Sunday, December 7, 2014

The tadpole shrimp

Tadpole shrimp - Triops cancriformis 

Tadpole shrimps are small crustaceans that were already alive 300 million years ago. They are living fossils that have outlived the trilobites, dinosaurs and mammoths and millions of other species.

They live in ponds and pools, an unstable habitat because they often dry up during parts of the year.

The tadpole shrimp found in the Maltese islands, which goes by the scientific name of Triops cancriformis, has existed unchanged for the past 200 million years or so. It is the oldest living species known.

Triops cancriformis is found in Europe, the Middle East and Japan. In many parts of Europe it has lost its habitat and has decreased considerably. In some areas it is endangered and strictly protected. Only two populations are known in the UK.

The tadpole shrimps I have seen in the Maltese islands have been about six centimetres in length although it is not unknown for members of this species to grow up to eleven centimetres long.

In Malta it has become very rare mainly because of destruction of its habitat. Many of the sites in which it used to be found have disappeared and it has also disappeared from most of its old but still existing sites.

Like the frog which shares the pools in which it lives, the tadpole shrimp has a very fast life cycle. It becomes a mature adult within two or three weeks of hatching. This allows it to complete its life cycle before the rain pools in which they live dry up.

The eggs are very resistant to drought and extreme temperatures and can survive for many years in the dried mud or dust at the bottom of a pool waiting for the right hatching conditions. They can even survive digestive juices and can pass through the digestive system of a bird unharmed. It is probably the ability of the eggs to survive under very difficult conditions that has enables the various species of triops to survive for so many millions of years.

Tadpole shrimps feed on small invertebrates, microscopic particles and plants. They absorb oxygen through their legs and can be seen moving their legs rhythmically all the time to move the water around them. They usually swim with their shield upwards but when oxygen is scarce they swim upside down with their feet close to the surface of the water where there is usually more oxygen.

The name Triops is derived from two Greek words meaning three and eyes. It got its name because these species have a pair of compound eyes and a third eye known as the ‘naupliar eye’ in between. 

This article was published in The Times of Malta on 3 April 2014.

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