Conquest of the King Crab – discussing the invasive status of Lithodid crabs in Antarctica
It has long been believed that there are no crabs in Antarctica. However, in 2006 at Palmers Deep (Antarctic shelf), scientists disproved this theory. Three species of crabs, all from the family Lithodidae, or king crab, were discovered with population densities of over 10,000 per kilometre squared. Since this discovery, further studies have uncovered 22 different species of crustaceans within the Southern Ocean. Little is known about these species, as some are endemic to this location. From this finding two opposing theories have been generated.
“Invasion Theory” suggests there were previously no crustaceans located within the Southern Ocean, but global warming has allowed organisms to recolonize previously uninhabitable areas. The cooling that created the Antarctic began 50 million years ago, triggered by movement of the plates; the Antarctic Circumpolar Current (ACC) was created and theorized to act almost as a physical barrier to the movement of species. Temperatures within the Southern Ocean show a current annual variation of only 2.5°C due to this current, making them highly stable in regards to temperature, varying around 0°C. These conditions are alleged to have caused the disappearance of shell-crushing predatory crustaceans; becoming locally extinct 15 million years ago, during the Miocene period. Only five crustacean species were thought to remain in the Southern Ocean, all coming from the shrimp family.
In the past 50 years temperatures within the Southern Ocean have risen by 1°C, and are projected to climb by 2°C more in the next 80 years. The Lithodid crabs discovered in Palmers deep were restricted to areas exceeding temperature of 1.4°C, despite surface water temperatures of the Southern Ocean only reaching 1°C. Contrary to oceans around the world, where temperatures decrease with depth, temperatures within the Southern Ocean increase with depth, as warmer waters are pushed down by the circulation of the ACC. The crabs were located at 850m and are currently absent in the extensively surveyed colder, shallower surrounding areas; despite some of the species present being able to colonise depths of 250m in warmer locations.
No fossil evidence has ever been found for crabs being located in Antarctica, supporting the idea of a geologically sudden intrusion by these organisms.
Video showing an expedition into the Southern Ocean to survey “invasive” crab populations in order to understand their impacts. (Source-NSF/Allinson Randolph, FIT)
In contrast to the “Invasion Theory”, the “Endurance Theory” is widely accepted as the probable cause. This states that instead of lithodid crabs, and other crustaceans, disappearing from the Southern ocean, they have always been there; limited technological advancements and ability to survey deep-sea zones is blamed for these gaps in knowledge. The first discovery of crabs were made through remotely operated vehicles, a relatively recent invention. Of the large populations of lithodid crabs located at Palmers Deep, many were reproducing or available to reproduce. Like many organisms within the deep, cold, Southern ocean, crabs posses a slow development period and can often live for decades. The rich, well-developed community at Palmers deep had not just appeared over a small time scale. Geological records also show the slopes where lithodid crabs have been uncovered were available for organisms to colonise for 9000 years, yet little has been recorded here. The fact species have evolved to be entirely unique to this location shows populations must have been located in these areas for long time scales. Large gaps in distributions are found to coincide with areas of limited study, showing distribution patterns of lithodid crabs within the Southern Ocean to be controlled by our knowledge, not actual populations.
The fossil evidence attributed to supporting the invasion hypothesis can also be called into question. Crab fossils are rare, particularly those of lithodid crabs; these often occupy the deep ocean; the pressure at these depths crushes the shells before they can form fossils. A factor unique to the poles, which also prevents fossils forming, is ice scrape, as ice bergs grind along the sea floor, breaking up live and dead organisms. Only 2 Lithodid fossils have ever been found worldwide, and neither of these was located in the poles.
What does this mean?
An understanding of the species located within the Southern ocean is vitally important in order to evaluate how current ocean temperature rises will affect communities. Antarctica was recently stated to be “the fastest warming area on the planet”. However, this is only an example of the warming that is occurring in other seas. Organisms that occupy cold environments may have to retreat further and further south to keep within safe temperature ranges, creating true invasive species within the southern ocean. Ability to track these invasive species and understand their effect on populations is limited by the present lack of knowledge of these communities. Intrusions of predators can have serious impacts on the delicate ecosystems found in Antarctica. Diversity is greatly reduced in areas where the lithodid crabs are present, endemic or not, and this can have a large impact on foodwebs.
Populations of the density and size of those found at Palmers Deep, formed of one of the worlds largest arthopods, have taken many years to locate and understand. Smaller populations may be living throughout the Southern Ocean. Of the believed 20,000 species that may be present on the Southern Ocean floor, less than half have been named or even located. Griffiths stated “Global warming could have already changed these populations, but… we’ve only just started to track these changes”. He recommends large scale studies into the species present here, so we may better understand the effects of climate change and future changes to our oceans.