Tadpole shrimp (Triops cancriformis)

Synonyms: Apus cancriformis
KingdomAnimalia
PhylumArthropoda
ClassCrustacea
OrderNotostraca
FamilyTriopsidae
GenusTriops (1)
SizeBody length (in captivity): up to 6 cm (2)
Body length (in the wild): up to 11 cm (2)

Classified as Endangered in Great Britain, and fully protected under the Wildlife and Countryside Act 1981 (3).

Triops cancriformis is a living fossil; it has not changed in appearance since the Triassic period, 220 million years ago, and is therefore the oldest living animal species known (2). With its flattened shield that covers the head- and leg-bearing parts of the body, Triops resembles a small horseshoe crab. The segmented abdomen bears two projections (4). All notostracans (tadpole shrimps) have 2 internal compound eyes plus the larval nauplius eye (Triops means 'three eyes'). In females, the 11th pair of legs is modified and carries the eggs (2).

The species is widely distributed in Europe through to Russia, and from the Middle East to India. In Great Britain it was recorded during the 18th and 19th centuries from Kent, Bristol, Dorset and Worcestershire, and until September 2004 it was thought to be restricted to just one pond in the New Forest in Hampshire. However, it was then found in pools in Caerlaverock in southwest Scotland (5). Genetic studies on the species may reveal that the species found in Britain has great international importance (3).

This species lives in temporary ponds that dry out in summer (3).

Due to the uncertain nature of the ponds in which this species occurs, it has an extremely fast life-cycle, and develops from an egg to an adult in just two or three weeks (depending on temperature) after the pond becomes wet again (3). When the pond dries out, the resistant eggs can remain dormant for decades before hatching when the pond is re-flooded (2).

The tadpole shrimp digs in the sediment with its shield to find food (6), it feeds on small invertebrates, microscopic particles and plants (3). It usually swims with its shield upwards (4), but when oxygen levels become very low they may swim 'upside-down' with their legs close to the surface of the water (6).

This species is very sensitive to veterinary compounds that are released into the water from livestock dung. Predation by introduced predators such as ducks and fish, as well as pollution and the introduction of alien plants are also likely to have affected this species (3).

The tadpole shrimp is targeted as a priority species for conservation action under the UK Biodiversity Action Plan (UK BAP). The Species Action Plan produced as a result, aims to maintain the population in the New Forest and develop captive populations to allow research, increase public awareness and appreciation into the species, and in the long-term enable potential reintroductions of the tadpole shrimp into parts of its former range (3). Measures taken to conserve this species will also be likely to benefit other species that inhabit temporary ponds, such as the dung beetle Aphodius niger (3). The New Forest is an SSSI (Site of Special Scientific Interest), an SPA (Special Protection Area), a RAMSAR site (a Wetland of International Importance) and a candidate SAC (Special Area for Conservation). English Nature's Species Recovery Programme is guiding research into the species, which is promoted by the Triops Conservation Group (3).

Information authenticated by Dr Erich Eder, University of Vienna.

  1. National Biodiversity Network Species Dictionary (July 2002) http://www.nhm.ac.uk/nbn/
  2. Triops CC (July 2002): http://www.triops.cc/
  3. UK BAP (July 2002): http://www.ukbap.org.uk
  4. Sterry, P. (1997) Complete British Wildlife Photoguide. Harper Collins Publishers, London.
  5. Morrell, B. (2005) Centre News. Wildfowl and Wetlands, 151: 30.
  6. Large branchiopod website (July 2002): http://mailbox.univie.ac.at/erich.eder/UZK/index2.html#notostraca