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The Monday excerpt: What king crabs tell us about the crisis of climate change

As editor of the superb new collection of essays in Dispatches from Continent Seven: An anthology of Antarctic science, Rebecca Priestley has chosen wisely and wittily. Her book includes a frightening vision of natural disaster by Kathryn Smith, who examines how a rapidly warming ocean has encouraged the invasion of the complete bastards of the sea – King Crabs…

Hundreds of metres below the surface of the freezing ocean surrounding Antarctica the sea floor is teeming with life. The animals living here have no idea that an army is on the brink of invading their tranquil environment. The army is composed of king crabs. Until 2003, there were no crabs in this fragile Antarctic ecosystem. Now, driven by warming waters, their arrival heralds a major upset.

The unique communities living on the continental shelf off Antarctica are found in no other place on Earth. Delicate brittle stars, beautiful sea stars, vibrant sea lilies and giant sea spiders are among the spectacular inhabitants. The animals live side by side, with almost no predators to upset the balance.

For millions of years, the cold water temperatures in the Antarctic have stopped most predators from surviving in this harsh environment, but this is rapidly changing. Climate change is increasing temperatures across our planet, and the Antarctic is no exception. Sea temperatures are rising at a faster rate here than almost anywhere else.

With the increasing temperatures come new residents. Animals that have been absent from the continental shelf around Antarctica for millions of years are quickly returning.

A prime example are crabs. In every part of the world except Antarctica, crabs are one of the major predators in sea-floor communities. Their strong crushing claws are deadly to snails, brittle stars and other slow-moving animals. On the continental slope and continental shelf surrounding Antarctica, icy water has kept the crabs away. Crabs naturally take up magnesium into their blood from sea water and can normally control the levels present. However, at very low temperatures they lose this ability and the magnesium builds up, acting like an anaesthetic and eventually causing the crab to die.

Water temperatures around Antarctica are now warming to levels that crabs can tolerate. Although they and other predators have long been absent from the area, one group, king crabs, have been living in the neighbouring deep ocean, where water temperatures have historically been warmer. Now, as shelf temperatures increase, they are beginning to move. In 2003, king crabs were seen on the continental slope off Antarctica for the first time. Since then growing numbers have been reported. The crabs are seemingly marching up the continental slope and towards the continental shelf with nothing to stop them.

If king crabs move on to the shelf they will be presented with a smorgasbord of invertebrates. King crabs do not care much what they eat. Any animal that falls into their path makes a delicious treat. In the Antarctic the native inhabitants are particularly at risk. In other parts of the world, animals living on the sea floor have thick shells or hard skeletons to protect them against predators such as crabs, but in

Antarctica they have evolved without any major predators for millions of years, and so their defences are limited. Their very thin shells, soft bodies and light skeletons make them an easy target for the rapidly approaching king crabs.

When these crabs arrive, they are very likely to have a huge impact on the unique inhabitants. These defenceless animals may well become yet another casualty of climate change.

King crabs are not alone in their invasion of Antarctica. Several non-native species of plants and invertebrates have also made their way south and are in the process of settling into the cold barren landscape. Although such invasions are rare, they occur most commonly around the western Antarctic Peninsula where tourism is high.

Most shallow-water and land-based alien species arrive with the help of humans. Each year tens of thousands of people visit the Antarctic for tourism or science. In the 2014/15 season more than 46,000 arrived by cruise ship alone. The region is also seeing a growth in commercial fishing, both legal and illegal. Shallow-water marine animals and plants can be carried on the hull or in the ballast water of a fishing boat, cruise ship or research vessel. For example, a species of shallow-water crab has recently been found frequenting the coastline of Deception Island. This island is just off the western Antarctic Peninsula and one of the most popular stops for cruise ships and other vessels. It is likely it was one of these boats that carried the crab to Deception Island from southern Chile, although it may also have been borne by ocean currents or on floating debris.

Plants and other terrestrial species usually arrive in Antarctica attached to the footwear, clothing or equipment of tourists or scientists. Seeds, buds, suckers and spores of plants, including moss, have been found on many pieces of expedition equipment. Alien species of plant can also arrive aboard aircraft, or be carried by wind or migrating birds.

Both on land and in the water, alien species can wreak havoc among the vulnerable native ecosystems. Because of this, one of the aims of the Antarctic Treaty is to protect the continent from such invaders. Where possible, non-native species are removed. Most alien plants that have been found growing around the western Antarctic Peninsula have now been successfully eradicated, stopping them from spreading. However, invertebrate species such as flies, mites and worms are more difficult to remove because of their mobility. Large areas of habitat would need to be destroyed, and as a result to date no attempts have been made to wipe them out.

As we’ve seen, climate change is making conditions increasingly favourable for alien species in Antarctica – in shallow water and on land, as well as on the sea floor. The rising temperatures make it easier and easier for new species arriving here to stay. Consequently, it is no surprise that biological invasions are now thought to be the biggest threat to the continent’s conservation. As Antarctica continues to warm and ship traffic around it increases, the threat of invasions will only rise. Human activities may one day make the unique ecosystems of Antarctica, which have existed in isolation for millions of years, disappear completely.

“The March of the King Crabs” by Kathryn Smith features in Dispatches from Continent Seven: An anthology of Antarctic science, edited by Rebecca Priestley, and available at Unity Books.

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