Rainbow nature: life in brilliant blue
From ocean to sky, blue is seemingly everywhere - but it's one of the most difficult colours for nature to produce using pigments.
Animals usually appear blue to the human eye because of microscopic structures in their skin, feathers, scales or shells, or because they take on the colour when they eat certain foods.
Even among plants, blue is relatively rare: less than 10% of the world's flowering plant species produce blue blooms.
Discover 10 examples of bold blue wildlife that show off this colour to striking effect.
Blue-footed booby (Sula nebouxii)
Proudly strutting around potential mates, the male blue-footed booby makes the most of its bold footwear.
More than a style statement, blue serves an important purpose for this bird, which is native to the Pacific coasts of Central and South America.
Females are thought to pick mates with the brightest feet, because the intensity of colour reflects the male's health.
The blue colour is produced by pigments found in the fish the birds eat. Studies have shown males that had recently eaten fresh fish had brighter feet and stronger immune systems.
A male embarks on a complicated courtship ritual by giving the female a stick or a stone. It then tips its beak, tail and wings to the sky and prances around the female with a high-step, showing off its unusual feet.
Blue sea star (Linckia laevigata)
The blue in starfish is created by carotenoids, common pigments responsible for producing some red, yellow and orange colours. The blue colour is a result of these pigments combining with a protein.
Lobsters are blue for the same reason. They turn red when cooked because the blue carotenoid and protein complex breaks down, releasing the red carotenoids.
The sea star can be different colours, including yellow or green. The colour is thought to depend on differences in the protein.
Emu eggs (Dromaius novaehollandiae)
Colours in eggshells are created by chemical compounds called tetrapyrroles. There are only two of these chemicals found in bird eggs, one blue-green and the other brown.
The pigments are deposited while the eggs are forming in the bird's oviduct.
It has recently been confirmed that the same pigments exist in fossilised eggshells of some extinct birds and dinosaurs - proving that laying colourful eggs is not a recent phenomenon.
Hyacinth macaw (Anodorhynchus hyacinthinus)
When birds look blue, it is a trick of the light - their feathers are actually full of black melanin.
Blue feathers are an example of structural colour - where microscopic structures affect how light is reflected. The plumage colour we see is the result of the way light interacts with both the microscopic structures and the melanin.
Tiny pockets of air inside the feathers preferentially reflect blue light, because blue light has shorter wavelengths. The blue colour is intensified because dark melanin granules absorb the other wavelengths of light.
Sky blue mushroom (Entoloma hochstetteri)
Azulene pigments are responsible for the sky blue mushroom's bold hue. The fungus is blue all over, with a red tint on the gills caused by its spores. Researchers are still examining how and why the strong colour is produced.
The fungus is so widespread in New Zealand it is part of Maori folklore and even features on the country's $50 note.
Common blue damselfly (Enallagma cyathigerum)
Often seen drifting through the air, the common blue damselfly has a distinctive pattern on its body. Males have blue abdomens with black spots, while females have green-yellow or blue abdomens with more variable markings.
The species is widespread in Britain, but is often confused with other species that share the male’s blue and black colouring. The damselfly can be distinguished by the mushroom-shaped black spot on its abdomen.
While closely related, damselflies and dragonflies are different. Unlike dragonflies, most damselflies rest with their wings close to their bodies, and their eyes are small and on the sides of their heads.
Although they look like worms, caecilians are actually legless amphibians.
Spending most of their lives underground, they have bodies that are specially adapted for burrowing. They create channels through loose or moist soil by pushing their strong and bony, bullet-shaped heads through it.
Given that caecilians spend much of their lives in darkness, it is remarkable that some of them have developed strong colours. While many caecilians are drab, others have colours ranging from purple to orange, or feature vivid stripes.
Although the function of their bright colours is still unknown, scientists think the hues may serve as a warning to potential predators that dig them up. Like all amphibians, caecilians secrete toxins through their skin.
British bluebell (Hyacinthoides non-scripta)
Carpets of intense blue transform British woods in spring. The flowers responsible are bluebells, and they occur in their millions. They burst into bloom to make the most of the sunlight available before the opening tree canopy leaves the woodland floor in shade.
Although the native British bluebell, Hyacinthoides non-scripta, is found along the edge of western Europe, about half the world’s population is in the UK.
The population in this region is under threat from interbreeding with non-native Spanish bluebells, Hyacinthoides hispanica, which creates hybrid plants.
Blue pollen is one clue that the bluebells you’re looking at aren’t native British ones. British bluebells have creamy white pollen and a strong, sweet smell.
Native and non-native bluebells are also shaped differently: British bluebell petals curve backwards at the tips, and the flowers hang to one side of the main stem.
Common blue butterfly (Polyommatus icarus)
This pastel-coloured species is the most widespread blue butterfly in Britain.
Males are brighter shades than females, and their blue wings are bordered with black or brown and a thin, white fringe. Wing colours of females vary from brown to blue, depending on the butterfly's location.
Like in the hyacinth macaw, structural colour is responsible for the blue hue of the wings. Flavonoid pigments are also present - they absorb UV and create wing patterns visible to creatures that can see in the UV range. Common blue butterflies can see these patterns, though humans cannot.
The flavonoids present in the adults' wings come from the plants the species consumed as caterpillars.
Blue poison dart frog (Dendrobates azureus)
The dazzling hue of the blue poison dart frog warns predators of deadly toxins lurking within its skin.
Frogs of the Dendrobatidae family can be yellow, gold, red, green, or blue. They are found in the rainforests of Central and South America, and have been introduced on some Hawaiian islands.
Jewel-like colours are produced by chromatophores, irregularly-shaped cells that either contain pigment or reflect light waves.
Flamboyant skin tones deter predators, and some species are toxic enough to kill a human. The frogs do not produce their own toxins, but take them in from their prey or the environment.