Every once in a while, a species thought to be extinct makes a sudden reappearance. The coelacanth is the most famous example. Dating back 70 million years, the fish were known only from fossils until 1938. However, they have survived in deep water around the Comoros and parts of Indonesia. Now it is the turn of a more modest creature, albeit on a smaller scale.

The tadpole shrimp has been around 220 million years. Found on every continent except Antarctica, there are several species. Despite the name, they are not a true shrimp but are freshwater crustaceans. They have remained the same for millions of years and are often described as living fossils with shield-like carapaces and paired phyllopod appendages. It looks like a small horseshoe crab. Tadpole shrimp are omnivores, eating aquatic plants, small invertebrates, and organic particles in sediment.

The species, Triops cancriformis, was on the verge of disappearing from the UK, and the Scottish population was considered extinct. The only other UK location is one pond in the New Forest of Hampshire. However, four years ago it was identified at Caerlaverock, in a single pond.

The shrimp live in a very specific habitat - pools that dry up in the summer and reform with rainy weather, known as ephemeral wetlands. Other pond inhabitants, those that compete with shrimp and others that prey on them, cannot withstand drying out. But the tadpole shrimp lays cyst-like eggs that lie dormant - in a state known as diapause - until conditions are favourable. Heavy rain in August has prompted a rapid hatching of eggs and increased shrimp numbers. The egg cysts remain viable up to 20 years.

To take advantage of seasonal opportunities, tadpole shrimp have a rapid lifecycle. The time from egg to adult can take two to three weeks, a single female can produce thousands of resistant egg cysts, and the entire lifespan is a mere 4-6 weeks.

In the UK, tadpole shrimp are listed as endangered. Due to their dependence on ephemeral ponds and freshwater, any encroachment on these areas can be detrimental. The introduction of predators, such as fish or birds, pollution, and other changes are all threats. Changing sea level can also affect ponds by raising salinity to levels too high for tadpole shrimp.

There are commercial forms of Triops, used for research and teaching. However, the European species is considered the oldest living animal, while the usual form is a related US species.