Just Add Water

Tuesday 9th July 2024

… a blog by Craig Macadam, Conservation Director at Buglife.

It might not look like much but the handful of sediment in my hand probably contains 50 or so eggs of one of the rarest invertebrates in Britain, and one of the oldest known animal species in the world. 220 million years ago, while dinosaurs roamed the land, these amazing crustaceans were swimming around their feet in temporary pools and wetlands.

Tadpole Shrimp (Triops cancriformis) are prehistoric and look it, with a shield shaped carapace; they look like a miniature cross between a Horseshoe Crab and an extinct trilobite. Lacking modern gills, they breathe with primitive leaf-like extensions on their legs. The eggs in my handful of dirt are tiny, barely a millimetre in length, and orange which allows you to just about pick them out from the darker sediment.

They live in seasonally brackish (slightly salty) or freshwater pools, which dry out in the summer. When the pools dry out all predators and competitors are killed off however the Tadpole Shrimps have adapted to survive. Their pools may only last a few weeks before drying out, and then it may be years or even decades before the water returns.  As a result of their short lived habitats Tadpole Shrimps have a very rapid development, maturing from an egg to adulthood in just two to three weeks. Utilising such an inhospitable and difficult habitat is believed to be one of the reasons why this group of animals has been able to survive for so many millions of years.

Tadpole Shrimps are able to survive in temporary pools due to the unique properties of their eggs.  When they are laid a proportion of the eggs hatch, and the rest go into ‘diapause’ – this means the eggs dry out and their development is stopped.  In diapause Tadpole Shrimp eggs are very durable and can survive up to an amazing 27 years. The eggs can also endure extreme temperatures, immersion in salt water, and can be eaten and excreted by an animal without harm!  Once these eggs are rehydrated and the environmental conditions are right, the diapause will end and the eggs will hatch forming a new generation of shrimps.

In the UK, Tadpole Shrimps are currently found in only two locations, one in the New Forest, Hampshire and the other in Scotland near the Solway Firth. The only other record in the last 100 years is from the Kirkcudbrightshire coast, however this population is believed to have been lost in the 1960s as a result of coastal erosion.

Larry spreading Tadpole Shrimp eggs © Craig Macadam

With a limited distribution, these crustaceans are intrinsically at risk of extinction. The introduction of predators, rising sea levels, water pollution and invasive aquatic plants could all sound the death knell for either of the populations.

That’s where my handful of dirt comes in. Buglife, RSPB Scotland, the Wildfowl and Wetlands Trust and ecologist Larry Griffin are working together as part of the Species on the Edge programme to re-establish a second population of Tadpole Shrimp on the Solway Coast.

Over the past year, Larry has reared Tadpole Shrimps hatched from sediment collected at the remaining Solway site. The sediment was heated to 60 degrees to kill off any pathogens present, then a small amount was re-wetted, resulting in tiny Tadpole Shrimp hatching from the eggs within 48 hours. The shrimps grew rapidly and busily dug pits in which to lay their eggs. This sediment was then heated again and stored over the winter.

Today, we spread 10 kilograms of egg-laden sediment into the bare earth of the now dry, seasonally wet coastal pools at the RSPB’s Mersehead reserve on the Kircudbrightshire coast. Such a simple action, but crucial for the future of this prehistoric species. The rest of the recipe for recovery is simple – Just Add Water. When the next rains arrive we’ll eagerly await news of the first Tadpole Shrimps to be seen on this stretch of coast in 60 years.

Main Image Credit: Tadpole Shrimp eggs in handful of sediment © Craig Macadam