Two marine biologists have been probing the murky depths of Carew Tidal Pond in search of a rare worm.

The Loch Ness monster this creature is not. It measures just 5mm and, unlike Nessie, its presence has now been established by Francis Bunker and Jon Moore.

A single specimen of the worm was found in Carew during a national survey of lagoons by the Natural History Museum in 1997.

This prompted the latest survey. Francis and Jon have been commissioned by the Countryside Council for Wales to study the tentacled lagoon worm population at Carew.

Alkamaria romijini - its Latin name - may be lowly, but it is protected by law, affording Carew Tidal Pond some of the highest environmental protection in the land.

Francis and Jon, who are both based in Pembrokeshire, have been licenced under the Wildlife and Country-side Act to survey the population's distribution and abundance.

They established the presence of the worms and carefully released the creatures back into wild after counting them.

Saline lagoons, like the one at Carew and Pickleridge, Dale, are a rare and vanishing habitat. More than 70% have been lost in the UK during the last 50 years.

The specialised creatures that have adapted to live in saline lagoons are now some of the rarest in Britain and include the tentacled lagoon worm, says Francis, of Bentlass, Hundleton.

"Little is known of these amazing creatures which live in a tube made of silt and change sex from male to female as they grow," he says.