In the world of wildlife, the word ‘camouflage’ can mean a lot of things. It sometimes refers to mimicry, which means being seen but incorrectly identified – think grasshoppers pretending to be dead leaves. The attributes we associate most with camouflage, however, are things which make animals hard to see at all; this is known as crypsis.
A clear advantage
Invisibility may sound like the stuff of fantasy, but it’s surprisingly common in our seas. True transparency is tricky to achieve – any food eaten will be visible, and eyes must be opaque in order to function. But partial transparency can still be useful, especially in shallower waters where light is plentiful.
Even the stealthiest animals are visible from above by the shadow they cast on the seabed, and from below by their silhouette. Since more light can pass through their bodies, transparent species have a diffuse outline from both angles, which makes it harder for predators and prey to identify them.
Local examples of this superpower include the common shrimp, the moon jellyfish, and the aptly named transparent goby. Eels also have a glass-like developmental stage which is thought to help them survive the long trip from their birthplaces at sea to the rivers where they mature into adults.
If transparency isn’t an option then clever use of colour could be the answer. Sardines, herrings, and sea bass are all darker along their spines, which is called countershading. In natural lighting the tops of objects are illuminated while their undersides fall into shadow. These fish are darker on top and lighter underneath, creating an optical illusion that helps them blend into their environment.
Seahorses, and flatfish like plaice, go one step further: they use pigment-filled skin cells called chromatophores to match the colour and pattern of their surroundings. Cuttlefish are masters of this trick thanks to additional control over cells called papillae, which they can raise to alter the texture of their skin too.
Dress to impress
Some of our marine species get crafty and enhance their natural camouflage with items from their environment. Dahlia anemones have sticky warts on their bodies which pick up pieces of shell, sand, and gravel. When their tentacles are retracted the anemones can be almost entirely obscured by these pieces of debris.
Several species of spider crab are called ‘decorators’ due to their love of accessorising. The great spider crab, for instance, has a shell with a rough and knobbly surface; this acts like velcro, allowing the crab to disguise itself with pieces of seaweed and sponge. Over time, these decorations can become a home for tiny animals like hydroids, making each crab a walking habitat!
Photo: Great spider crab Photo Credit: Amy Lewis ©