Evolution: Crabs go deep to avoid...( Researchers from the National Oceano...)

Researchers from the National Oceanography Centre, Southampton, have drawn together 200 years' worth of oceanographic knowledge to investigate the distribution of a notorious deep-sea giant the king crab. The results, published this week in the Journal of Biogeography, reveal temperature as a driving force behind the speciation and radiation of a major seafloor predator; globally, and over tens of millions of years of Earth's history.

In deep seas all over the world, around 100 species of king crabs live largely undiscovered. The fraction that we have found includes some weird and wonderful examples - Paralomis seagrantii has its eight walking legs and claws entirely covered in long fur-like setae; while related group Lithodes megacanthus grows to lengths of 1.5 metres, and has 15-20-cm long defensive spines covering its body. At temperatures of around 1- 4C, these crabs thrive in some of the colder waters on Earth; living and growing very slowly, probably to very old ages. Only in the cooler water towards the poles are king crabs found near the water surface though temperatures found around some parts of the Antarctic (below 1C) are too extreme for their survival.

A paper, published 15 years ago in Nature is thought to show that king crabs evolved from shell-bound hermit crabs - similar to the familiar shoreline animals. Soft-bodied, but shell-free intermediate forms are found only in the shallow waters off Japan, Alaska, and Western Canada.

By looking at 200 years' worth of records from scientific cruises and museum collections, Sally Hall and Sven Thatje from the University of Southampton's School of Ocean and Earth Science at the National Oceanography Centre discovered that the soft-bodied forms can live at temperatures about ten degrees higher than the hard-bodied forms, but that both groups can only reproduce when temperature is between 1C up to 13-15C.

"It seems that most shallow-water represen
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