Horseshoe crabs have ten eyes – the largest pair is used only for finding mates.
Horseshoe Crab – Vision © Sophia Volzke
A quick glance at the horseshoe will show the crab’s two compound lateral eyes.
This is unusual because no other living animal from the Crab’s sub-family, Chelicerate, possesses compound eyes.
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Whirligig beetles have two pairs of eyes — one pair for looking underwater and one for looking above the waterline.
Horseshoe crabs have existed in essentially the same form for the past 135 million years. Their blood is blue in colour and provides a valuable test that protect us humans from the toxins that cause septic shocks.
The Diving Bell Spider (Argyroneta aquatica) is the only known spider in the world that lives entirely underwater. Like other arachnids, it must breathe air, so it creates a dome-shaped web between aquatic plants and fills the ‘diving bell’ with air carried from the surface.
They live almost entirely within the bells, darting out to catch prey that touches the bell or the silk threads that anchor it.
The largest kind of crustacean, the giant spider crab of Japan, measures up to 3.7 meters (12 feet) across between its outstretched claws. The smallest crustaceans, such as water fleas, can be smaller than 0.2 millimeter (1⁄125 inch) long.
Mantis Shrimps are the only animals to have hyperspectral colour vision and are considered to have the most complex eyes in the animal kingdom. They can see ultraviolet, visible and infra-red light and different planes of polarized light. The shape of their eyes allows them to see things with three different parts of the eye at once.
Harlequin Mantis Shrimp © p@ragon
They are aggressive predators and feed mainly on small fish, other crustaceans and molluscs that they are able to strike with acceleration of up to 23 metres per second. The shock wave from the force can be enough to stun or even kill the prey. Some larger species with hammer-like claws have even been known to break the glass of aquariums.
Crabs’ eyes are on stalks like periscopes that perceive polarised light and have a view of the whole 360 degree panorama around them.
It supports providing compass information for navigation and enables them to judge distance while the stalks fold away into protective grooves when the crab enters the burrow.
Diving beetles can hunt for food underwater where land-living insects cannot compete.
In Antarctica where only two per cent of the land is ever free of ice, the largest organism that does not rely directly on the sea for its food is a species of midge.
15 % of crabs are left-handed, the same percentage as humans.
Ghost Crab © Sophia Volzke
The Ghost Crab (Ocypode Cordimanna) is common to the sandy beaches of the Australian East Coast. The nocturnal crab has a translucent creamy coloured body which is where it’s name comes from. The bigger claw of this young male indicates the one preferred.
