Unless you step into an elevator that whisks you to the top of one of the planet's tallest buildings, it can be hard to appreciate the pressures that we are normally under. Most terrestrial species spend the majority of their lives oblivious to the 1 kg of air pressing down upon each square centimetre at sea level. But pressure is a pre-eminent factor in the survival of many marine species. Alastair Brown from the University of Southampton, UK, explains that little is known about the impact of high pressure on the physiology of many of the species that make their homes at depth. He says, ‘There appears to be a physiological bottleneck imposed on shallow-water creatures by decreasing temperature and increasing pressure between 1000 and 3000 m deep’, adding that few stray deeper. But there was little evidence for how increasing pressure might compromise survival.
With an entire ocean of species to choose from, Brown and Sven Thatje decided to focus on one member of the king crab family, Lithodes maja, which is content at depths ranging from 4 to 790 m around the northern European coast and North America. ‘The crabs’ latitudinal range in shallow water appears to be limited by the physiological impacts of temperature: too cold towards the Arctic and too warm towards the Tropics’, says Brown; but it wasn't clear why the crabs could go no deeper. After driving crabs, which had been collected by Bengt Lundve and Tony Roysson, 1800 km from Sweden to the National Oceanography Centre Southampton, Brown fitted heart rate monitors to the crustaceans’ shells and then installed the animals in high-pressure vessels before gradually increasing the pressure in 2.5 MPa steps (the equivalent of dropping them down 250 m) to 20 MPa – corresponding to a depth of 2000 m.
Monitoring the animals, Brown saw their heart rate fall from about 35 beats min−1 at the shallowest water pressures to ∼17 beats min−1 at the highest pressure. And when he analysed the crabs’ oxygen consumption, he was amazed to see it increase gradually up to a pressure of 12.5 MPa – the equivalent of a depth of 1250 m – before crashing dramatically as the pressure continued rising. ‘The decrease in metabolic rate [oxygen consumption] at high pressures greater than 12.5 MPa likely represents a distinct physiological response indicating extreme stress’, says Brown.
And when James Morris and Chris Hauton investigated how the crabs’ bodies responded to the stress, they saw evidence that the crustacean's nervous system might be impaired at high pressure while the animals also produced proteins that are designed to protect the body from the effects of stress. However, Andrew Oliphant, David Pond and Elizabeth Morgan found no sign that the animals altered the structure of their cell membranes (in order to maintain function at higher pressures) or switched to anaerobic respiration before their oxygen consumption dwindled.
Explaining that the metabolic cost of coping with high pressure probably accounts for the crabs’ distribution, Brown says, ‘We think that L. maja’s pressure tolerance is limited by the effects of high pressure on neurotransmission, which interferes with heart function and thus limits the ability to supply sufficient oxygen to cells to meet the elevated metabolic demand at high pressure’. And he urges scientists that are concerned about the impact of climate change on marine species to add pressure to the list of factors that they consider when attempting to predict population shifts. ‘Such an approach is crucial for accurately projecting biogeographic responses to changing climate’, he warns.
