At the bottom of the world, a creature no larger than a thumbnail holds together one of Earth's last great wild ecosystems — and the question of whether it can also sustain a booming human supplement industry is now being answered, quietly, by supermarket buyers. Coles and Woolworths have withdrawn krill oil products from their shelves, joining a growing cohort of major international retailers who are stepping back from a $154 million market under pressure from environmental advocates. The Antarctic krill fishery closed early for the second year running after hitting its 620,000-tonne limit, a