permanent bodies of water.
They lay their eggs in the waterbody before it dries up. These eggs are very tough and resistant to both drying out and freezing, which is important as they may have to lie in the sediment for many months (or even years!) until the next rainfall. The eggs can also be spread to new waterbodies by the wind or by animals, like waterfowl, when the sediment containing little tadpole shrimp eggs sticks to their legs.
When the waterbody fills up again after rainfall, these eggs hatch into juvenile tadpole shrimp (called metanauplii). If conditions are right, they can reach sexual maturity within about two weeks. They only have a short lifespan so need to make the most of it.
Adults are most likely to be found in September or October, before the water begins to dry.
The tadpole shrimp was classified as ‘Threatened – Nationally Endangered’ in the most recent conservation status assessment (in 2018), meaning it faces a high risk of extinction in the short term. Information on actual numbers of tadpole shrimp populations in New Zealand is patchy, however, as their unusual lifestyle makes them tricky to survey.
We do know they’re facing challenges that are causing their range to decline:
• The temporary ponds they need are often either filled in or modified as part of land use change (often associated with agriculture).
• Increased water use with activities like irrigation can change the hydrology of these habitats, making them unsuitable habitat for tadpole shrimp.
• Stock access can degrade the habitat via “pugging” – essentially, the ground gets churned up by lots of tromping hooves, compacting the soil below.
• Wetland drainage causes a loss of habitat for a huge range of species, including these guys.
• Invasive plant species (like grey willows) can lead to structural changes in the habitat, which can make it unsuitable for tadpole shrimp.
• Climate change will also impact this species and its habitat. For example, it will change the presence and cycles of water in a habitat (its hydrologic cycle), and increase the growth rates and distribution of several invasive species.
And if you happen to be near a temporary pond, tarn, or wetland and spot a tiny prehistoric critter, please report it to your nearest DOC office.
You can also help increase public awareness of this species by spreading the word, like so many tiny tadpole shrimp eggs on water bird’s legs. Not only does this let people know they should look out for them and protect their habitats, you’ll enrich your friends’ lives by letting them know such a creature exists.