They may not have the visual impact of a desolate-looking polar bear stranded on an ice floe, but we already have a contender for this year's ecological icon - migrating coral reefs. Japanese corals are heading north towards colder waters.

The corals seem to be running for their lives. The International Union for Conservation of Nature declared, in 1998, that four colonies of coral in Japanese waters were "vulnerable" or "near threatened". The same four colonies have been tracked moving at unprecedented speeds of up to 14 kilometres per year towards the North Pole.

Even before the full study was published in an issue of Geophysical Research Letters, the science journal Nature had flagged up the speed of
the migration as "stunning" in a report on its website.

Corals are composed of hundreds of thousands of organisms called polyps. Each millimetre-sized polyp secretes calcium carbonate, which eventually forms a reef. Though corals are usually static, they are not bound to the reef as a whole; if it has to, the population will up and leave. This happens when polyps find better waters to grow in. Those left behind in the original, problematic areas die out.

Corals initially show distress through "bleaching". This happens when they expel the microscopic algae that give them their colour. In normal conditions, the algae live in a mutually beneficial, symbiotic relationship with the coral.

When things get tough, however, the coral conserves its resources by kicking out the colourful organisms. In times of austerity, corals wear only white. Last year, a study carried out in the Caribbean (it's not all bad being a scientist) found signs of extreme distress in 80 per cent
of corals. In some areas, 40 per cent had died. Bleaching is not a long-term solution because the algae provide the coral with nutrients - glucose, glyceroland amino acids - that it can't manage without.

Egg energy

Some corals employ an alternative coping strategy: a sex change. By switching from female to male, mushroom coral manages to survive environmental stresses that cause other species of coral to become bleached and die.

It's an elegant solution to the problem of diminishing resources. Producing eggs costs more energy than producing sperm, so a male-dominated colony is more frugal, increasing its chances of toughing out the hard times. As the pressure lessens, some of the corals flip back to being female and life returns to normal.

Sometimes, however, the pressure cannot be reduced. As a result of higher sea temperatures, 2010 was one of the worst years on record for coral bleaching. Every coral-carrying sea suffered bleaching events, a phenomenon that had occurred only once before - in 1998.

In the past century, the waters around the migrating Japanese corals have heated by as much as 2.4° Celsius: enough to make the corals pack their bags and get out. Perhaps an exodus at 14 kilometres a year doesn't quite count as a majestic, sweeping migration, but it's another sure sign that change is coming.