Algae are aquatic plants that range in size from shrublike seaweed to microscopic cells. At the smaller end of the scale are the diatoms, some species of which are circular while others are cigar-shaped. The nucleus of each diatom is surrounded by a porous wall of silica, the hard mineral that also forms quartz crystals and sand grains. This protective layer allows diatoms to live not only in water but in ice at the surface of the polar seas. Diatoms in water are eaten by krill, the little crustaceans that compose the diet of filter-feeding whales. The food chain from microscopic plants to blue whales is thus a rather short one.