Researchers from the University of 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 divergence 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 have been 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- 4ºC, 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 1ºC) 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 Dr Sven Thatje from the
University of Southampton’s School of Ocean and Earth Science at the
National Oceanography Centre, Southampton 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 1ºC up to 13-15ºC.
"It seems that most shallow-water representatives of this family are
trapped in the coastal regions of the North Pacific because the higher
sea surface temperatures further south prevent them from reproducing
successfully and spreading," said Dr Thatje.
In order to leave this geographic bottleneck and spread around the
world, the shallow water ancestors of current deep-sea groups had to go
deep and adapt to the challenges of life in the deep sea. The process
of adaptation to constant low temperatures (1-4ºC) prevailing in the
deep sea seems to have narrowed the temperature tolerance range of the
crabs where they have emerged to the surface waters in the Southern
Hemisphere. With differences of only a couple of degrees in temperature
affecting the distribution of the king crab, it is difficult to predict
the consequences of range expansion in the warming waters around the
Antarctic Peninsular region.
King crabs are of great commercial value, and fisheries are
established in high latitude regions of both hemispheres.
"Understanding their evolutionary history and ecology is key to
supporting sustainable fisheries of these creatures," said research
student Sally Hall. She adds: "Recent range extensions of king crabs
into Antarctica, as well as that of the red king crab Paralithodes
camtchaticus in the Barents Sea and along the coast off Norway
emphasise the responsiveness of this group to rapid climate change."
This study reveals 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.
The study has been supported by the National Environment Research
Council (UK) through a PhD studentship to Sally Hall, and a Research
Grant from the Royal Society awarded to Dr Thatje.
Source : University of Southampton