In August, after the long weeks of cloudless blue skies, and heat that shimmered over the cracked mud of the merse, the rain came. The jet stream had looped into another orientation, and the rain fell day after day for a week. Dr Larry Griffin emailed me: “the rain after the heat has brought the eggs out of stasis in at least two pools” and so he was ready to take a small group to have a look.
Larry is the Principal Species Research Officer at the Wildfowl and Wetlands Trust (WWT) Caerlaverock Reserve on the north coast of the Solway Firth, near Dumfries, and Caerlaverock is one of only two places in the UK where the rare and elusive tadpole shrimp, Triops cancriformis, is – occasionally – found. 
The modern story of Triops is a story of discovery and loss and re-discovery. But this is an animal that has scarcely changed since the Triassic period – its fossils date back two hundred million years. It is a freshwater Crustacean, belonging to the Order Notostraca, and looks very similar to (but is unrelated to) a small horseshoe crab as it trundles around on the bottom of a pond: its head and thorax are covered by a carapace, like a shield, so that from above its legs and mouthparts are scarcely visible.
Was it always the case that Triops lived, fed and bred in freshwater pools? This is a dangerous life-strategy yet despite, or because of, this danger the animals evolved a means of surviving when the pools dried out. Today, they are found only in ephemeral pools in the New Forest and on the saltmarshes (merse) and wet pastures around Caerlaverock. They can live for two to three months, or until the tide inundates the merse – and when the pools dry out under the hot summer sun, the adults die and disappear. But in the drying sediment their eggs live on, yet ‘switched off’ in a state of diapause.
When the rain comes and the pond re-wets, the eggs are stimulated to hatch – and the larvae feed and grow very quickly, so that adults are ready to lay eggs within as little as two to three weeks from hatching. As Larry says, “They can flash in and out of existence – it’s just luck whether you find them. It needs an inquisitive person … in the right place at the right time, who thinks ‘Oooh, what’s that?’ ”
This explains why the Triops’ story has been so exciting and challenging; for me even more so because this special animal survives just across the Solway from where I live, and I now have a chance to see the creatures alive, rather than – many years ago – pickled in formalin in an undergraduate Invertebrate Zoology class.
It was more than 100 years ago that Triops (then known as Apus) was found on the merse, by F. Balfour-Brown. Forty years later, in 1948, in a paper to Nature, he wrote (or narrated – the style of scientific writing was much more like story-telling than it is now):
“In September 1907 I discovered two shallow grassy pools on the Preston sea merse, near Southwick, Kirkcudbrightshire, in which Apus was present. In one of these it was so abundant that when I raised my eleven-inch ring net out of the water it was half full of specimens, mostly full-grown. I searched many other pools in the same area but without finding it and, returning to the same pools a few days later, I found the edges covered with the shells and very few specimens left in the water. The gulls had discovered this mass of food and had destroyed most of the Apus. I have visited the area many times during the last forty years but not until this month, working the merse near the mouth of the Southwick burn, have I again seen Apus. My son found three specimens in a pool which then yielded us about thirty or more, and several other pools near the first produced small numbers, mostly immature.”
These pools were subsequently lost through erosion of the merse, but Tadpole Shrimps were found again, nearly 20 kms away, in 2004.
Larry grinned as he told me about his own discovery. “I’d been out looking for natterjack tadpoles, it was quite a late season. I thought I might as well have a look in that pool – and there they [the Triops] were, tons of them!”
He wasn’t immediately sure what he had found, until he took some of the animals back to the lab. “It was a gradual dawning… I was looking in the back of a Collins book, I think it was – there was a drawing, a silhouette. And then I had a memory of someone from the RSPB, who had been talking about trying to introduce them somewhere – and then it was ‘hang on, this is that incredibly rare thing’!”
Collaboration with Dr Ruth Feber , from Oxford University’s WILDCru, and Professor Colin Adams of Glasgow University’s Scottish Centre for Ecology and the Natural Environment (SCENE), and their colleagues, led to discussions, then further investigations and research at WWT Caerlaverock.
Colin and colleagues collected sediment from the bottom of 86 ponds at Caerlaverock. It was known that the eggs or cysts needed to be desiccated to enter the quiescent, diapause, state, so samples were dried out and then incubated in fresh water at 20oC. Samples from just two ponds produced living larvae (nauplii). And then, “During the preparation of this [scientific] paper, on 29 July 2010, a population of c. 100+ adults of various sizes up to 3cm were discovered by chance in another ephemeral pond 80m west of the original 2004 site on the Caerlaverock upper salt marshes. This pool was largely created by tractor wheel ruts and cattle movements in an area of gorse (Ulex sp.) and rushes (Juncus effusus).” 
As a result of this serendipitous discovery, just three ponds at Caerlaverock were found to be home to Tadpole Shrimps!
It’s thought that eggs are spread in several ways – through the guts of animals, on the feet of cattle, deer, or geese, “Anything traipsing around, really,” Larry says. This includes the tyres of tractors and quad bikes, and because the Tadpole Shrimps are hermaphrodite, “It just needs one egg, and after it hatches and survives, you could then get hundreds.”
Sometimes the shrimps themselves have been found, other times it has been the presence of the eggs in washed and filtered sediment samples. Larry reckons there could be stores of eggs of different ages in some of the sites. “I feel a lot better knowing that it’s not just in the one pool!”
He has collected mud from the ponds, as a safety precaution and also as part of his research. There’s a shelf in his office piled with polythene bags that contain mud-samples dating back to 2008. “I wet them every September – so far, they’re still hatching out. This year will be the tenth year [to test them].”
Larry uses tanks of de-chlorinated tap-water, bubbled with air for a day to oxygenate it, then throws in the mud-sample containing eggs. “It’s really low-tech! I feed them on fish food and a bit of sea-weed to provide some iodine. They show good behaviour! They learn that when I come into the office, I’ll feed them. They come up to the surface – and I sprinkle the food on the surface, and they turn over and swim on their backs so they can take it with their little legs.”
The adult shrimps lay their eggs in little scrapes in the mud. From an egg less than 0.5mm diameter, an adult can grow up to 7 cm long when reared in the lab. It reaches maturity by passing through several larval stages, each of which grows then moults its skin, or exuvium, before passing on to the next stage.
How often does Larry find the Triops out on the merse? “Most years. It depends on the weather, but if conditions are right, you can easily see them crawling around. One clue is the exuviae, you find them wind-blown at the end of a pond.”
When Larry emailed me after the rain, it was because he had seen these papery skins at the edges of a pond and found some nauplii. But finding a date for even our small group of privileged people to join him proved impossible – and the pools dried out again. This was not to be the year when I would see living Triops in the wild.
Are they living on the other Solway saltmarshes, at Rockcliffe, say, or at Campfield or Newton Arlosh? After all, these are no distance as the goose flies (and large flocks of geese do fly between these sites).
People have looked, and Bart Donato, of Natural England, who has a great interest in Rockcliffe, says the shrimps have never been found there. But it needs someone to be there at exactly the right time, someone who can recognise what they are seeing.
Now, though, there’s a new molecular diagnostic technique available, to test environmental samples for a range of ‘eDNA’ – fragments of genes that are species-characteristic and which may have been left ‘lying around’ in the environment. Graham Sellers from Hull University has recently developed this ‘DNA bar-coding’ technique for Triops and, working with Larry, has established the existence of the shrimp in six ponds at Caerlaverock. 
Perhaps this technique can now be used for pools on the other Solway saltmarshes?
It would be exciting, and a relief, to find that the story of the rare and elusive Triops can be expanded to a few more chapters: that this enigmatic survivor will remain an important addition to the ‘shrimps’ of the Solway.
Meanwhile, I’m hoping for a hot dry summer in 2019, followed by a spell of rain …
1.Triops cancriformis is classified as Endangered, listed as a priority BAP species, and specially protected under Schedule 5 of the Wildlife and Countryside Act 1981.
2. Ruth Feber et al. June 2011. Ecology and conservation of the Tadpole Shrimp, Triops cancriformis, in Britain. In British Wildlife, p334
3. Colin Adams et al. 2014. Short range dispersal by a rare, obligate freshwater crustacean Triops cancriformis (Bosc). In Aquatic Conservation: Marine & Freshwater Ecosystems. 24: 48–55
4. Graham Sellers et al. 2017. A new molecular diagnostic tool for surveying and monitoring Triops cancriformis populations. PeerJ 5:e3228; DOI 10.7717/peerj.3228