Fairy shrimps Chirocephalus diaphanus are, like tadpole shrimps, specialists of temporary ponds that periodically dry out. They are not true shrimps, though they are crustaceans, and belong to the Anostraca which is a group that also includes the brine shrimps used as fish food in the pet trade and as ‘sea monkey’ pets.
They are long creatures with eyes on stalks, a long thin abdomen with 2 ‘tails’ and swim upside down by beating their phyllopodia which are actually their legs, which as in most crustacean have attached gills. They swim around in the water column, often at the surface, unlike other freshwater crustaceans, usually at a leisurely pace, but can move pretty swiftly when they feel threatened. They can be quite colourful when observed up close with a mixture of reds, pinks, purples and greens on a browny background.
They can be found in all sorts of temporary freshwater bodies, but seem to favour those that have some disturbance, such as from livestock or vehicles going through, in fact on Salisbury Common are found in small numbers in many of the depressions and ruts made by tanks moving around. In the UK they are generally found in ponds in the South and South East of the country as the warm summers and generally drier conditions lead to the formation of more suitable temporary ponds than in the wetter North and Western areas of the country.
Their specialisation to living in temporary pools has required a number of adaptations. They have drought resistant eggs that can lie dormant in the dry pond and when it is refilled with rainwater some will hatch to take advantage of the good conditions, while other will not, acting as an sort of insurance policy in case those that do hatch do not manage to reproduce. They are not as selective in the conditions on which they will hatch as tadpole shrimps, tolerating a wide range of pH, oxygen levels and in particular temperature, which allows them to hatch if they pond is refilled at cooler times of year. This means unlike the tadpole shrimp, fairy shrimps are often an annual occurrence in their temporary ponds, usually appearing in autumn, providing their pond has refilled. The eggs do not hatch if the pond has not dried out before favourable conditions have returned as this could been predators have colonised the pond and they would be vulnerable due to their slow free swimming habits. The small eggs can be moved around easily on larger animals, such as on birds feet, which can transport them to new ponds where they can form new colonies and those on Salisbury Common are moved from depression to depression by the constant movement of tanks, allowing a colony to persist across a number of small bodies of water.
Once the eggs hatch they can reach maturity in as little as six weeks in favourable conditions, allowing the population to reproduce and persist in quite ponds with changeable conditions and prone to rapidly drying out, though typically they complete their lifecycle in 3 months. They hatch into naupllii and moult and grow until they reach their adult form up to 4cm long. The males have a pair of tusks which are actually modified antennae used to grip the female during reproduction and the females can be recognised by their egg sacks hanging off their abdomen just behind the legs.
They feed on algae, tiny zooplankton invertebrates and detritus which they filter out of the water with their phyllopodia as they swim around.
Unfortunately the ponds on which they depend are now in short supply and they have disappeared from many areas where they were previously recorded, and the remaining populations are vulnerable to their ponds completely drying out due to succession or being destroyed by man or in some cases water remaining in the pond too long each year and allowing colonisation or introduction of predatory fish. Due to this rarity and vulnerability they are protected in the UK under the Wildlife and Countryside Act and require a schedule 5 license to disturb or catch them.